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1-2-3 RuleA means of avoiding winds associated with a tropical cyclone by taking into account the forecast track error of the National Weather Service over a 10 year period which is approximately 100 nm in 24 hours, 200 nm for 48 hours and 300 nm in 72 hours. The forecast track error is added to the 34 knot wind radii to compute the danger area. The wind radii may be found within Tropical Cyclone Forecast Advisory (TCM) forecasts. 100-year FloodA statistic that indicates the magnitude of flood which can be expected to occur on average with a frequency of once every 100 years at a given
point or reach on a river. The 100-year flood is usually developed from a statistical distribution that is based on historical floods.
This is also called a base flood.100-year Flood PlainThe flood plain that would be inundated in the event of a 100-year flood.A IndexA daily index of geomagnetic activity derived as the average of
the eight 3-hourly a indices.Absolutely Stable AirAn atmospheric condition that exists when the environmental lapse rate is less than the moist adiabatic lapse rate.
Absolutely Unstable AirAn atmospheric condition that exists when the environmental lapse rate is greater than the dry adiabatic lapse rate.AbutmentThe part of a valley or canyon wall against which a dam is constructed. Right and left abutments are those on respective sides of an observer looking downstream.Abutment SeepingReservoir water that moves through seams or pores in the natural abutment material and exits as seepage.Accessory CloudA cloud which is dependent on a larger cloud system for development and continuance. Roll clouds, shelf clouds, and wall clouds are examples of accessory clouds.AccretionThe growth of a precipitation particle by the collision of a frozen particle with a supercooled liquid water droplet which freezes upon impact.Acid PrecipitationPrecipitation, such as rain, snow or sleet, containing relatively high concentrations of acid-forming chemicals that have been released into the atmosphere and combined with water vapor; harmful to the environment.Acre-footThe amount of water required to cover one acre to a depth of one foot. An acre-foot equals 326,851 gallons, or 43,560 cubic feet.Action StageThe stage which, when reached by a rising stream, represents the level where the NWS or a partner/user needs to take some type of mitigation action in preparation for possible significant hydrologic activity. The appropriate action is usually defined in a weather forecast office (WFO) hydrologic services manual. Action stage can be the same as forecast issuance stage (see / forecast issuance stage/).Active(abbrev. ACTV). In solar-terrestrial terms, solar activity levels with at least one geophysical event or
several larger radio events (10cm) per day (Class M Flares)Active Conservation StorageIn hydrologic terms, the portion of water stored in a reservoir that can be released for all useful purposes such as municipal water supply, power, irrigation, recreation, fish, wildlife, etc. Conservation storage is the volume of water stored between the inactive pool elevation and flood control stage. Active Dark Filament (ADF)In solar-terrestrial terms, an Active Prominence seen on the Disk.Active LongitudeIn solar-terrestrial terms, the approximate center of a range of heliographic longitudes in which Active Regions are more numerous and more flare-active than the average.Active ProminenceIn solar-terrestrial terms, a prominence displaying material motion and changes
in appearance over a few minutes of time.Active Prominence Region (APR)In solar-terrestrial terms, a portion of the solar limb displaying active prominences.Active Region (AR)In solar-terrestrial terms, a localized, transient volume of the solar atmosphere in which plages, sunspots, faculae, flares, etc. may be observed.Active Storage CapacityIn hydrologic terms, the total amount of reservoir capacity normally available for release from a reservoir below the maximum storage level. It is total or reservoir capacity minus inactive storage capacity. More specifically, it is the volume of water between the outlet works and the spillway crest. Active Surge Region (ASR)In solar-terrestrial terms, an Active Region that exhibits a group or
series of spike-like surges that rise above the limb.ADAPTATION (ADAPTABLE) PARAMETERGenerally, data related to a specific WSR-88D unit. These data may consist of meteorological or hydrological parameters or of geographic boundaries, political boundaries, system configuration, telephone numbers (auto dial), or other like data. Such data may be generated at either a centralized location or locally at the WSR-88D unit. Additive Data A group of coded remarks that includes pressure tendency, amount of precipitation, and maximum/minimum temperature during specified periods of
timeAdiabatic Lapse RateThe rate of decrease of temperature experienced by a parcel of air when it is lifted in the atmosphere under the restriction that it cannot exchange heat with its environment. For parcels that remain unsaturated during lifting, the (dry adiabatic) lapse rate is 9.8°C per kilometer.Adiabatic ProcessA process which occurs with no exchange of heat between a system and its environment.Adirondack Type Snow Sampling SetIn hydrologic terms, a snow sampler consisting of a 5-foot fiberglass tube, 3 inches in diameter, with a serrated-edge steel cutter at one end and a twisting handle at the other. This sampler has a 60-inch snow depth capacity. Advanced Baseline Imager (ABI)A moderate resolution imaging system on the GOES and Himawari family of satellites.Advanced Hydrologic Prediction Service (AHPS)A web-based suite of accurate and information-rich forecast products. They display the magnitude and uncertainty of occurrence of floods or droughts, from hours to days and months in advance. These graphical products are useful information and planning tools for many economic and emergency managers.Advection(Abbrev. ADVCTN)- Transport of an atmospheric property by the wind.Advection FogA fog that forms when warm air flows over a cold surface and cools from below until saturation is reached.Aeration ZoneA portion of the lithosphere in which the functional interstices of permeable rock or earth are not filled with water under hydrostatic pressure. The interstices either are not filled with water or are filled with water that is no held by capillarity.AeroallergensAny of a variety of allergens such as pollens, grasses, or dust carried by winds.AerosolA system of colloidal particles dispersed in a gas, such as smoke or fog.AFREDAbbreviation for the A Index for Fredericksburg.AfterbayIn hydrologic terms, the tail race of a hydroelectric power plant at the outlet of the turbines. The term may be applied to a short stretch of stream or conduit, or to a pond or reservoir.AgglomerateAn ice cover of floe formed by the freezing together of various forms of ice. Air Mass ThunderstormGenerally, a thunderstorm not associated with a front or other type of synoptic-scale forcing mechanism. Air mass thunderstorms typically are associated with warm, humid air in the summer months; they develop during the afternoon in response to insolation, and dissipate rather quickly after sunset. They generally are less likely to be severe than other types of thunderstorms, but they still are capable of producing downbursts, brief heavy rain, and (in extreme cases) hail over 3/4 inch in diameter.
Since all thunderstorms are associated with some type of forcing mechanism, synoptic-scale or otherwise, the existence of true air-mass thunderstorms is debatable.Air Pollution PotentialThe meteorological potential for air pollution problems, considered without regard to the presence or absence of actual pollution sources.Air Quality ModelMathematical or conceptual model used to estimate present or future air quality.Air Transportable Mobile UnitA modularized transportable unit containing communications and observational equipment necessary to support a meteorologist preparing on-site forecasts at a wildfire or other incident.Airborne Snow Survey ProgramIn hydrologic terms, Center (NOHRSC) program that makes airborne snow water equivalent and soil moisture measurements over large areas of the country that are subject to severe and chronic snowmelt flooding.AIRMETAirman's Meteorological advisory (WA)Alaska CurrentA North Pacific Ocean current flowing counterclockwise in the Gulf of Alaska. It is the northward flowing (warm) division of the Aleutian CurrentAlbedoReflectivity; the fraction of radiation striking a surface that is reflected by that surface.Alberta ClipperA fast moving low pressure system that moves southeast out of Canadian Province of Alberta (southwest Canada) through the Plains, Midwest, and Great Lakes region usually during the winter. This low pressure area is usually accompanied by light snow, strong winds, and colder temperatures. Another variation of the same system is called a "Saskatchewan Screamer".ALERTAutomated Local Event Reporting in Real Time. Network of automatic raingauges that transmit via VHF radio link when precipitation occurs. Some sites
are also equipped with other sensors such as temperature, wind, pressure, river stage or tide level.Alert StageThe stage which, when reached by a rising stream, represents the level where appropriate officials (e.g., county sheriff, civil defense officials, or bypass gate operators) are notified of the threat of possible flooding. (Used if different from action stage, and at the discretion of the WFO or river forecast center [RFC].) The term "alert stage" is to be used instead of warning stage. Monitor stage or caution stage may be used instead of alert stage in some parts of the country.Aleutian CurrentAn eastward flowing North Pacific Ocean current which lies north of the North Pacific Current. Aleutian LowA semi-permanent, subpolar area of low pressure located in the Gulf of Alaska near the Aleutian Islands. It is a generating area for storms and migratory lows often reach maximum instensity in this area. It is most active during the late fall to late spring. During the summer, it is weaker, retreating towards the North Pole and becoming almost nonexistent. During this time, the North Pacific High pressure system dominates.Along-slope Wind SystemA closed, thermally driven diurnal mountain wind circulation whose lower branch blows up or down the sloping sidewalls of a valley or mountain. The upper branch blows in the opposite direction, thereby closing the circulation.AltimeterAn instrument that indicates the altitude of an object above a fixed level. Pressure altimeters use an aneroid barometer with a scale graduated in altitude instead of pressure.Altimeter SettingA correction of the station pressure to sea level used by aviation. This correction takes into account the standard variation of pressure with height and the influence of temperature variation with height on the pressure. The temperatures used correspond to the standard atmosphere temperatures between the surface and sea level. AmbientOf the surrounding area or environment.AmplifierA device used to increase the strength of an analog signalAmplitudeThe maximum magnitude of a quantity. Often used to refer to the maximum height of a wave.AMVER Automated Mutual Assistance Vessel Rescue System. A system operated by the U.S. Coast Guard which computes the nearest available rescue vessels for vessels in distress using vessel track and position reports supplied by participating vessels. AMVER/SEASA software program created by the National Weather Service intended to efficiently generate AMVER and VOS reports as part of a cooperative effort.Anchor IceIn hydrologic terms, submerged frazil ice attached or anchored to the river bottom, irrespective of its formation.Anchor Ice DamAn accumulation of anchor ice which acts as a dam and raises the water level.AnemometerAn instrument used for measuring the speed of the wind.Aneroid BarometerAn instrument for measuring atmospheric pressure in which a needle, attached to the top of an evacuated box, is deflected as changes in atmospheric pressure cause the top of the box to bend in or out.
AngelsRadar echoes caused by birds, insects, and localized refractive index discontinuities.Angle of ReflectionThe angle at which a reflected ray of energy leaves a reflecting surface. It is measured between the outgoing ray and a perpendicular to the surface at the point of incidence (i.e., where the ray strikes).Annual Exceedance Probability (AEP)The probability that a stream reach will have a flow of a certain magnitude in any given year.Antedecent Precipitation Index(Abbrev. API) - an index of moisture stored within a drainage basin before a storm.ANTENNA GAINThe measure of effectiveness of a directional antenna as compared to an isotropic radiator, maximum value is called antenna gain by convention. AnthelionA luminous white spot that appears on the parhelic circle at the same altitude as the sun and 180 degrees from it in azimuth. Anthropogenic SourceA pollutant source caused or produced by humans.AnticyclogenesisThe formation or intensification of an anticyclone or high pressure center.AnticycloneA large-scale circulation of winds around a central region of high atmospheric pressure, clockwise in the Northern Hemisphere, counterclockwise in the Southern HemisphereAntilles CurrentA current which originates in the vicinity of the Leeward Islands as part of the Atlantic North Equatorial Current.Anvil Crawler[Slang], a lightning discharge occurring within the anvil of a thunderstorm, characterized by one or more channels that appear to crawl along the underside of the anvil. They typically appear during the weakening or dissipating stage of the parent thunderstorm, or during an active MCS.Anvil DomeA large overshooting top or penetrating top.Anvil RolloverSlang for a circular or semicircular lip of clouds along the underside of the upwind part of a back-sheared anvil, indicating rapid expansion of the anvil.AP IndexIn solar-terrestrial terms, an averaged planetary A Index based on data from a set of specific
stations.AphelionThe point on the annual orbit of a body (about the sun) that is farthest from the sun; at present, the earth reaches this point (152 million kilometer from the sun) on about 5 July. Opposite of perihelion.API MethodIn hydrologic terms, a statistical method to estimate the amount of surface runoff which will occur from a basin from a given rainstorm based on the antecedent precipitation index, physical characteristics of the basin, time of year, storm duration, rainfall amount, and rainfall intensity. ApogeeThe farthest distance between the moon and earth or the earth and sun.Apparent TemperatureA measure of human discomfort due to combined heat and humidity (e.g., heat index).Apparent WindThe speed and true direction from which the wind appears to blow with reference to a moving point. Sometimes called RELATIVE WIND.AquicludeIn hydrologic terms, a formation which contains water but cannot transmit it rapidly enough to furnish a significant supply to a well or spring.AquiferIn hydrologic terms, permeable layers of underground rock, or sand that hold or transmit groundwater below the water table that will yield water to a well in sufficient quantities to produce water for beneficial use.AquifugeIn hydrologic terms, a geologic formation which has no interconnected openings and cannot hold or transmit water. Arch Filament System (AFS)In solar-terrestrial terms, a bright, compact plage crossed by a system of
small, arched filaments, which is often a sign of rapid or continued growth in an Active Region.Arctic Sea SmokeSteam fog, but often specifically applied to steam fog rising from small open water within sea ice. Area Forecast DiscussionThis National Weather Service product is intended to provide a well-reasoned discussion of the meteorological thinking which went into the preparation of the Zone Forecast Product. The forecaster will try to focus on the most particular challenges of the forecast. The text will be written in plain language or in proper contractions. At the end of the discussion, there will be a list of all advisories, non-convective watches, and non-convective warnings. The term non-convective refers to weather that is not caused by thunderstorms. An intermediate Area Forecast Discussion will be issued when either significant forecast updates are being made or if interesting weather is expected to occur.Area Hydrologic Discussion (AHD)A short range, episodic, discussion and graphic which highlights locations across the nation that may be impacted by rapid-onset flooding, using National Water Model and other guidance.Area of InfluenceIn hydrologic terms, the area covered by the drawdown curves of a given pumping well or combination of wells at a particular time. Area SourceAn array of pollutant sources, so widely dispersed and uniform in strength that they can be treated in a dispersion model as an aggregate pollutant release from a defined area at a uniform rate. Compare line source and point source.Area Wide Hydrologic Prediction System(Abbrev. AWHPS) - A computer system which automatically ingests areal flash flood guidance values and WSR-88D products and displays this data and other hydrologic information on a map background.Area-Capacity CurveIn hydrologic terms, a graph showing the relation between the surface area of the water in a reservoir, the corresponding volume, and elevation. Artesian WellIn hydrologic terms, a well drilled into a confined aquifer with enough hydraulic pressure for the water to flow to the surface without pumping. Also called a flowing well.ASSOCIATED PRINCIPAL USERA Principal User with dedicated communications to a WSR-88D unit.AtmosphereThe air surrounding and bound to the Earth.Atmospheric Boundary LayerSame as Boundary Layer - in general, a layer of air adjacent to a bounding surface. Specifically, the term most often refers to the planetary boundary layer, which is the layer within which the effects of friction are significant. For the earth, this layer is considered to be roughly the lowest one or two kilometers of the atmosphere. It is within this layer that temperatures are most strongly affected by daytime insolation and nighttime radiational cooling, and winds are affected by friction with the earth's surface. The effects of friction die out gradually with height, so the "top" of this layer cannot be defined exactly.Atmospheric Circulation ModelA mathematical model for quantitatively describing, simulating, and analyzing the structure of the circulation in the atmosphere and the underlying causes. Sometimes referred to as Atmospheric General Circulation Models or AGCMs.Atmospheric PressureThe pressure exerted by the earth's atmosphere at any given point, determined by taking the product of the gravitational acceleration at the point and the mass of the unit area column of air above the point.Atmospheric RadiationInfrared radiation (energy in the wavelength interval of 3- 80 micrometer) emitted by or being propagated through the atmosphere. It consists of both upwelling and downwelling components. Compare with terrestrial radiation.AttenuationIt refers to the reduction of the radar beam power due to the reflection or absorption of energy when it strikes a target. The greatest attenuation occurs
when the radar beam goes through very heavy rain.ATTENUATIONAny process in which the flux density (power) of a beam of energy is dissipated. Augmented reportA meteorological report prepared by an automated surface weather observing system for transmission with certified observers signed on to the
system to add information to the report.Aurora BorealisAlso known as the northern lights; the luminous, radiant emission from the upper atmosphere over middle and high latitudes, and centred around the earth's magnetic poles. These silent fireworks are often seen on clear winter nights in a variety of shapes and colors.Automated Event Reporting Gage(also see Tipping Bucket Rain Gage); for river stage gages, IFLOWS pressure transducer type gages can be programmed to report if water surface rises or falls by a predetermined amount. Automated ReportA meteorological report prepared by an automated surface weather observing system for transmission, and with no certified weather observers
signed on to the system.Automated Surface Observing SystemThe ASOS program is a joint effort of the National Weather Service (NWS), the Federal Aviation Administration (FAA), and the Department of Defense (DOD). Completed in the mid-1990s, the ASOS systems serve as the nation's primary surface weather observing network. ASOS is designed to support weather forecast activities and aviation operations and, at the same time, support the needs of the meteorological, hydrological, and climatological research communities.Autumnal EquinoxThe equinox at which the sun approaches the Southern Hemisphere, marking the start of astronomical autumn in the Northern Hemisphere. The time of this occurrence is approximately September 22. On that day, daylight is everywhere 12 hours. Compare with vernal equinox, offset by six months.AvalancheA mass of snow, rock, and/or ice falling down a mountain or incline. In practice, it usually refers to the snow avalanche. In the United States, the term snow slide is commonly used to mean a snow avalanche.Avalanche AdvisoryA preliminary notification that conditions may be favorable for the development of avalanches in mountain regions.Azimuth AngleThe direction or bearing toward which a sloping surface faces (e.g., a north-facing slope has an azimuth angle of 360°; a northeast-facing slope, an azimuth angle of 45°).Azores CurrentOne of the currents of the North Atlantic subtropical gyre.
Azores HighAlternate term for Bermuda High - a semi-permanent, subtropical area of high pressure in the North Atlantic Ocean off the East Coast of North America that migrates east and west with varying central pressure. Depending on the season, it has different names. When it is displaced westward, during the Northern Hemispheric summer and fall, the center is located in the western North Atlantic, near Bermuda. In the winter and early spring, it is primarily centered near the Azores in the eastern part of the North Atlantic. Also known as Azores High. Back-building ThunderstormA thunderstorm in which new development takes place on the upwind side (usually the west or southwest side), such that the storm seems to remain stationary or propagate in a backward direction.Back-sheared Anvil [Slang], a thunderstorm anvil which spreads upwind, against the flow aloft. A
back-sheared anvil often implies a very strong updraft and a high severe weather potential.BackfireA fire started to stop an advancing fire by creating a burned area in its path.
BackscatterThe portion of power scattered back in the incident direction.Backwater CurveIn hydrologic terms, the longitudinal profile of the surface of a liquid in a non-uniform flow in an open channel, when the water surface is not parallel to the invert owing to the depth of water having been increased by the interposition of an obstruction such as a dam or weir. The term is sometimes used in a generic sense to denote all water surface profiles; or for profiles where the water is flowing at depths greater than the critical.Backwater EffectIn hydrologic terms, the effect which a dam or other obstruction has in raising the surface of the water upstream from it.Backwater FloodingHydrologic terms, upstream flooding caused by downstream conditions such as channel restriction and/or high flow in a downstream confluence stream.BANDPASS FILTERA filter whose frequencies are between given upper and lower cutoff values, while substantially attenuating all frequencies outside these values (this band). Bank StorageIn hydrologic terms, water absorbed and stored in the void in the soil cover in the bed and banks of a stream, lake, or reservoir, and returned in whole or in part as the level of water body surface falls.Bankfull StageAn established gage height at a given location along a river or stream, above which a rise in water surface will cause the river or stream to overflow the lowest natural stream bank somewhere in the corresponding reach. The term "lowest bank" is however, not intended to apply to an unusually low place or a break in the natural bank through which the water inundates a small area. Bankfull stage is not necessarily the same as flood stage.Banner CloudA cloud plume often observed to extend downwind behind isolated mountain peaks, even on otherwise cloud-free days.Barber Pole[Slang], a thunderstorm updraft with a visual appearance including cloud striations that are
curved in a manner similar to the stripes of a barber pole. The structure typically is most pronounced on the
leading edge of the updraft, while drier air from the rear flank downdraft often erodes the clouds on the
trailing side of the updraft. Baroclinic leaf shieldA cloud pattern on satellite images - frequently
noted in advance of formation of a low pressure center.Baroclinic ZoneA region in which a temperature gradient exists on a constant pressure surface. Baroclinic zones are favored areas for strengthening and weakening systems; barotropic systems, on the other hand, do not exhibit significant changes in intensity. Also, wind shear is characteristic of a baroclinic zone.BarometerAn instrument that measures atmospheric pressure.Barometric PressureThe pressure of the atmosphere as indicated by a barometer.Barotropic SystemA weather system in which temperature and pressure surfaces are coincident, i.e., temperature is uniform (no temperature gradient) on a constant pressure surface. Barotropic systems are characterized by a lack of wind shear, and thus are generally unfavorable areas for severe thunderstorm development. See baroclinic zone.
Usually, in operational meteorology, references to barotropic systems refer to equivalent barotropic systems - systems in which temperature gradients exist, but are parallel to height gradients on a constant pressure surface. In such systems, height contours and isotherms are parallel everywhere, and winds do not change direction with height.
As a rule, a true equivalent barotropic system can never be achieved in the real atmosphere. While some systems (such as closed lows or cutoff lows) may reach a state that is close to equivalent barotropic, the term barotropic system usually is used in a relative sense to describe systems that are really only close to being equivalent barotropic, i.e., isotherms and height contours are nearly parallel everywhere and directional wind shear is weak.BarrageIn hydrologic terms, any artificial obstruction placed in water to increase water level or divert it. Usually the idea is to control peak flow for later release. Barrier JetA jet-like wind current that forms when a stably-stratified low-level airflow approaches a mountain barrier and turns to the left to blow parallel to the longitudinal axis of the barrier.
Bartel's Rotation NumberThe serial number assigned to 27-day rotation
periods of solar and geophysical parameters. Rotation 1 in this
sequence was assigned arbitrarily by Bartel to begin in January
1833.BASE DATAThose digital fields of reflectivity, mean radial velocity, and spectrum width data in spherical coordinates provided at the finest resolution available from the radar.Base FloodIn hydrologic terms, the national standard for floodplain management is the base, or one percent chance flood. This flood has at least one chance in 100 of occurring in any given year. It is also called a 100 year flood.BASE PRODUCTSThose products that present some representation of the base data. This representation may not necessarily be either in full resolution or depict the full area of coverage. Base products can be used to generate a graphic display or further processing. Base ReflectivityBase Reflectivity is the default image. Taken from the lowest (½° elevation) slice, it is the primary image used to "see what's out there". There are two versions of Base Reflectivity image; the short range version which extends out to 124 nautical miles (143 statute miles/230 kilometers) and the long range version which extends out to 248 nautical miles (285 statute miles/460 kilometers). This image is available upon completion of the ½° elevation scan during each volume scanBase StationIn hydrologic terms, a computer which accepts radio signals from ALERT gaging sites, decodes the data, places the data in a database, and makes the data available to other users. Base WidthIn hydrologic terms, the time duration of a unit hydrograph.BaseflowIn hydrologic terms, streamflow which results from precipitation that infiltrates into the soil and eventually moves through the soil to the stream channel. This is also referred to as ground water flow, or dry-weather flow. Basin RechargeIn hydrologic terms, rainfall that adds to the residual moisture of the basin in order to help recharge the water deficit. i.e; water absorbed into the soil that does not take the form of direct runoff.BathymetryThe science of measuring depths of the oceans, lakes, seas, etc.Beach ErosionThe movement of beach materials by some combination of high waves, currents and tides, or wind.BEAM FILLINGThe measure of variation of hydrometeor density throughout the radar sampling volume. If there is no variation in density, the beam is considered to be filled.BEAM WIDTHAngular width of antenna pattern. Usually that width where the power density is one-half that of the axis beam. (Half-Power or 3 dB point)Bear's Cage[Slang], a region of storm-scale rotation, in a thunderstorm, which is wrapped in heavy
precipitation. This area often coincides with a radar hook echo and/or mesocyclone, especially one
associated with an HP storm.
The term reflects the danger involved in observing such an area visually, which must be done at close range
in low visibility.Beaufort ScaleThe Beaufort wind scale is a system used to estimate and report wind speeds when no measuring apparatus is available. It was invented in the early 19th Century by Admiral Sir Francis Beaufort of the British Navy as a way to interpret winds from conditions at sea. Since that time, the scale has been modernized for effects on land.
Beaufort Force 0 - Wind less than 1 kt, Calm, Sea surface smooth and mirror-like. Smoke rises vertically.
Beaufort Force 1 - Wind 1-3 kt, Light Air, Scaly ripples, no foam crests. Smoke drift indicates wind direction, still wind vanes.
Beaufort Force 2 - Wind 4-6 kt, Light Breeze, Small wavelets, crests glassy, no breaking waves. Wind felt on face, leaves rustle, vanes begin to move.
Beaufort Force 3 - Wind 7-10 kt, Gentle Breeze, Large wavelets, crests begin to break, scattered whitecaps. Leaves and small twigs constantly moving, light flags extended.
Beaufort Force 4 - Winds 11-16 kt, Moderate Breeze, Small waves 1 -4 ft. becoming longer, numerous whitecaps. Dust, leaves, and loose paper lifted, small tree branches move.
Beaufort Force 5 - Winds 17-21 kt, Fresh Breeze, Moderate waves 4 -8 ft taking longer form, many whitecaps, some spray. Small trees in leaf begin to sway.
Beaufort Force 6 - Winds 22-27 kt, Strong Breeze, Larger waves 8 -13 ft, whitecaps common, more spray. Larger tree branches moving, whistling in wires.
Beaufort Force 7 - Winds 28-33 kt, Near Gale, Sea heaps up, waves 13 -20 ft, white foam streaks off breakers. Whole trees moving, resistance felt walking against wind.
Beaufort Force 8 - Winds 34-40 kt Gale, Moderately high (13 -20 ft) waves of greater length, edges of crests begin to break into spindrift, foam blown in streaks. Whole trees in motion, resistance felt walking against wind.
Beaufort Force 9 - Winds 41-47 kt, Strong Gale, High waves (20 ft), sea begins to roll, dense streaks of foam, spray may reduce visibility. Slight structural damage occurs, slate blows off roofs.
Beaufort Force 10 - Winds 48-55 kt, Storm, Very high waves (20 -30 ft) with overhanging crests, sea white densely blown foam, heavy rolling, lowered visibility. Seldom experienced on land, trees broken or uprooted, "considerable structural damage".
Beaufort Force 11 - Winds 56-63 kt, Violent Storm, Exceptionally high (30 -45 ft) waves, foam patches cover sea, visibility more reduced.
Beaufort Force 12 -Winds 64+ kt, Hurricane, Air filled with foam, waves over 45 ft, sea completely white with driving spray, visibility greatly reduced. Beaver('s) Tail[Slang], a particular type of inflow band with a relatively broad, flat appearance
suggestive of a beaver's tail. It is attached to a supercell's general updraft and is oriented roughly parallel to
the pseudo-warm front, i.e., usually east to west or southeast to northwest. As with any inflow band, cloud
elements move toward the updraft, i.e., toward the west or northwest. Its size and shape change as the
strength of the inflow changes. See also inflow stinger.Bed LoadIn hydrologic terms, sand, silt, gravel, or soil and rock detritus carried by a stream on or immediately above its bed. The particles of this material have a density or grain size such as to preclude movement far above or for a long distance out of contact with the stream bed under natural conditions of flow. Beginning of FreezupIn hydrologic terms, date on which ice forming a stable winter ice cover is first observed on the water surfaceBeginning of the BreakupIn hydrologic terms, date of definite breaking, movement, or melting of ice cover or significant rise of water level. Benchmark(Abbrev. BM) - In hydrologic terms, a permanent point whose known elevation is tied to a national network. These points are created to serve as a point of reference. Benchmarks have generally been established by the USGS, but may have been established by other Federal or local agencies. Benchmarks can be found on USGS maps.Bergeron ProcessThe process by which ice crystals in a cloud grow at the expense of supercooled liquid water droplets.Bergy BitA piece of ice which has broken away from an iceberg, extending 1-5 meters above the sea surface and 100-300 square meters in area. Can also be the remains of a melting iceberg.Bermuda HighA semi-permanent, subtropical area of high pressure in the North Atlantic Ocean off the East Coast of North America that migrates east and west with varying central
pressure. Depending on the season, it has different names. When it is displaced westward, during the Northern Hemispheric summer and fall, the center is located in the western North
Atlantic, near Bermuda. In the winter and early spring, it is primarily centered near the Azores in the eastern part of the North Atlantic. Also known as Azores High.Best TrackA subjectively-smoothed representation of a tropical cyclone's location and intensity
over its lifetime. The best track contains the cyclone's latitude, longitude, maximum
sustained surface winds, and minimum sea-level pressure at 6-hourly intervals.
Best track positions and intensities, which are based on a post-storm assessment
of all available data, may differ from values contained in storm advisories. They also
generally will not reflect the erratic motion implied by connecting individual center fix
positions.Black Ice1. Slang reference to patchy ice on roadways or other transportation surfaces that cannot easily be seen.
2. In hydrologic terms, transparent ice formed in rivers and lakes.Blocked FlowFlow approaching a mountain barrier that is too weak or too stable to be carried over the barrier.Blue Watch or Blue Box[Slang], a severe thunderstorm watch.BlusterySame as Breezy; 15 to 25 mph winds.Border IceIn hydrologic terms, an ice sheet in the form of a long border attached to the bank or shore.; shore ice. Boundary LayerIn general, a layer of air adjacent to a bounding surface. Specifically, the term most often refers to the planetary boundary layer, which is the layer within which the effects of friction are significant. For the earth, this layer is considered to be roughly the lowest one or two kilometers of the atmosphere. It is within this layer that temperatures are most strongly affected by daytime insolation and nighttime radiational cooling, and winds are affected by friction with the earth's surface. The effects of friction die out gradually with increasing height, so the "top" of this layer cannot be defined exactly.
There is a thin layer immediately above the earth's surface known as the surface boundary layer (or simply the surface layer). This layer is only a portion of the planetary boundary layer, and represents the layer within which friction effects are more or less constant throughout (as opposed to decreasing with height, as they do above it). The surface boundary layer is roughly 10 meters thick (from the surface up to 10 m above the ground), but again the exact depth is indeterminate. Like friction, the effects of insolation and radiational cooling are strongest within this layer.Bounded Weak Echo Region (BWER)(Also known as a vault.) Radar signature within a
thunderstorm characterized by a local minimum in radar reflectivity at low levels which extends
upward into, and is surrounded by, higher reflectivities aloft. This feature is associated with
a strong updraft and is almost always found in the inflow region of a thunderstorm. It cannot be seen
visually.Bow EchoA radar echo which is linear but bent outward in a bow shape. Damaging straight-line winds often occur near the "crest" or center of a bow echo. Areas of circulation also can develop at either end of a bow echo, which sometimes can lead to tornado formation - especially in the left (usually northern) end, where the circulation exhibits cyclonic rotation.Bowen RatioFor any moist surface, the ratio of heat energy used for sensible heating (conduction and convection) to the heat energy used for latent heating (evaporation of water or sublimation of snow). The Bowen ratio ranges from about 0.1 for the ocean surface to more than 2.0 for deserts; negative values are also possible. It is named for Ira S. Bowen (1898-1978), an American astrophysicist.Box ModelA computer model used to calculate air pollution concentrations. A box model is based on the assumption that pollutants are emitted into a box through which they are immediately and uniformly dispersed. The sides and bottom of the box are defined by the sidewalls and floor of the valley being studied.Brackish IceIn hydrologic terms, ice formed from brackish water.Braided StreamIn hydrologic terms, characterized by successive division and rejoining of streamflow with accompanying islands. A braided stream is composed of anabranches.Brash IceIn hydrologic terms, accumulation of floating ice made up of fragments not more than 2 meters across; the wreckage of other forms of ice. BreachIn hydrologic terms, the failed opening in a dam.BreakersWaves that break, displaying white water. Depends on wave steepness and bottom bathymetry.BreakupIn hydrologic terms, the time when a river whose surface has been frozen from bank to bank for a significant portion of its length begins to change to an open water flow condition. Breakup is signaled by the breaking of the ice and often associated with ice jams and flooding.Breakup DateIn hydrologic terms, date on which a body of water is first observed to be entirely clear of ice and remains clear thereafter. Breakup JamIn hydrologic terms, an ice jam that occurs as a result of the accumulation of broken ice pieces.Breakup PeriodIn hydrologic terms, the period of disintegration of an ice cover.Breezy15 to 25 mph windsBright Surge on the Disk (BSD)In solar-terrestrial terms, a bright gaseous stream (surge) emanating from the chromosphere.Bright Surge on the Limb (BSL)In solar-terrestrial terms, a large gaseous stream (surge) that moves outward more than 0.15 solar radius above the limb.BrightnessA basic visual sensation describing the amount of light that appears to emanate from an object, or more precisely, the luminance of an object Brocken SpecterAn optical phenomenon sometimes occurring at high altitudes when the image of an observer placed between the sun and a cloud is projected on the cloud as a greatly magnified shadow. The shadow's head is surrounded by rings of color, called a glory.Broken LevelA layer of the atmosphere with 5/8 to 7/8 sky cover (cloud cover).Bubble HighA mesoscale area of high pressure, typically associated with cooler air from the
rainy downdraft area of a thunderstorm or a complex of thunderstorms. A gust front or outflow
boundary separates a bubble high from the surrounding air.Bubbler GageIn hydrologic terms, a water stage recording device that is capable of attaching to a LARC for data automation purposes. Bulk Richardson NumberA non-dimensional (i.e., no units) number relating vertical stability to vertical shear (generally, stability divided by shear). High values indicate unstable and/or weakly-sheared environments; low values indicate weak instability and/or strong vertical shear. Generally, values in the range of around 50 to 100 suggest environmental conditions favorable for supercell development.Buttress DamButtress dams are comprised of reinforced masonry or stonework built against concrete. They are usually in the form of flat decks or multiple arches. They require about 60 percent less concrete than gravity dams, but the increased form work and reinforcement steel required usually offset the savings in concrete. Many were built in the 1930's when the ratio of labor cost to materials was comparatively low. However, this type of construction is not competitive with other types of dams when labor costs are high.BWERAbbreviation for Bounded Weak Echo Region; a radar signature within a thunderstorm
characterized by a local minimum in radar reflectivity at low levels which extends upward into, and is
surrounded by, higher reflectivities aloft. This feature is associated with a strong updraft and is
almost always found in the inflow region of a thunderstorm. It cannot be seen visually.CAPEConvective Available Potential Energy. A measure of the amount of energy available for convection. CAPE is directly related to the maximum potential vertical speed within an updraft; thus, higher values indicate greater potential for severe weather. Observed values in thunderstorm environments often may exceed 1000 joules per kilogram (J/kg), and in extreme cases may exceed 5000 J/kg.
However, as with other indices or indicators, there are no threshold values above which severe weather becomes imminent. CAPE is represented on an upper air sounding by the area enclosed between the environmental temperature profile and the path of a rising air parcel, over the layer within which the latter is warmer than the former. (This area often is called positive area.) See also CIN.Capillary FringeIn hydrologic terms, the soil area just above the water table where water can rise up slightly through the cohesive force of capillary action. This layer ranges in depth from a couple of inches, to a few feet, and it depends on the pore sizes of the materials. The capillary fringe is also called the capillary zone.Capillary WavesWaves caused by the initial wind stress on the water surface causes what are known as capillary waves. These have a wavelength of less than 1.73 cm, and the force that tries to restore them to equilibrium is the cohesion of the individual molecules. Capillary waves are important in starting the process of energy transfer from the air to the water. Capillary ZoneUsed interchangably with Capillary Fringe; the soil area just above the water table where water can rise up slightly through the cohesive force of capillary action. This layer ranges in depth from a couple of inches, to a few feet, and it depends on the pore sizes of the materials.Capping InversionAlternate term for Cap; a layer of relatively warm air aloft, usually several thousand feet above the ground, which suppresses or delays the development of thunderstorms. Air parcels rising into this layer become cooler than the surrounding air, which inhibits their ability to rise further and produce thunderstorms. As such, the cap often prevents or delays thunderstorm development even in the presence of extreme instability. However, if the cap is removed or weakened, then explosive thunderstorm development can occur.
The cap is an important ingredient in most severe thunderstorm episodes, as it serves to separate warm, moist air below and cooler, drier air above. With the cap in place, air below it can continue to warm and/or moisten, thus increasing the amount of potential instability. Or, air above it can cool, which also increases potential instability. But without a cap, either process (warming/moistening at low levels or cooling aloft) results in a faster release of available instability - often before instability levels become large enough to support severe weather development.Carbon DioxideCO2; a colorless and odorless gas which is the fourth most abundant constituent of dry air.Carrington LongitudeA system of fixed longitudes rotating with the sunCatalina EddyA Catalina Eddy (coastal eddy) forms when upper level large-scale flow off Point Conception interacts with the complex topography of the Southern California coastline. As a result, a counter clockwise circulating low pressure area forms with its center in the vicinity of Catalina Island. This formation is accompanied by a southerly shift in coastal winds, a rapid increase in the depth of the marine layer, and a thickening of the coastal stratus. Predominately these eddies occur between April and September with a peak in June. A typical Catalina eddy will allow coastal low clouds and fog to persist into the afternoon. A strong Catalina eddy may extend to 6000 feet and these clouds will move through the inland valleys and reach as far as Palmdale.Catchment AreaIn hydrologic terms, an area having a common outlet for its surface runoff (also see Drainage Area or Basin, Watershed).CategoricalA National Weather Service precipitation descriptor for a 80, 90, or 100 percent chance of measurable precipitation (0.01 inch). See Precipitation Probability (PoP)Caution StageThe stage which, when reached by a rising stream, represents the level where appropriate officials (e.g., county sheriff, civil defense officials, or bypass gate operators) are notified of the threat of possible flooding. Alert stage or caution stage are used instead of caution stage in some parts of the country.Ceiling(Abbrev. CIG) - The height of the cloud base for the lowest broken or overcast cloud layer. CeilometerA device using a laser or other light source to determine the height of a cloud base. An optical ceilometer uses triangulation to determine the height of a spot of light projected onto the base of the cloud; a laser ceilometer determines the height by measuring the time required for a pulse of light to be scattered back from the cloud base.CellConvection in the form of a single updraft, downdraft, or updraft/downdraft couplet, typically seen as a vertical dome or tower as in a towering cumulus cloud. A typical thunderstorm consists of several cells.
The term "cell" also is used to describe the radar echo returned by an individual shower or thunderstorm. Such usage, although common, is technically incorrect.CelsiusThe standard scale used to measure temperature in most areas outside the United States. On this scale, the freezing point of water is 0°F and the boiling point is 100°F. To convert a Fahrenheit temperature to Celsius, subtract 32 from it and then multiply by 5/9:
°C = (°F - 32) * 5/9 CEMCivil Emergency Message. A message issued by the National Weather Service in coordination with Federal, state or local government to warn the
general public of a non-weather related time-critical emergency which threatens life or property, e.g. nuclear accident, toxic chemical spill, etcCenterGenerally speaking, the vertical axis of a tropical cyclone, usually defined by the
location of minimum wind or minimum pressure. The cyclone center position can
vary with altitude. In advisory products, refers to the center position at the surface.Centimeter BurstA solar radio burst in the centimeter wavelength range.Central Meridian Passage (CMP)In solar-terrestrial terms, the passage of an Active Region or other
feature across the longitude meridian that passes through the
apparent center of the solar disk.CENTROIDThe center of mass of a storm.ChanceA National Weather Service precipitation descriptor for 30, 40, or 50 percent chance of measurable precipitation (0.01 inch). When the precipitation is convective in nature, the
term scattered is used. See Precipitation Probability (PoP). ChannelIn hydrologic terms, also known as Watercourse; an open conduit either naturally or artificially created which periodically, or continuously contains moving water, or forms a connecting link between two bodies of water. River, creek, run, branch, anabranch, and tributary are some of the terms used to describe natural channels. Natural channels may be single or braided. Canal and floodway are some of the terms used to describe artificial channels.Channel InflowIn hydrologic terms, water, which at any instant, is flowing into the channel system form surface flow, subsurface flow, base flow, and rainfall that has directly fallen onto the channel.Channel LeadIn hydrologic terms, an elongated opening in the ice cover caused by a water current.Channel RoutingIn hydrologic terms, the process of determining progressively timing and shape of the flood wave at successive points along a river.Channeled High WindsIn mountainous areas or in cities with tall buildings, air may be channeled through constricted passages producing high winds. Santa Ana winds and winds through passes from the cold Alaskan interior to the sea are examples of these winds. Channeled high winds are local in nature but can be extremely strong. These winds generally occur in well-defined areas.ChannelizationIn hydrologic terms, the modification of a natural river channel; may include deepening, widening, or straightening.Chemistry ModelA computer model used in air pollution investigations that simulates chemical and photochemical reactions of the pollutants during their transport and diffusion.ChromosphereIn solar-terrestrial terms, the layer of the solar atmosphere above the photosphere and
beneath the transition region and the corona.Chromospheric EventsIn solar-terrestrial terms, flares that are just Chromospheric Events without Centimetric Bursts or Ionospheric Effects. (SID) (Class C flare)Civil Emergency Message(Abbrev. CEM) - A message issued by the National Weather Service in coordination with Federal, state or local government to warn the general public of a non-weather related time-critical emergency which threatens life or property, e.g. nuclear accident, toxic chemical spill, etc.Class I AreasGeographic areas designated by the Clean Air Act where only a small amount or increment of air quality deterioration is permissible.Clear Air Turbulence(CAT) - In aviation, sudden severe turbulence occurring in cloudless regions that causes violent buffeting of aircraft.Clear IceA thin coating of ice on terrestrial objects, caused by rain that freezes on impact. The ice is relatively transparent, as opposed to rime ice, because of large drop size, rapid accretion of liquid water, or slow dissipation of latent heat of fusion.Clear SlotWith respect to severe thunderstorms, a local region of clearing skies or reduced cloud cover, indicating an intrusion of drier
air; often seen as a bright area with higher cloud bases on the west or southwest side of a wall
cloud. A clear slot is believed to be a visual indication of a rear flank downdraft.Client AgencyAs used in connection with reimbursable National Weather Service (NWS) fire
weather services, a public fire service or wildlands management agency, Federal or non-Federal,
which requires and uses NWS fire and forestry meteorological servicesClimateThe composite or generally prevailing weather conditions of a region, throughout the year, averaged over a series of years.Climate ChangeA non-random change in climate that is measured over several decades or longer. The change may be due to natural or human-induced causes.Climate Diagnostics Bulletin(CDB) - The monthly CPC Bulletin reports on the previous months' status of the ocean-atmosphere climate system and provides various seasonal ENSO-related outlooks. It is issued by the fifteenth of the month.Climate Diagnostics Center(CDC) - The mission of NOAA's Climate Diagnostics Center is to identify the nature and causes for climate variations on time scales ranging from a month to centuries.Climate ModelMathematical model for quantitatively describing, simulating, and analyzing the interactions between the atmosphere and underlying surface (e.g., ocean, land, and ice).Climate OutlookA climate outlook issued by the CPC gives probabilities that conditions, averaged over a specified period, will be below-normal, normal, or above-normal.Climate Prediction CenterThis Center is one of several centers under the National Centers for Environmental Prediction (NCEP) part of the National Weather Service (NWS) in the National Oceanic and Atmospheric Administration (NOAA). The Center serves the public by assessing and forecasting the impacts of short-term climate variability, emphasizing enhanced risks of weather-related extreme events, for use in mitigating losses and maximizing economic gains.Climate SystemThe system consisting of the atmosphere (gases), hydrosphere (water), lithosphere (solid rocky part of the Earth), and biosphere (living) that determine the Earth's climate.ClimometerAn instrument that measures angles of inclination; used to measure cloud ceiling heights.Closed BasinA basin draining to some depression or pond within its area, from which water is lost only by evaporation or percolation. A basin without a surface outlet for precipitation falling precipitation.Closed Basin Lake FloodingFlooding that occurs on lakes with either no outlet or a relatively small one. Seasonal increases in rainfall cause the lake level to rise faster than it can drain. The water may stay at flood stage for weeks, months, or years.Closed LowA low pressure area with a distinct center of cyclonic circulation which can be completely encircled by one or more isobars or height contour lines. The term usually is used to distinguish a low pressure area aloft from a low-pressure trough. Closed lows aloft typically are partially or completely detached from the main westerly current, and thus move relatively slowly (see Cutoff Low).Cloud CeilingSame as Ceiling; the height of the cloud base for the lowest broken or overcast cloud layer. Cloud Condensation NucleiSmall particles in the air on which water vapor condenses and forms cloud droplets.Cloud LayerAn array of clouds whose bases are at approximately the same level.Cloud MovementThe direction toward which a cloud is movingCloud StreetsRows of cumulus or cumulus-type clouds aligned parallel to the low-level flow. Cloud streets sometimes can be seen from the ground, but are seen best on satellite photographs.ClutterRadar echoes that interfere with observation of desired signals on the radar display.CoalescenceThe process by which water droplets in a cloud collide and come together to form raindrops.Coastal WatersIncludes the area from a line approximating the mean high water along the mainland or island as far out as 100 nautical miles including the bays, harbors and sounds.Coastal Waters Forecast (CWF)The marine forecast for areas, including bays, harbors, and sounds, from a line approximating the mean high water mark (average height of high water over a 19-year period) along the mainland or near shore islands extending out to as much as 100 NM.
Coastal/Lakeshore Flood AdvisoryMinor flooding is possible (i.e., over and above normal high tide levels. Coastal/Lakeshore Flood Advisories are issued using the Coastal/Lakeshore Hazard Message (CFW) product. Coastal/Lakeshore Flood WarningFlooding that will pose a serious threat to life and property is occurring, imminent or highly likely. Coastal/Lakeshore Flood Warnings are issued using the Coastal/Lakeshore Hazard Message (CFW) product.Coastal/Lakeshore Flood WatchFlooding with significant impacts is possible. Coastal/Lakeshore Flood Watches are issued using the Coastal/Lakeshore Hazard Message (CFW) product.Coastal/Lakeshore Flooding(i) (Oceanic) Coastal Flooding is the inundation of land areas caused by sea waters over and above normal tidal action. This flooding may impact the immediate oceanfront, gulfs, bays, back bays, sounds, and tidal portions of river mouths and inland tidal waterways. (ii) Lakeshore Flooding is the inundation of land areas adjacent to one of the Great Lakes caused by lake water exceeding normal levels. Lakeshore flooding impacts the immediate lakefront, bays, and the interfaces of lakes and connecting waterways, such as rivers.COEIn hydrologic terms, Corps of Engineers.Coherent RadarA radar that utilizes both signal phase and amplitude to determine target characteristics.Cold AdvectionTransport of cold air into a region by horizontal winds.Cold Air AvalancheDownslope flow pulsations that occur at more or less regular intervals as cold air builds up on a peak or plateau, reaches a critical mass, and then cascades down the slopes.Cold Air FunnelA funnel cloud or (rarely) a small, relatively weak tornado that can develop from a small shower or thunderstorm when the air aloft is unusually cold (hence the name). They are much less violent than other types of tornadoes.Collection EfficiencyThe fraction of droplets approaching a surface that actually deposit on that surface.Columnar IceIn hydrologic terms, ice consisting of columnar shaped grain. The ordinary black ice is usually columnar-grained.Combined SeasGenerally referred to as SEAS. Used to describe the combination or interaction of wind waves and swells in which the separate components are not distinguished. This includes the case when swell is negligible or is not considered in describing sea state. Specifically, Seas2 = S2+W2 where S is the height of all swell components and W is the height of the wind wave components. When used, SEAS should be considered as being the same as the significant wave height.
Comma EchoA thunderstorm radar echo which has a comma-like shape. It often appears during latter
stages in the life cycle of a bow echo Complex Gale/Storm In the high seas and offshore forecasts, an area for which gale/storm force winds are forecast or are occurring but for which no single center is the principal generator of these winds.Complex TerrainTypically used to refer to mountainous terrain. In general usage, it may also refer to coastal regions and heterogeneous landscapes.CompositeAn average that is calculated according to specific criteria. For example, one might want a composite for the rainfall at a given location for all years where the temperature was much above average.Composite HydrographA stream discharge hydrograph which includes base flow, or one which corresponds to a net rain storm of duration longer than one unit period.Comprehensive Flare Index (CFI)In solar-terrestrial terms, the indicative of solar flare importance.
Concentric RingsThese are common in the most intense hurricanes. They usually mark the end the period of intensification. These hurricanes then maintain quasi-constant intensity or
weaken. When the inner eye is completely dissipated, more intensification may occur.CondensationIn general, the physical process by which a vapor becomes a liquid or solid;
the opposite of evaporation, although on the molecular scale, both processes are always
occurring.Condensation FunnelA funnel-shaped cloud associated with rotation and consisting of condensed water droplets (as opposed to smoke, dust, debris, etc.).Conditionally Unstable AirAn atmospheric condition that exists when the environmental lapse rate is less than the dry adiabatic lapse rate but greater than the moist adiabatic lapse rate.Cone of DepressionIn hydrologic terms, the depression, roughly conical in shape, produced in a water table, or other piezometric surface, by the extraction of water from a well at a given rate. The volume of the cone will vary with the rate of withdrawal of water. Also called the Cone of Influence. Cone of InfluenceSame as Cone of Depression; in hydrologic terms, the depression, roughly conical in shape, produced in a water table, or other piezometric surface, by the extraction of water from a well at a given rate. The volume of the cone will vary with the rate of withdrawal of water. Confined Ground WaterIn hydrologic terms, ground water held under an aquiclude or an aquifuge, called artesian if the pressure is positive.ConfluenceA pattern of wind flow in which air flows inward toward an axis oriented parallel to the general direction of flow. It is the opposite of difluence. Confluence is not the same as convergence. Winds often accelerate as they enter a confluent zone, resulting in speed divergence which offsets the (apparent) converging effect of the confluent flow.Congestus(or Cumulus Congestus) - same as towering cumulus. Congressional Organic Act of 1890The act that assigned the responsibility of river and floor forecasting for the benefit of the general welfare of the Nation’s people and economy to the Weather Bureau, and subsequently the National Weather Service.Conjugate PointsTwo points on the earth's surface, at opposite ends of a
geomagnetic field lineConservation StorageIn hydrologic terms, storage of water for later release for usual purposes such as municipal water supply, power, or irrigation in contrast with storage capacity used for flood control.Consolidated Ice CoverIn hydrologic terms, ice cover formed by the packing and freezing together of floes, brash ice and other forms of floating ice.Constant Pressure ChartAlternate term for Isobaric Chart; a weather map representing conditions on a surface of equal atmospheric pressure. For example, a 500 mb chart will display conditions at the level of the atmosphere at which the atmospheric pressure is 500 mb. The height above sea level at which the pressure is that particular value may vary from one location to another at any given time, and also varies with time at any one location, so it does not represent a surface of constant altitude/height (i.e., the 500 mb level may be at a different height above sea level over Dallas than over New York at a given time, and may also be at a different height over Dallas from one day to the next).ContentsIn hydrologic terms, the volume of water in a reservoir. Unless otherwise indicated reservoir content is computed on the basis of a level pool and does not include bank storage. Continental Air MassA dry air mass originating over a large land area. Contrast with tropical air mass.Continental ShelfThe zone bordering a continent and extending to a depth, usually around 100 FM, from which there is a steep descent toward greater depth.ConvectionGenerally, transport of heat and moisture by the movement of a fluid.
In meteorology, the term is used specifically to describe vertical transport of heat and moisture in the atmosphere, especially by updrafts and downdrafts in an unstable atmosphere. The terms "convection" and "thunderstorms" often are used interchangeably, although thunderstorms are only one form of convection. Cbs, towering cumulus clouds, and ACCAS clouds all are visible forms of convection. However, convection is not always made visible by clouds. Convection which occurs without cloud formation is called dry convection, while the visible convection processes referred to above are forms of moist convection.Convective Boundary LayerThe unstable boundary layer that forms at the surface and grows upward through the day as the ground is heated by the sun and convective currents transfer heat upwards into the atmosphere.Convective CloudsThe vertically developed family of clouds are cumulus and cumulonimbus. The height of their bases range from as low as 1,000 feet to a bit more than 10,000 feet.
Clouds with extensive vertical development are positive indications of unstable air. Strong upward currents in vertically developed clouds can carry high concentrations of supercooled
water to high levels where temperatures are quite cold. Upper portions of these clouds may be composed of water and ice.Convective Condensation Level(abbrev. CCL)- The level in the atmosphere to which an air parcel, if heated from below, will rise dry adiabatically, without becoming colder than its environment just before the parcel becomes saturated. See Lifted Condensation Level (LCL).Convective Inhibition(CIN or B-) - A numerical measure of the strength of "capping," typically used to assess thunderstorm potential. Specifically, it represents the cumulative effect of atmospheric layers the
are warmer than the parcel moving vertically along the adiabat. Low level parcel ascent is often
inhibited by such stable layers near the surface. If natural processes fail to destabilize the lower
levels, an input of energy from forced lift (a front, an upper level shortwave, etc.) will be required
to move the negatively buoyant air parcels to the point where they will rise freely. Since CIN is proportional to the amount of kinetic energy that a parcel loses to buoyancy while it is colder than the surrounding environment, it contributes to the downward momentum.Convective Outlook(sometimes called AC) - A forecast containing the area(s) of expected thunderstorm
occurrence and expected severity over the contiguous United States, issued several times daily by the SPC.
The terms approaching, slight risk, moderate risk, and high risk are used to describe severe thunderstorm
potential. Local versions sometimes are prepared by local NWS offices.Convective OverdevelopmentConvection that covers the sky with clouds, thereby cutting off the sunshine that produces convection.Convective TemperatureThe approximate temperature that the air near the ground must warm to in order for surface-based convection to develop, based on analysis of a sounding.
Calculation of the convective temperature involves many assumptions, such that thunderstorms sometimes develop well before or well after the convective temperature is reached (or may not develop at all). However, in some cases the convective temperature is a useful parameter for forecasting the onset of convection.ConvergenceA contraction of a vector field; the opposite of divergence. Convergence in a horizontal wind field indicates that more air is entering a given area than is leaving at that level. To compensate for the resulting "excess," vertical motion may result: upward forcing if convergence is at low levels, or downward forcing (subsidence) if convergence is at high levels. Upward forcing from low-level convergence increases the potential for thunderstorm development (when other factors, such as instability, are favorable). Compare with confluence.Conveyance LossIn hydrologic terms, the loss of water from a conduit due to leakage, seepage, evaporation, or evapo-transpiration.Cooling Degree Days(Abbrev. CDD) - A form of Degree Day used to estimate energy requirements for air conditioning or refrigeration. Typically, cooling degree days are calculated as how much warmer the mean temperature at a location is than 65°F on a given day. For example, if a location experiences a mean temperature of 75°F on a certain day, there were 10 CDD (Cooling Degree Days) that day because 75 - 65 = 10.Cooperative ObserverAn individual (or institution) who takes precipitation and temperature observations-and in some cases other observations such as river stage, soil temperature, and evaporation-at or near their home, or place of business. Many observers transmit their reports by touch-tone telephone to an NWS computer, and nearly all observers mail monthly reports to the National Climatic Data Center to be archived and published. Coordinated Universal Time (UTC)By international agreement, the local time at the prime meridian, which passes through Greenwich, England. Prior to 1972, this time was called Greenwich Mean Time (GMT) but is now referred to as Coordinated Universal Time or Universal Time Coordinated (UTC). It is a coordinated time scale, maintained by the Bureau International des Poids et Mesures (BIPM). It is also known a "Z time" or "Zulu Time".
More about UTC, and a table to convert UTC to your local time is
posted at:
http://www.srh.noaa.gov/jetstream/remote/radarfaq.htm#utcCore Punch[Slang], a penetration by a vehicle into the heavy precipitation core of a thunderstorm.
Core punching is not a recommended procedure for storm spotting.Coriolis ForceA fictitious force used to account for the apparent deflection of a body in motion with respect to the earth, as seen by an observer on the earth. The deflection (to the right in the Northern Hemisphere) is caused by the rotation of the earth.Corn Snow IceIn hydrologic terms, rotten granular ice. Corner EffectsA small-scale convergence effect that can be quite severe. It occurs around
steep islands and headlands.Coronal HoleIn solar-terrestrial terms, an extended region of the corona, exceptionally low in density
and associated with unipolar photospheric regions.Coronal TransientsIn solar-terrestrial terms, a general term for short-time-scale changes in the corona, but principally used to describe outward-moving plasma clouds.Correlated ShearAn output of the mesocyclone detection algorithm indicating a 3-dimensional shear region (i.e. vertically correlated) that is not symmetrical. CorrelationA measure of similarity between variables of functions.County Warning and Forecast AreaThe group of counties for which a National Weather Service Forecast Office is responsible for issuing warnings and weather forecasts.County Warning AreaThe group of counties for which a National Weather Service Forecast Office is responsible for issuing warnings.Coupled Atmosphere-Ocean ModelSame as Coupled Model; in the context of climate modeling this usually refers to a numerical model which simulates both atmospheric and oceanic motions and temperatures and which takes into account the effects of each component on the other. Coupled ModelIn the context of climate modeling this usually refers to a numerical model which simulates both atmospheric and oceanic motions and temperatures and which takes into account the effects of each component on the other.CoupletAdjacent maxima of radial velocities of opposite signs. CreekA small stream of water which serves as the natural drainage course for a drainage basin of nominal, or small size. The term is a relative one as to size, some creeks in the humid section would be called rivers if they occurred in the arid portion. Crepuscular RaysThe alternating bands of light and dark (rays and shadows) seen at the earth's surface when the sun shines through clouds.CrestHighest point in a wave.
In hydrologic terms, (1) The highest stage or level of a flood wave as it passes a point. (2) The top of a dam, dike, spillway, or weir, to which water must rise before passing over the structure. Crest GageA gage used to obtain a record of flood crests at sites where recording gages are installed.Crest WidthIn hydrologic terms, the thickness or width of a dam at the level of the crest (top) of the dam. The term "thickness" is used for gravity and arch dams and "width" for other types of dams.Critical DepthIn hydrologic terms, The depth of water flowing in an open channel or conduit, partially filled, corresponding to one of the recognized critical velocities.CrochetIn solar-terrestrial terms, a sudden deviation in the sunlit geomagnetic field (H component;
see geomagnetic elements) associated with large solar
flare X-ray emissionCrop Moisture IndexIn 1968, Palmer developed the index to assess short-term crop water conditions and needs across major crop-producing regions. This index is a useful tool in forecasting short-term drought conditions.Cross-Valley Wind SystemA thermally driven wind that blows during daytime across the axis of a valley toward the heated sidewall.Crown FireA fire where flames travel from tree to tree at the level of the tree's crown or top.Cubic Feet per Second(Abbrev. CFP) - In hydrologic terms, a unit expressing rates of discharge. One cubic foot per second is equal to the discharge through a rectangular cross section, 1 foot wide by 1 foot deep, flowing at an average velocity of 1 foot per second. It is also approximately 7.48 gallons per second. Cumulus CongestusA large, towering cumulus cloud with great vertical development, usually with a cauliflower-like appearance, but lacking the characteristic anvil of a cumulonimbus.CurrentA horizontal movement of water. Currents may be classified as tidal and nontidal. Tidal currents are caused by gravitational interactions between the sun, moon, and earth and are a part of the same general movement of the sea that is manifested in the vertical rise and fall, called TIDE. Tidal currents are periodic with a net velocity of zero over the tidal cycle. Nontidal currents include the permanent currents in the general circulatory systems of the sea as well as temporary currents arising from more pronounced meteorological variability. The SET of a current is the direction toward which it flows; the DRIFT is its speed.Current MeterIn hydrologic terms, device used to measure the water velocity or current in a river.
Cyclogenesis(Abbrev. CYCLGN) - The formation or intensification of a cyclone or low-pressure storm system.Cyclone(abbrev. CYC) - A large-scale circulation of winds around a central region of low atmospheric pressure, counterclockwise in the Northern Hemisphere, clockwise in the Southern Hemisphere.D RegionIn solar-terrestrial terms, a daytime layer of the earth's ionosphere approximately 50 to
90 km in altitude.Daily Climatological ReportAs the name indicates, this climatological product is issued daily by each National Weather Service office. Most of the climatological data in this report are
presented in a tabular form; however, some narrative statements may also be used in the product. The report is organized so that similar items are grouped together (i.e., temperature,
precipitation, wind, sunrise and sunset times, etc.).Daily Flood PeakIn hydrologic terms, the maximum mean daily discharge occuring in a stream during a given flood event.Dam FailureIn hydrologic terms, catastrophic event characterized by the sudden, rapid, and uncontrolled release of impounded water.Dark Surge on Disk (DSD)In solar-terrestrial terms, dark gaseous ejections visible in H-alpha.Dart LeaderA faint, negatively charged channel that travels more or less directly and continuously from cloud to ground.DATANETIn hydrologic terms, it is the hydrologic Data Network Analysis Software.Day LengthDuration of the period from sunrise to sunset.Dead StorageIn hydrologic terms, the volume in a reservoir below the lowest controllable level.Debris CloudA rotating "cloud" of dust or debris, near or on the ground, often appearing
beneath a condensation funnel and surrounding the base of a tornado.
This term is similar to dust whirl, although the latter typically refers to a circulation which contains
dust but not necessarily any debris. A dust plume, on the other hand, does not rotate. Note that a
debris cloud appearing beneath a thunderstorm will confirm the presence of a tornado, even in the
absence of a condensation funnel.DecadalOccurring over a 10-year period, such as an oscillation whose period is roughly 10 years ("Pacific Decadal Oscillation").DeclinationThe latitude that the sun is directly over at a given time. The declination is ~23°N at the summer solstice, ~23°S at the winter solstice, and 0° (over the equator) at the spring and autumn equinoxes.Deep Percolation LossIn hydrologic terms, water that percolates downward through the soil beyond the reach of plant roots. Deep SeepageIn hydrologic terms, infiltration which reaches the water table. Deep wellIn hydrologic terms, a well whose pumping head is too great to permit use of a suction pump.DeepeningA decrease in the central pressure of a surface low pressure system. The storm is intensifying.Deformation ZoneThe change in shape of a fluid mass by variations in
wind, specifically by stretching and/or shearing. Deformation is a primary factor in frontogenesis (evolution of fronts) and frontolysis
(decay of fronts). Deformed IceIn hydrologic terms, a general term for ice which has been squeezed together and forced upwards and downwards in places. Subdivisions are rated
ice, ridge ice, hummocked ice, and other similar deformations.DegradationIn hydrologic terms, the geologic process by means of which various parts of the surface of the earth are worn down and carried away and their
general level lowered, by the action of wind and water.Degree DayA measure that gauges the amount of heating or cooling needed for a building using 65 degrees
as a baseline. Electrical, natural gas, power, and heating, and air conditioning industries utilize heating and cooling degree information to calculate their needs. For more specific definitions and how to calculate degree days, see the definitions for Heating Degree Days and Cooling Degree Days.DeltaIn hydrologic terms, an alluvial deposit, often in the shape of the Greek letter "delta", which is formed where a stream drops its debris load on entering a
body of quieter water.Delta TChange in temperature.
1) A simple representation of the mean lapse rate within a layer of the atmosphere, obtained by calculating the difference between observed temperatures at the bottom and top of the layer. Delta Ts often are computed operationally over the layer between pressure levels of 700 mb and 500 mb, in order to evaluate the amount of instability in mid-levels of the atmosphere. Generally, values greater than about 18 indicate sufficient instability for severe thunderstorm development.
2) The difference in temperature between the surface of a lake and 850mb, typically used to determine
lake effect snow potential.DendritesIn hydrologic terms, thin branch-like growth of ice on the water surface. DendriticIn hydrologic terms, the form of the drainage pattern of a stream and it's tributaries when it follows a treelike shape, with the main trunk, branches, and
twigs corresponding to the main stream, tributaries, and subtributaries, respectively, of the stream.Dense Fog AdvisoryIssued when fog reduces visibility to 1/8 mile or less over a widespread area.
For marine products: An advisory for widespread or localized fog reducing visibilities to regionally or locally defined limitations not to exceed 1 nautical mile.
Dense Smoke AdvisoryAn advisory for widespread or localized smoke reducing visibilities to regionally or locally defined limitations not to exceed 1 nautical mile. Density CurrentIn hydrologic terms, a flow of water maintained by gravity through a large body of water, such as a reservoir or lake, and retaining its unmixed identity
because of a difference in density.Density of SnowIn hydrologic terms, the ratio, expressed as a percentage, of the volume which a given quantity of snow would occupy if it were reduced to water, to
the volume of the snow. When a snow sampler is used, it is the ratio expressed as percentage of the scale reading on the sampler to
the length of the snow core or sample. DEPDepartureDepletion CurveIn hydrologic terms, the part of the hydrograph extending from the point of termination of the Recession Curve to the subsequent rise or alternation of
inflow due to additional water becomming available for stream flow.DepressionA region of low atmospheric pressure that is usually accompanied by low clouds and precipitation. The term is also sometimes used as a reference to a Tropical Depression.Depression StorageIn hydrologic terms, the volume of water contained in natural depressions in the land surface, such as puddles.Depth of RunoffIn hydrologic terms, the total runoff from a drainage basin, divided by its area. For convenience in comparing runoff with precipitation, the term is
usually expressed in inches of depth during a given period of time over the drainage area or acre-feet per square mile. Derecho(Pronounced day-RAY-cho), a widespread and usually fast-moving windstorm associated with convection. Derechos include any family of downburst clusters produced by an extratropical MCS, and can produce damaging straight-line winds over areas hundreds of miles long and more than 100 miles across.Derived ProductsProcessed base data on the Doppler radar.DesertificationA tendency toward more prominent desert conditions in a region.Design CriteriaIn hydrologic terms, the hypothetical flood used in the sizing of the dam and the associated structures to prevent dam failure by overtopping, especially
for the spillway and outlet works. Detention BasinsStructures built upstream from populated areas to prevent runoff and/or debris flows from causing property damage and loss of life. They are normally dry, but are designed to attenuate storm flows or detain mud/debris during and immediately after a runoff event. They have no spillway gates or valves and do not store water on a long-term basis. Typical detention times for storm flows are on the order of 24 to 72 hours, but may be as long as 5 to 10 days. Basins designed for detention of mud and rock debris are periodically excavated to maintain their storage capacity.Detention StorageIn hydrologic terms, the volume of water, other than depression storage, existing on the land surface as flowing water which has not yet reached the
channel.DetritusIn hydrologic terms, (1) the heavier mineral debris moved by natural watercourses, usually in bed-load form. (2) the sand, grit, and other coarse material
removed by differential sedimentation in a relatively short period of detention.Developing Gale/StormIn the high seas and offshore forecasts, a headline used in the warnings section to indicate that gale/storm force winds are not now occurring but are expected before the end of the forecast period. DewMoisture that has condensed on objects near the ground, whose temperatures have fallen below the dewpoint temperature.Dew Point(Abbrev. DWPT) - A measure of atmospheric moisture. It is the temperature to which air must be cooled in order to reach saturation (assuming air pressure and moisture content are constant). A higher dew point indicates more moisture present in the air. It is sometimes referred to as Dew Point Temperature, and sometimes written as one word (Dewpoint).Dew Point DepressionThe difference in degrees between the air temperature and the dew point.Dew Point FrontA narrow zone (mesoscale feature) of extremely sharp moisture gradient and little temperature gradient. It separates moist air from dry air. Severe weather can be
associated with this front. It is also known as a "dryline" or "dry front".DEWPOn a buoy report, the dewpoint temperature taken at the same height as the air temperature measurement.Diagnostic ModelA computer model used to calculate air pollution concentrations. A diagnostic model produces a wind field over an area by interpolating from actual wind observations.Differential MotionCloud motion that appears to differ relative to other nearby cloud elements,
e.g. clouds moving from left to right relative to other clouds in the foreground or background. Cloud
rotation is one example of differential motion, but not all differential motion indicates rotation. For
example, horizontal wind shear along a gust front may result in differential cloud motion without the
presence of rotation.Differential RotationIn solar-terrestrial terms, the change in solar rotation rate with latitude. Low
latitudes rotate at a faster angular rate (approx. 14 degrees per
day) than do high latitudes (approx. 12 degrees per day). Diffuse IceIn hydrologic terms, poorly defined ice edge limiting an area of dispersed ice; usually on the leeward side of an area of floating ice.Difluence(or diffluence) - A pattern of wind flow in which air moves outward (in a "fan-out" pattern) away from a central axis that is oriented parallel to the general direction of the flow. It is the opposite of confluence.
Difluence in an upper level wind field is considered a favorable condition for severe thunderstorm development (if other parameters are also favorable). But difluence is not the same as divergence. In a difluent flow, winds normally decelerate as they move through the region of difluence, resulting in speed convergence which offsets the apparent diverging effect of the difluent flow.Direct Flood DamageIn hydrologic terms, the damage done to property, structures, goods, etc., by a flood as measured by the cost of replacement and repairs.Direct HitA close approach of a tropical cyclone to a particular location. For locations on the
left-hand side of a tropical cyclone's track (looking in the direction of motion), a direct
hit occurs when the cyclone passes to within a distance equal to the cyclone's
radius of maximum wind. For locations on the right-hand side of the track, a direct
hit occurs when the cyclone passes to within a distance equal to twice the radius of
maximum wind. Compare indirect hit, strike. Direct RunoffIn hydrologic terms, the runoff entering stream channels promptly after rainfall or snowmelt. Superposed on base runoff, it forms the bulk of the
hydrograph of a flood.Direct Solar RadiationThe component of solar radiation received by the earth's surface only from the direction of the sun's disk (i.e. it has not been reflected, refracted or scattered).Directional ShearThe component of wind shear which is due to a change in wind direction with
height, e.g., southeasterly winds at the surface and southwesterly winds aloft. A veering wind with
height in the lower part of the atmosphere is a type of directional shear often considered important
for tornado development.Disappearing Solar Filament (DSF)In solar-terrestrial terms, the sudden (timescale of minutes to hours) disappearance of a solar filament (prominence).DischargeIn hydrologic terms, the rate at which water passes a given point. Discharge is expressed in a volume per time with units of L3/T. Discharge is often
used interchangeably with streamflow.Discharge CurveIn hydrologic terms, a curve that expresses the relation between the discharge of a stream or open conduit at a given location and the stage or elevation
of the liquid surface at or near that location. Also called Rating Curve and Discharge Rating Curve.Discharge TableIn hydrologic terms,
1. A table showing the relation between two mutually dependant quantities or variable over a given range of magnitude.
2. A table showing the relation between the gage height and the discharge of a stream or conduit at a given gaging station. Also called a Rating Table. DisdrometerEquipment that measures and records the size distribution of raindrops. DispersionThe process of separating radiation into various wavelengths.Diurnal CyclesVariations in meteorological parameters such as temperature and relative humidity over the course of a day which result from the rotation of the
Earth about its axis and the resultant change in incoming and outgoing radiation.Diurnal Temperature RangeThe temperature difference between the minimum at night (low) and the
maximum during the day (high).DivergenceThe expansion or spreading out of a vector field; usually said of horizontal winds. It is the opposite of convergence. Divergence at upper levels of the atmosphere enhances upward motion, and hence the potential for thunderstorm development (if other factors also are favorable).DiversionIn hydrologic terms, the taking of water from a stream or other body of water into a canal, pipe, or other conduit.DivideIn hydrologic terms, the high ground that forms the boundary of a watershed. A divide is also called a ridge.Dividing StreamlineIn the blocked flow region upwind of a mountain barrier, the streamline that separates the blocked flow region near the ground from the streamlines above which go over the barrier.Dividing Streamline HeightThe height above ground of the dividing streamline, as measured far upwind of a mountain barrier. See dividing streamline.Domestic ConsumptionIn hydrologic terms, the quantity, or quantity per capita, of water consumed in a municipality or district for domestic uses or purposes during a given
period, generally one day. It is usually taken to include all uses included within the term Municipal Use of Water and quantity
wasted, lost, or otherwise unaccounted for.Domestic Use of waterIn hydrologic terms, the use of water primarily for household purposes, the watering of livestock, the irrigation of gardens, lawns, shrubbery, etc.,
surrounding a house or domicile.Doppler RadarRadar that can measure radial velocity, the instantaneous component of motion parallel to
the radar beam (i.e., toward or away from the radar antenna).Down-Valley WindA thermally driven wind directed down a valley's axis, usually occurring during nighttime; part of the along-valley wind system.Downslope FlowA thermally driven wind directed down a mountain slope and usually occurring at night; part of the along-slope wind system.DownstreamIn the same direction as a stream or other flow, or toward the direction in which the flow is
moving.Downstream SlopeIn hydrologic terms, the slope or face of the dam away from the reservoir water. This slope requires some kind of protection (e.g.; grass) from the
erosive effects of rain and surface flowDownwelling RadiationThe component of radiation directed toward the earth's surface from the sun or the atmosphere, opposite of upwelling radiation.Drainage AreaIn hydrologic terms, an area having a common outlet for its surface runoff (also see Watershed and Catchment Area).Drainage BasinIn hydrologic terms, a part of the surface of the earth that is occupied by a drainage system, which consists of a surface stream or a body of impounded
surface water together with all tributary surface streams and bodies of impounded surface water.Drainage DensityIn hydrologic terms, the relative density of natural drainage channels in a given area. It is usually expressed in terms of miles of natural drainage or
stream channel per square mile of area, and obtained by dividing the total length of stream channels in the area in miles by the area
in square miles.Drainage DivideIn hydrologic terms, the boundary line, along a topographic ridge or along a subsurface formation, separating two adjacent drainage basins.DrainerA valley or basin from which air drains continuously during nighttime rather than becoming trapped or pooled.
Drains (Relief Wells)In hydrologic terms, a vertical well or borehole, usually downstream of impervious cores, grout curtains or cutoffs, designed to collect and direct
seepage through or under a dam to reduce uplift pressure under or within a dam. A line of such wells forms a "drainage curtain".DredgingIn hydrologic terms, the scooping, or suction of underwater material from a harbor, or waterway. Dredging is one form of channel modification. It is
often too expensive to be practical because the dredged material must be disposed of somewhere and the stream will usually fill
back up with sediment in a few years. Dredging is usually undertaken only on large rivers to maintain a navigation channel.Drifting IceIn hydrologic terms, pieces of floating ice moving under the action of wind and/ or currents.DrizzlePrecipitation consisting of numerous minute droplets of water less than 0.5 mm (500 micrometers) in diameter.Drop-size DistributionThe distribution of rain drops or cloud droplets of specified
sizes.Drought AssessmentsAt the end of each month, CPC issues a long-term seasonal drought assessment. On Thursdays of each week, the CPC together with NOAA National Climatic Data Center, the United States Department of Agriculture, and the National Drought Mitigation Center in Lincoln, Nebraska, issues a weekly drought assessment called the United States Drought Monitor. These assessments review national drought conditions and indicate potential impacts for various economic sectors, such as agriculture and forestry.Drought IndexIn hydrologic terms, computed value which is related to some of the cumulative effects of a prolonged and abnormal moisture deficiency. (An index of
hydrological drought corresponding to levels below the mean in streams, lakes, and reservoirs.)Dry Adiabatic Lapse RateThe rate at which the temperature of a parcel of dry air decreases as the parcel is lifted in the atmosphere. The dry adiabatic lapse rate (abbreviated DALR) is 5.5°F per 1000 ft or 9.8°C per km.Dry LineA boundary separating moist and dry air masses, and an important factor in
severe weather frequency in the Great Plains. It typically lies north-south across
the central and southern high Plains states during the spring and early summer,
where it separates moist air from the Gulf of Mexico (to the east) and dry desert
air from the southwestern states (to the west). The dry line typically advances
eastward during the afternoon and retreats westward at night. However, a strong
storm system can sweep the dry line eastward into the Mississippi Valley, or
even further east, regardless of the time of day.
A typical dry line passage results in a sharp drop in humidity (hence the name),
clearing skies, and a wind shift from south or southeasterly to west or
southwesterly. (Blowing dust and rising temperatures also may follow, especially
if the dry line passes during the daytime. These changes occur in reverse order
when the dry line retreats westward. Severe and sometimes tornadic
thunderstorms often develop along a dry line or in the moist air just to the east of
it, especially when it begins moving eastward.Dry Line BulgeA bulge in the dry line, representing the area where dry air is advancing most strongly at lower levels. Severe weather potential is increased near and ahead of a dry line bulge.Dry Line StormAny thunderstorm that develops on or near a dry line.Dry ThunderstormGenerally a high-based thunderstorm when lightning is observed, but little if any precipitation reaches the ground. Most of the rain produced by the thunderstorm evaporates into relatively dry air beneath the storm cell. May also be referred to as "dry lightning". Dry Weather FlowIn hydrologic terms, streamflow which results from precipitation that infiltrates into the soil and eventually moves through the soil to the stream channel.
This is also referred to as baseflow, or ground water flow.Dst IndexA geomagnetic index describing variations in the equatorial
ringcurrentDuration CurveIn hydrologic terms, a cumulative frequency curve that shows the percent of time during which specified units of items (e.g. discharge, head,
power,etc.) were equaled or exceeded in a given period. It is the integral of the frequency diagram.Duration of Ice CoverIn hydrologic terms, The time from freeze-up to break-up of an ice cover.Duration of SunshineThe amount of time sunlight was detected at a given point.Dust DevilA small, rapidly rotating wind that is made visible by the dust, dirt or debris it picks up. Also called a whirlwind, it develops best on clear, dry, hot
afternoonsDust PlumeA non-rotating "cloud" of dust raised by straight-line winds. Often seen in a microburst or
behind a gust front.Dynamic IceIn hydrologic terms, pressure due to a moving ice cover or drifting ice. Pressure occuring at movement of first contact termed Ice Impact PressureDynamic Wave Routing Model (DWOPER)A computerized hydraulic routing program whose algorithms incorporate the complete one-dimensional equations of unsteady flowEEastE REGIONIn solar-terrestrial terms, a daytime layer of the earth's ionosphere roughly between the
altitudes of 85 and 140 km.E-19, Report on River Gage StationIn hydrologic terms, a report to be completed every 5 years providing a complete history of a river station and all gages that have been used for public
forecasts since the establishment of the station.E-19a, Abridged Report on River Gage StaIn hydrologic terms, an abridged version of an E-19, an E-19a updates the E-19 as additional information, or changes occur at the station during the
intervening five year period. An E-19a is to be completed anytime a significant change occurs at a forecast point. An E-19a is also
used to take the place of an E-19 in documenting any gage history, or information of any non-forecast point (i.e; data point).E-3, Flood Stage ReportIn hydrologic terms, a form that a Service Hydrologist/ Hydrology Focal Point completes to document the dates in which forecast points are above
flood stage, as well as the crest dates and stages. Discussion of the flood event must also be included in the E-5, Monthly Report of
River and Flood conditions. An E-3 report is sent to Regional Headquarters, the appropriate RFC, as well as the Office of
Hydrology (OH). E-5, Monthly Report of River and Flood cIn hydrologic terms, a monthly narrative report covering flooding which occurred over the past month. Flood stage, flood crest and dates in which
flooding occurred is covered within this report for each data point which was in flood. If the flooding involved a forecast point, an
E-3 must be filled out as well. If no flooding has occurred within the past month, a climatic summary of the past month can be
included as well as other interesting non-flood events, such as water supply, ice jams and the occurrence of drought. An E-5 report
is sent to Regional Headquarters, the appropriate RFC, as well as the Office of Hydrology (OH).E-7, Flood Damage ReportIn hydrologic terms, a report to be completed anytime there is reported flood damage or loss of life as a direct result of flooding. An E-7 report is sent
to Regional Headquarters, as well as the Office of Hydrology (OH).EasterliesAny winds with components from the east.Easterly WaveA low level disturbance of tropical origins. Easterly waves can develop into tropical cyclones. However, tropical cyclone development is not required in order for these systems to produce significant amounts of rainfall. The easterly waves are primarily a summer phenomenon.Ebb CurrentThe movement of a tidal current away form the coast or down an estuary.EBNDEastboundEBSEmergency Broadcast SystemEccentricityA dimensionless quantity describing the elliptical shape of a planet's orbit.EchoEnergy back scattered from a target (precipitation, clouds, etc.) and received by and displayed on a radar screen.Echo TopsThe height above ground of the center of the radar beam using the tilt, or scan, that contains the highest elevation where reflectivities greater than 18 dBZ can be detected.ECMFEuropean Center for Meteorology Forecast model.ECMWFEuropean Center for Medium-Range Weather Forecasts. Operational references in forecast discussions typically refer to the ECMWF's medium-range numerical forecast model, which runs out to 10 days.EddySwirling currents of air at variance with the main current.EDTEastern Daylight TimeEFCTEffectEffective PorosityIn hydrologic terms, the ratio, usually expressed as a percentage, of the volume of water or other liquid which a given saturated volume of rock or soil
will yield under any specified hydraulic condition, to the given volume of soil or rock.Effective Precipitation1) That part of the precipitation that produces runoff.
2) A weighted average of current and antecedent precipitation that is "effective" in correlating with runoff.
3) That part of the precipitation falling on an irrigated area that is effective in meeting the consumptive use requirements.Effective Terrestrial RadiationThe difference between upwelling infrared or terrestrial radiation emitted from the earth and the downwelling infrared radiation from the atmosphere Effective TopographyThe topography as seen by an approaching flow, which may include not only the actual terrain but also cold air masses trapped within or adjacent to the actual topography.Effluent SeepageIn hydrologic terms, diffuse discharge of ground water to the ground surfaceEffluent StreamIn hydrologic terms, any watercourse in which all, or a portion of the water volume came from the Phreatic zone, or zone of saturation by way of
groundwater flow, or baseflowEl NiñoA warming of the ocean current along the coasts of Peru and Ecuador that is generally associated with dramatic changes in the weather patterns of the region; a major El Niño event generally occurs every 3 to 7 years and is associated with changes in the weather patterns worldwide.ElementOne of the basic conditions of the atmosphere discussed in this FMH (wind, visibility, runway visual range, weather, obscurations, sky condition,
temperature and dewpoint, and pressure). See parameterELEVElevationElevated ConvectionConvection occurring within an elevated layer, i.e., a layer in which the lowest portion is based above the earth's surface. Elevated convection often occurs when air near the ground is relatively cool and stable, e.g., during periods of isentropic lift, when an unstable layer of air is present aloft.
In cases of elevated convection, stability indices based on near-surface measurements (such as the lifted index) typically will underestimate the amount of instability present. Severe weather is possible from elevated convection, but is less likely than it is with surface-based convection.ELSWElsewhereELYEasterlyEmbankmentIn hydrologic terms, fill material, usually earth or rock, placed with sloping sides and usually with length greater than height. All dams are types of
embankmentsEMBDDEmbeddedEMCEnvironmental Modeling CenterEmergency Action PlanIn hydrologic terms, a predetermined plan of action to be taken to reduce the potential for property damage and loss of life in an area affected by a
dam break or excessive spillway.Emergency ServicesIn hydrologic terms, services provided in order to minimize the impact of a flood that is already happening. These measures are the responsibility of city,
or county emergency management staff and the owners or operators of major, or critical facilities. Some examples of emergency
services are flood warning and evacuation, flood response, and post flood activities.Emerging Flux Region (EFR)In solar-terrestrial terms, an area on the sun where new magnetic flux is
erupting.EmissivityThe ability of a surface to emit radiant energy compared to that of a black body at the same temperature and with the same area.EMLElevated Mixed LayerENDGEndingEnergy DissipatorIn hydrologic terms, a structure which slows fast-moving spillway flows in order to prevent erosion of the stream channel.Energy Helicity IndexAn index that incorporates vertical shear and instability, designed for the
purpose of forecasting supercell thunderstormsEngineer's LevelA telescope which is attached to a spirit-tube level, all revolving around a vertical axis and is mounted on a tripod. An Engineer's
Level is used for determining the difference in elevation between two points. The telescope on the level has a vertical cross hair and
a horizontal cross hair. Once the instrument is leveled, the sighting through the horizontal cross hair represents a horizontal plane of
equal elevation.Enhanced VA pattern seen on satellite infrared photographs of thunderstorms, in which a thunderstorm anvil exhibits a V-shaped region of colder cloud tops extending downwind from
the thunderstorm core. The enhanced V indicates a very strong updraft, and therefore a higher potential for severe weather. Enhanced V should not be confused with V notch, which is a
radar signature.Enhanced Wording1. An option used by the SPC in tornado and severe thunderstorm watches when the potential for strong/violent tornadoes, or unusually widespread damaging straight-line winds, is high. The text that accompanies a watch of this type will include the line "THIS IS A PARTICULARLY DANGEROUS SITUATION."
2. Strong wording or emphasis used in a zone forecast issued by a National Weather Service Forecast Office highlighting a potential condition (e.g., "some thunderstorms may be severe").ENHNCDEnhancedENSEMBLEA collection of numerical model results that show slightly different possible outcomes. Ensemble ForecastMultiple predictions from an ensemble of slightly different initial conditions and/or various versions of models. The objectives are to improve the accuracy of the forecast through averaging the various forecasts, which eliminates non-predictable components, and to provide reliable information on forecast uncertainties from the diversity amongst ensemble members. Forecasters use this tool to measure the likelihood of a forecast.Ensemble Hydrologic ForecastingIn hydrologic terms, a process whereby a continuous hydrologic model is successively executed several times for the same forecast period by use of
varied data input scenarios, or a perturbation of a key variable state for each model run. A common method employed to obtain a
varied data input scenario is to use the historical meteorological record, with the assumption that several years of observed data
covering the time period beginning on the current date and extending through the forecast period comprises a reasonable estimate
of the possible range of future conditions.EnsemblesReference to a set of computer models run under the concept of Ensemble Forecasting: multiple predictions from an ensemble of models with slightly different initial conditions used as input and/or slightly different versions of models. The objectives are to improve the accuracy of the forecast through averaging the various forecasts, which eliminates non-predictable components, and to provide reliable information on forecast uncertainties from the diversity amongst ensemble members. Forecasters use this tool to measure the likelihood of a forecast. ENSOAbbreviation for El Niño-Southern Oscillation, a reference to the state of the Southern Oscillation.ENSO Diagnostic DiscussionThe CPC issues the ENSO Diagnostic Discussion around the middle of the month. The discussion addresses the current oceanic and atmospheric conditions in the Pacific and the seasonal climate outlook for the following one to three seasons.ENTREntireEntrainment ZoneA shallow region at the top of a convective boundary layer where fluid is entrained into the growing boundary layer from the overlying fluid by the collapse of rising convective plumes or bubbles.Entrance RegionThe region upstream from a wind speed maximum in a jet stream (jet max), in which air is approaching (entering) the region of maximum winds, and therefore is accelerating. This acceleration results in a vertical circulation that creates divergence in the upper-level winds in the right half of the entrance region (as would be viewed looking along the direction of flow).
This divergence results in upward motion of air in the right rear quadrant (or right entrance region) of the jet max. Severe weather potential sometimes increases in this area as a result. See also exit region, left exit region.EntropyThe amount of energy that is not available for work during a certain process.Environment CanadaThe Canadian federal government department responsible for issuing weather forecasts and weather warnings in Canada.Environmental Lapse RateThe rate of decrease of air temperature with height, usually measured with a radiosonde.Environmental Temperature SoundingAn instantaneous or near-instantaneous sounding of temperature as a function of height. This sounding or vertical profile is usually obtained by a balloon-borne instrument, but can also be measured using remote sensing equipment.EPAEnvironmental Protection AgencyEPCTGExpectingEPVEquivalent Potential VorticityEqui-Potential LineIn hydrologic terms, a line, in a field of flow, such that the total head is the same for all points on the line, and therefore the direction of flow is
perpendicular to the line at all points.Equilibrium DrawdownIn hydrologic terms, the ultimate, constant drawdown for a steady rate of pumped discharge.Equilibrium Level(EL) - On a sounding, the level above the level of free convection (LFC) at which the temperature of a rising air parcel again equals the temperature of the environment. The height of the EL is the height at which thunderstorm updrafts no longer accelerate upward. Thus, to a close approximation, it represents the height of expected (or ongoing) thunderstorm tops.Equilibrium Surface DischargeIn hydrologic terms, the steady rate of surface discharge which results from a long-continued, steady rate of net rainfall, with discharge rate equal to net
rainfall rateEquilibrium TimeIn hydrologic terms, the time when flow conditions become substantially equal to those corresponding to equilibrium discharge or equilibrium
drawdown.EquinoxThe time when the sun crosses the earth's equator, making night and day of approximately equal length all over the earth and occurring about March 21 (the spring or vernal equinox) and September 22 (autumnal equinox).Equivalent Potential TemperatureThe equivalent potential temperature is the temperature a parcel at a specific pressure level and temperature would have if it were raised to 0 mb, condensing all moisture from the parcel, and then lowered to 1000 mb.ERLYEarlyERNEasternErosionIn hydrologic terms, wearing away of the lands by running water, glaciers,winds, and waves, can be subdivided into three process: Corrasion,
Corrosion, and Transportation. Weathering, although sometimes included here, is a distant process which does not imply removal
of any materialEruptiveIn solar-terrestrial terms, solar activity levels with at least one radio event (10 cm) and
several chromospheric events per day (Class C Flares).Eruptive Prominence on Limb (EPL)In solar-terrestrial terms, a solar prominence that becomes activa-
ted and is seen to ascend from the sun.ESPExtended Streamflow PredictionESTEastern Standard TimeEstuaryIn hydrologic terms, the thin zone along a coastline where freshwater systems and rivers meet and mix with a salty ocean (such as a bay, mouth of a
river, salt marsh, lagoon).Esturine watersIn hydrologic terms, deepwater tidal habitats and tidal wetlands that are usually enclosed by land but have access to the ocean and are at least
occasionally diluted by freshwater runoff from the land (such as bays, mouths of rivers, salt marshes, lagoons). Esturine ZoneIn hydrologic terms, the area near the coastline that consists of esturaries and coastal saltwater wetlandsETA1. The Eta Model, now referred to as North Amercian Meso (NAM) an 84-hour numerical model of the atmosphere run four times daily by NCEP. This is one of the main forecast models used for short-term weather prediction in the United States.
2. Estimated Time of ArrivalEta ModelNow referred to as North Amercian Meso (NAM) is one of the operational numerical forecast models run at NCEP. The Eta is run four times daily, with forecast output out to 84 hours.EvaporationThe process of a liquid changing into a vapor or gas, usually water in meteorology.Evaporation PanIn hydrologic terms, a pan used to hold water during observations for the determination of the quantity of evaporation at a given location. Such pans are
of varying sizes and shapes, the most commonly used being circular or square.Evaporation RateIn hydrologic terms, the quantity of water, expressed in terms of depth of liquid water, which is evaporated from a given surface per unit of time. It is
usually expressed in inches depth, per day, month, or year.Evaporation-mixing FogFog that forms when the evaporation of water raises the dew point of the adjacent air.Evaporative Demand Drought Index (EDDI)An experimental drought monitoring and early warning guidance tool. It examines how anomalous the atmospheric evaporative demand is for a given location and across a time period of interest.EvaporimeterIn hydrologic terms, an instrument which measures the evaporation rate of water into the atmosphere.EvapotranspirationCombination of evaporation from free water surfaces and transpiration of
water from plant surfaces to the atmosphere.EVEEveningEWDEastwardEWWExtreme Wind Warning (EWW) inform the public of the need to take immediate shelter in an interior portion of a well-built structure due to the onset of extreme tropical cyclone winds. An EWW for extreme tropical cyclone winds should be issued when both of the following criteria are met:
a. Tropical cyclone is a category 3 or greater on the Saffir Simpson hurricane scale as
designated by NHC, CPHC or JTWC.
b. Sustained tropical cyclone surface winds of 100 knots (115 mph) or greater are occurring or are expected to occur in a WFO’s county warning area within one hour. Excess RainIn hydrologic terms, effective rainfall in excess of infiltration capacity.Excessive HeatExcessive heat occurs from a combination of high temperatures (significantly above normal) and high humidities. At certain levels, the human body cannot maintain proper internal temperatures and may experience heat stroke. The "Heat Index" is a measure of the effect of the combined elements on the body.Excessive Heat OutlookThis CPC product, a combination of temperature and humidity over a certain number of days, is designed to provide an indication of areas of the country where people and animals may need to take precautions against the heat during May to November.Excessive Heat WarningIssued within 12 hours of the onset of the following criteria: heat index of at least 105°F for more than 3 hours per day for 2 consecutive days, or heat index more than 115°F for any period of time.Excessive Heat WatchIssued by the National Weather Service when heat indices in excess of 105ºF (41ºC) during the day combined with nighttime low
temperatures of 80ºF (27ºC) or higher are forecast to occur for two consecutive days.Excessive Rainfall Outlook (ERO)A graphical product in which the Weather Prediction Center (WPC) forecasts the probability that rainfall will exceed flash flood guidance (FFG) within 40 kilometers (25 miles) of a point.EXCLDExcludeExclusive Flood Control Storage CapacityIn hydrologic terms, the space in a reservoir reserved for the sole purpose of regulating flood inflows to abate flood damageExit RegionThe region downstream from a wind speed maximum in a jet stream (jet max), in which air is moving away from the region of maximum winds, and therefore is decelerating. This deceleration results in divergence in the upper-level winds in the left half of the exit region (as would be viewed looking along the direction of flow).
This divergence results in upward motion of air in the left front quadrant (or left exit region) of the jet max. Severe weather potential sometimes increases in this area as a result. See also entrance region, right entrance region.ExosphereThe upper most layer of the earth's atmosphere; the only layer where atmospheric gases can escape into outer space.Experimental ProductAn experimental product is in the final stages of testing and evaluation. If the product proves accurate and valuable to users then the next step is to make it an operational product.Explosive DeepeningA decrease in the minimum sea-level pressure of a tropical cyclone of 2.5 mb/hr for
at least 12 hours or 5 mb/hr for at least six hours.EXTDExtend/ExtendedExtended Forecast DiscussionThis discussion is issued once a day around 2 PM EST (3 PM EDT) and is primarily intended to provide insight into guidance forecasts for the 3-
to 5-day forecast period. The geographic focus of this discussion is on the United States (including Alaska and Hawaii). Although portions of this narrative will parallel the
Hemispheric Map Discussion, a much greater effort is made to routinely relate the model forecasts and necessary modifications to weather forecasts, mainly in terms of temperature and
precipitation. Extraterrestrial RadiationThe theoretically-calculated radiation flux from the sun at the top of the atmosphere, before losses by atmospheric absorption.ExtratropicalA term used in advisories and tropical summaries to indicate that a cyclone has lost its "tropical" characteristics. The term implies both poleward displacement of the
cyclone and the conversion of the cyclone's primary energy source from the release of latent heat of condensation to baroclinic (the temperature contrast between warm and cold air
masses) processes. It is important to note that cyclones can become extratropical and still retain winds of hurricane or tropical storm force.Extratropical CycloneA cyclone in the middle and high latitudes often being 2000 kilometers in diameter and usually containing a cold front that extends toward the equator for hundreds of kilometers.Extratropical LowA low pressure center which refers to a migratory frontal cyclone of middle and higher latitudes. Tropical cyclones occasionally evolve into extratropical lows losing tropical characteristics and become associated with frontal discontinuity. Extreme Ultraviolet (EUV)A portion of the electromagnetic spectrum from
approximately 100 to 1000 angstroms.Extreme Wind WarningExtreme Wind Warning (EWW) inform the public of the need to take immediate shelter in an interior portion of a well-built structure due to the onset of extreme tropical cyclone winds. An EWW for extreme tropical cyclone winds should be issued when both of the following criteria are met:
a. Tropical cyclone is a category 3 or greater on the Saffir Simpson hurricane scale as
designated by NHC, CPHC or JTWC.
b. Sustained tropical cyclone surface winds of 100 knots (115 mph) or greater are occurring or are expected to occur in a WFO’s county warning area within one hour. Extremely Low Frequency (ELF)That portion of the radio frequency spectrum
from 30 to 3000 hertzEXTRMExtremeEXTSVExtensiveEyeThe relatively calm center in a hurricane that is more than one half surrounded by wall cloud. The winds are light, the skies are partly cloudy or even clear (the skies are usually free
of rain) and radar depicts it as an echo-free area within the eye wall. Eye WallIt is an organized band of cumuliform clouds that immediately surrounds the center (eye) of a hurricane. The fiercest winds and most intense rainfall typically occur near the
eye wall. VIP levels 3 or greater are typical. Eye wall and wall cloud are used synonymously, but it should not be confused with a wall cloud of thunderstorm. F RegionIn solar-terrestrial terms, the upper layer of the ionosphere, approximately 120 to 1500 km
in altitude. The F region is subdivided into the F1 and F2
regions. The F2 region is the most dense and peaks at altitudes
between 200 and 600 km. The F1 region is a smaller peak in
electron density, which forms at lower altitudes in the daytime.F ScaleAbbreviation for Fujita Scale, a system of rating the intensity of tornadoes; for detailed information, see the definition for that term.FaceIn hydrologic terms, the external surface of a structure, such as the surface of a dam.Fahrenheit(abbrev. F) The standard scale used to measure temperature in the United States. On this scale, the freezing point of water is 32°F and the boiling point is 212°F. To convert a Celsius temperature to Fahrenheit, multiply it by 9/5 and then add 32:
°F = (°C * 9/5) + 32Fall LineA skiing term, indicating the line of steepest descent of a slope.FallstreakSame as Virga; streaks or wisps of precipitation falling from a cloud but evaporating before reaching the ground. In certain cases, shafts of virga may precede a microburst.Federal Snow SamplerIn hydrologic terms, a snow sampler consisting of five or more sections of sampling tubes, one which has a steel cutter on the end. The combined
snowpack measuring depth is 150 inches. This instrument was formerly the Mount Rose Type Snow Sampling Set.Feeder BandsLines or bands of low-level clouds that move (feed) into the updraft region of a
thunderstorm, usually from the east through south (i.e., parallel to the inflow). Same as inflow bands.
This term also is used in tropical meteorology to describe spiral-shaped bands of convection
surrounding, and moving toward, the center of a tropical cyclone.FEMAFederal Emergency Management Agency. An agency of the federal government having responsibilities in hazard mitigation; FEMA also administers the National Flood
Insurance Program.Ferrel CellIn the general circulation of the atmosphere, the name given to the middle latitude cell marked by sinking motion near 30 degrees and rising motion near 60 degrees latitude.Fetch1. The area in which ocean waves are generated by the wind. Also refers to the length of the fetch area, measured in the direction of the wind.
2. In hydrologic terms,
- The effective distance which waves have traversed in open water, from their point of origin to the point where they break.
- 2. The distance of the water or the homogenous type surface over which the wind blows without appreciable change in direction.
FewA National Weather Service convective precipitation descriptor for a 10 percent chance of measurable precipitation (0.01 inch). Few is used interchangeably with isolated.Few CloudsAn official sky cover classification for aviation weather observations, descriptive of a sky cover of 1/8 to 2/8. This is applied only when obscuring phenomenon aloft are
present--that is, not when obscuring phenomenon are surface-based, such as fog.Field (Moisture) CapacityThe amount of water held in soil against the pull of gravityField Moisture DeficiencyThe quantity of water, which would be required to restore the soil moisture to field moisture capacity.Filament A mass of gas suspended over the photosphere by magnetic fields
and seen as dark lines threaded over the solar disk. A filament
on the limb of the sun seen in emission against the dark sky is
called a prominence.Filament ChannelA broad pattern of fibrils in the chromosphere, marking
where a filament may soon form or where a filament recently
disappeared.Fire WindA thermally driven wind blowing radially inward toward a fire, produced by horizontal temperature differences between the heated air above the fire and the surrounding cooler free atmosphere.FirebrandAny source of heat, natural or man made, capable of igniting wildland fuels; flaming or glowing fuel particles that can be carried naturally by wind, convection currents, or gravity into unburned fuels.Firn LineIn hydrologic terms, the highest level to which the fresh snow on a glacier's surface retreats during the melting season. The line separating the
accumulation area from the ablation areaFirst Law of ThermodynamicsThe law of physics that states that the heat absorbed by a system either raises the internal energy of the system or does work on the environment.Flanking LineA line of cumulus or towering cumulus clouds connected to and extending
outward from the most active part of a supercell, normally on the southwest side. The line normally
has a stair-step appearance, with the tallest clouds closest to the main stormFlareIn solar-terrestrial terms, a sudden eruption of energy on the solar disk lasting minutes to
hours, from which radiation and particles are emittedFlash Flood Guidance(FFG) Forecast guidance produced by the River Forecast Centers, often model output, specific to the potential for flash flooding (e.g., how much rainfall over a given area will be required to produce flash flooding).Flash Flood Statement(FFS) In hydrologic terms, a statement by the NWS which provides follow-up information on flash flood watches and warnings.Flash Flood TableIn hydrologic terms, a table of pre-computed forecast crest stage values for small streams for a variety of antecedent moisture conditions and rain
amounts. Soil moisture conditions are often represented by flash flood guidance values. In lieu of crest stages, categorical
representations of flooding, e.g., minor, moderate, etc. may be used on the tables.Float Recording Precipitation gageIn hydrologic terms, a rain gage where the rise of a float within the instrument with increasing rainfall is recorded. Some of these gages must be emptied
manually, while others employ a self-starting siphon to empty old rainfall amounts.FloeIn hydrologic terms, an accumulation of frazil flocs (also known as a "pan") or a single piece of broken iceFlood CategoriesTerms defined for each forecast point which describe or categorize the severity of flood impacts in the corresponding river/stream reach. Each flood category is bounded by an upper and lower stage (see Example 1). The severity of flooding at a given stage is not necessarily the same at all locations along a river reach due to varying channel/bank characteristics or presence of levees on portions of the reach. Therefore, the upper and lower stages for a given flood category are usually associated with water levels corresponding to the most significant flood impacts somewhere in the reach. The flood categories used in the NWS are:
*Minor Flooding* - minimal or no property damage, but possibly some public threat.
*Moderate Flooding* - some inundation of structures and roads near stream. Some evacuations of people and/or transfer of property to higher elevations.
*Major Flooding* - extensive inundation of structures and roads. Significant evacuations of people and/or transfer of property to higher elevations.
*Record Flooding* - flooding which equals or exceeds the highest stage or discharge at a given site during the period of record keeping.
Note: all three of the lower flood categories (minor, moderate, major) do not necessarily exist for a given forecast point. For example, at the level where a river reaches flood stage, it may be considered moderate flooding. However, at least one of these three flood categories must start at flood stage. Flood Control StorageIn hydrologic terms, storage of water in reservoirs to abate flood damageFlood CrestMaximum height of a flood wave as it passes a certain location.Flood Frequency CurveIn hydrologic terms, (1) A graph showing the number of times per year on the average, plotted as abscissa, that floods of magnitude, indicated by the
ordinate, are equaled or exceeded. (2) A similar graph but with recurrence intervals of floods plotted as abscissa.Flood Loss Reduction MeasuresIn hydrologic terms, the strategy for reducing flood losses. There are four basic strategies. They are prevention, property protection, emergency
services, and structural projects. Each strategy incorporates different measures that are appropriate for different conditions. In
many communities, a different person may be responsible for each strategy.Flood of RecordIn hydrologic terms, the highest observed river stage or discharge at a given location during the period of record keeping. (Not necessarily the highest
known stage.)Flood Potential Outlook(ESF on AFOS) (FPO for Acronym): In hydrologic terms, An NWS outlook that is issued to alert the public of potentially heavy rainfall that could send area rivers and streams into flood or
aggravate an existing flood.Flood PreventionIn hydrologic terms, measures that are taken in order to keep flood problems from getting worse. Planning, land acquisition, river channel maintenance,
wetlands protection, and other regulations all help modify development on floodplains and watersheds to reduce their susceptibility
to flood damage. Preventive measures are usually administered by the building, zoning, planning and/ or code enforcement offices
of the local government.Flood ProblemsIn hydrologic terms, problems and damages that occur during a flood as a result of human development and actions. Flood problems are a result from:
1) Inappropriate development in the floodplain (e.g., building too low, too close to the channel, or blocking flood flows); 2)
Development in the watershed that increases flood flows and creates a larger floodplain, or; 3) A combination of the previous two. Flood ProfileIn hydrologic terms, a graph of elevation of the water surface of a river in flood, plotted as ordinate, against distance, measured in the downstream
direction, plotted as abscissa. A flood profile may be drawn to show elevation at a given time, crests during a particular flood, or to
show stages of concordant flowsFlood StageAn established gage height for a given location above which a rise in water surface level begins to create a hazard to lives, property, or commerce. The issuance of flood (or in some cases flash flood) warnings is linked to flood stage. Not necessarily the same as bankfull stage.Flood Statement (FLS)In hydrologic terms, a statement issued by the NWS to inform the public of flooding along major streams in which there is not a serious threat to life or
property. It may also follow a flood warning to give later information.Flood WaveIn hydrologic terms, a rise in streamflow to a crest and its subsequent recession caused by precipitation, snowmelt, dam failure, or reservoir releasesFlooded IceIn hydrologic terms, ice which has been flooded by melt water or river water and is heavily loaded by water and wet snow.Flow Duration CurveIn hydrologic terms, a cumulative frequency curve that shows the percentage of time that specified discharges are equaled or exceeded.Flow SeparationThe process by which a separation eddy forms on the windward or leeward sides of bluff objects or steeply rising hillsides.Flowing WellIn hydrologic terms, a well drilled into a confined aquifer with enough hydraulic pressure for the water to flow to the surface without pumping. Also
called an Artesian well.FluenceTime integrated fluxFlurriesSnow flurries are an intermittent light snowfall of short duration (generally light snow showers) with no measurable accumulation (trace category).FoehnA warm, dry wind on the lee side of a mountain range, the warmth and dryness of the air being due to adiabatic compression as the air descends the mountain slopes. In the United States, the term chinook is used for Foehn winds in the Rocky and Sierra mountains.Foehn PauseA temporary cessation of the foehn at the ground due to the formation or intrusion of a cold air layer which lifts the foehn off the ground.foEsIn solar-terrestrial terms, the maximum ordinary mode radiowave frequency capable of reflec-
tion from the sporadic E region of the ionosphere.Forced ChannelingChanneling of upper winds along a valley's axis when upper winds are diverted by the underlying topography. Compare pressure-driven channeling.ForebayIn hydrologic terms, the water behind (upstream) of the dam.ForecastA statement of prediction.Forecast CrestIn hydrologic terms, the highest elevation of river level, or stage, expected during a specified storm event.Forecast GuidanceComputer-generated forecast materials used to assist the preparation of a forecast, such as numerical forecast models.Forecast Issuance StageThe stage which, when reached by a rising stream, represents the level where RFCs need to begin issuing forecasts for a non-routine (flood-only) forecast point. This stage is coordinated between WFO and RFC personnel and is not necessarily the same as action or alert stage. The needs of WFO/RFC partners and other users are considered in determining this stage. Forecast PeriodsOfficial definitions for NWS products:
Today...............................Sunrise to sunset
This afternoon..................noon till 6 p.m.
This evening.....................6 p.m. till sunset
Tonight.............................sunset till sunrise
Tomorrow.........................sunrise to sunset of the following dayForecast PointA location along a river or stream for which hydrologic forecast and warning services are provided by a WFO. The observed/forecast stage or discharge for a given forecast point can be assumed to represent conditions in a given reach (see /reach/).Forecast valid forThe period of time the forecast is in effect
beginning at a given day, date and time, and ending at a given day, date
and time.ForesightIn hydrologic terms, a sighting on a point of unknown elevation from an instrument of known elevation. To determine the elevation of the point in
question, the foresight is subtracted from the height of the instrument. FountainheadIn hydrologic terms, the upper end of a confined-aquifer conduit, where it intersects the land surface.FractureIn hydrologic terms, any break or rupture formed in an ice cover or floe due to deformation.Fracture ZoneIn hydrologic terms, an area which has a great number of fractures.Frazil IceIn hydrologic terms, fine spicules, plates, or discoids of ice suspended in water. In rivers and lakes, frazil is formed in supercooled, turbulent water.Freak WaveA wave of much greater height and steepness than other waves in the prevailing sea or swell system. See Rogue Wave.Free AtmosphereThe part of the atmosphere that lies above the frictional influence of the earth's surface.Free Ground WaterIn hydrologic terms, unconfined ground water whose upper boundary is a free water tableFreeboardIn hydrologic terms, the vertical distance between the normal maximum level of the water surface in a channel, resrvoir, tank, canal, etc., and the top of
the sides of a levee, dam, etc., which is provided so that waves and other movements of the liquid will not overtop the confining
structureFreezeA freeze is when the surface air temperature is expected to be 32°F or below over a widespread area for a climatologically significant period of time. Use of the term is usually restricted to advective situations or to occasions when wind or other conditions prevent frost. "Killing" may be used during the growing season when the temperature is expected to be low enough for a sufficient duration to kill all but the hardiest herbaceous crops.Freeze WarningIssued during the growing season when surface temperatures are expected to drop below freezing over a large area for an extended period of time, regardless whether or not frost develops.Freezeup dateIn hydrologic terms, the date on which the water body was first observed to be completely frozen overFreezing DrizzleA drizzle that falls as a liquid but freezes into glaze or rime upon contact with the cold ground or surface structures.Freezing Drizzle AdvisoryIssued when freezing rain or freezing drizzle is forecast but a significant accumulation is not expected. However, even small amounts of freezing rain or freezing drizzle may cause significant travel problems. Freezing FogA fog the droplets of which freeze upon contact with exposed objects and form a coating of rime and/or glaze. Freezing LevelThe altitude at which the air temperature first drops below freezing.Freezing RainRain that falls as a liquid but freezes into glaze upon contact with the ground.
Freezing Rain AdvisoryIssued when freezing rain or freezing drizzle is forecast but a significant accumulation is not expected. However, even small amounts of freezing rain or freezing drizzle may cause significant travel problems.Freezing SprayAn accumulation of freezing water droplets on a vessel caused by some appropriate combination of cold water, wind, cold air temperature, and vessel movement.Freezing Spray AdvisoryAn advisory for an accumulation of freezing water droplets on a vessel at a rate of less than 2 centimeters (cm) per hour caused by some appropriate combination of cold water, wind, cold air temperature, and vessel movement.Freezup jamIn hydrologic terms, ice jam formed as frazil ice accumulates and thickensFrench DrainIn hydrologic terms, an underground passageway for water through the interstices among stones placed loosely in a trenchFreshetthe annual spring rise of streams in cold climates as a result of snow melt; freshet also refers to a flood caused by rain or melting snow.Friction HeadIn hydrologic terms, the decrease in total head caused by frictionFriction LayerSame as Planetary Boundary Layer; the layer within the atmosphere between the earth's surface and 1 km above the surface; this is the layer where friction affects wind speed and wind direction.Frontal InversionA temperature inversion that develops aloft when warm air overruns the cold air behind a front.Frontogenesis1. The initial formation of a front or frontal zone. 2. In general, an increase in the horizontal gradient of an airmass property, principally density, and the development of the accompanying features of the wind field that typify a front.FrostbiteHuman tissue damage caused by exposure to intense cold.Frozen DewWhen liquid dew changes into tiny beads of ice. This occurs when dew forms and temperatures later drop below freezing.Fugitive DustDust that is not emitted from definable point sources such as industrial smokestacks. Sources include open fields, roadways, storage piles, etc.Fujita Scale(or F Scale) - A scale of tornado intensity in which wind speeds are inferred from an analysis of wind damage:
Rating | Wind, Damage |
F0 (weak) | 40-72 mph, light damage |
F1 (weak) | 73-112 mph, moderate damage |
F2 (strong) | 113-157 mph, considerable damage |
F3 (strong) | 158-206 mph, severe damage |
F4 (violent) | 207-260 mph, devestating damage |
F5 (violent) | 260-318 mph (rare), incredible damage |
All tornadoes, and most other severe local windstorms, are assigned a single number from this scale according to the most intense damage caused by the storm.Fujiwhara EffectA binary interaction where tropical cyclones within a certain distance (300-750 nm depending on the sizes of the cyclones) of each other begin to rotate about a
common midpoint.Full-Physics Numerical ModelA computer model used to calculate air pollution concentrations. A full-physics numerical model uses a full set of equations describing the thermodynamic and dynamic state of the atmosphere and can be used to simulate atmospheric phenomena.Funnel CloudA condensation funnel extending from the base of a towering cumulus or Cb, associated with a rotating column of air that is not in contact with the ground (and hence different from a tornado). A condensation funnel is a tornado, not a funnel cloud, if either a) it is in contact with the ground or b) a debris cloud or dust whirl is visible beneath it.FunnellingThe process whereby wind is forced to flow through a narrow opening between adjacent land areas, resulting in increased wind speed. GageIn hydrologic terms, 1) A device for indicating the magnitude or position of a thing in specific units, when such magnitude or position undergoes change,
for example: The elevation of a water surface, the velocity of flowing water, the pressure of water, the amount or intensity of
precipitation, the depth of snowfall, etc. (2) The act or operation of registering or measuring the magnitude or position of a thing
when these characteristics are undergoing change. (3) The operation, including both field and office work, of measuring the
discharge of a stream of water in a waterway.Gage DatumA horizontal surface used as a zero point for measurement of stage or gage height. This surface usually is located slightly below the lowest point of the stream bottom such that the gage height is usually slightly greater than the maximum depth of water. Because the gage datum is not an actual physical object, the datum is usually defined by specifying the elevations of permanent reference marks such as bridge abutments and survey monuments, and the gage is set to agree with the reference marks. Gage datum is a local datum that is maintained independently of any national geodetic datum. However, if the elevation of the gage datum relative to the national datum (North American Vertical Datum of 1988 or National Geodetic Vertical Datum of 1929) has been determined, then the gage readings can be converted to elevations above the national datum by adding the elevation of the gage datum to the gage reading.GaleAn extratropical low or an area of sustained surface winds of 34 (39 mph) to 47 knots (54 mph).Gale Warning A warning of sustained surface winds, or frequent gusts, in the range of 34 knots (39 mph) to 47 knots (54 mph) inclusive, either predicted or occurring, and not directly associated with a tropical cyclone.Gale WatchA watch for an increased risk of a gale force wind event for sustained surface winds, or frequent gusts, of 34 knots (39 mph) to 47 knots (54 mph), but its occurrence, location,and/or timing is still uncertain. GalleryIn hydrologic terms, a passageway within the body of a dam or abutment.GateIn hydrologic terms, a device in which a leaf or member is moved across the waterway from an external position to control or stop flow. There are
many different kinds of gates used on a damGaussian Plume ModelA computer model used to calculate air pollution concentrations. The model assumes that a pollutant plume is carried downwind from its emission source by a mean wind and that concentrations in the plume can be approximated by assuming that the highest concentrations occur on the horizontal and vertical midlines of the plume, with the distribution about these mid-lines characterized by Gaussian- or bell-shaped concentration profiles.Gaussian Puff ModelA model used to calculate air pollution concentrations. The model assumes that a continuously emitted plume or instantaneous cloud of pollutants can be simulated by the release of a series of puffs that will be carried in a time- and space-varying wind field. The puffs are assumed to have Gaussian or bell-shaped concentration profiles in their vertical and horizontal planes.GEMPAKGeneral Environmental Meteorological Package (programming language)GENGeneralGeneral CirculationThe totality of large-scale organized motion for the entire global atmosphere.General Circulation Models(GCMs) - These computer simulations reproduce the Earth's weather patterns and can be used to predict change in the weather and climate.General WindLand management agency term for winds produced by synoptic-scale pressure systems on which smaller-scale or local convective winds are superimposed.GeohydrologyIn hydrologic terms, the branch of hydrology relating to subsurface, or subterranean waters.Geomagnetic ElementsIn solar-terrestrial terms, the components of the geomagnetic field at the surface of the earth. In SESC use, the northward and eastward components are often called the H and D components, where the D component is expressed in gammas and is derived from D (the declination angle) using the small angle approximation.Geomagnetic FieldThe magnetic field observed in and around the earth.
The intensity of the magnetic field at the earth's surface is
approximately 0.32 gauss at the equator and 0.62 gauss at the
north poleGeomagnetic StormIn solar-terrestraial terms, a worldwide disturbance of the earth's magnetic field,
distinct from regular diurnal variations.
Minor Geomagnetic Storm: A storm for which the Ap index was
greater than 29 and less than 50.
Major Geomagnetic Storm: A storm for which the Ap index was
greater than 49 and less than 100.
Severe Geomagnetic Storm: A storm for which the Ap index was
100 or more.
Initial Phase: Of a geomagnetic storm, that period when there
may be an increase of the middle-latitude horizontal
intensity (H).
Main Phase: Of a geomagnetic storm, that period when the hori-
zontal magnetic field at middle latitudes is generally
decreasing.
Recovery Phase: Of a geomagnetic storm, that period when the
depressed northward field component returns to normal levels.Geophysical EventsIn solar-terrestrial terms, flares (Importance two or larger) with Centimetric
Outbursts (maximum of the flux higher than the Quiet Sun flux,
duration longer 10 minutes) and/or strong SID. Sometimes these
flares are followed by Geomagnetic Storms or small PCA. (Class M
Flares)GeophysicsIn hydrologic terms, the study of the physical characteristics and properties of the earth; including geodesy, seismology, meteorology, oceanography,
atmospheric electricity, terrestrial magnetism, and tidal phenomena.Geopotential HeightThe height above sea level of a pressure level. For example, if a station reports that the 500 mb height at its location is 5600 m, it means that the level of the atmosphere over that station at which the atmospheric pressure is 500 mb is 5600 meters above sea level. This is an estimated height based on temperature and pressure data.Geostationary SatelliteA satellite that rotates at the same rate as the earth, remaining over the same spot above the equator.Geostrophic WindA wind that is affected by coriolis force, blows parallel to isobars and whose strength is related to the pressure gradient (i.e., spacing of the isobars).GeosynchronousTerm applied to any equatorial satellite with an orbital
velocity equal to the rotational velocity of the earth. The net
effect is that the satellite is virtually motionless with respect
to an observer on the groundGlacierIn hydrologic terms, bodies of land ice that consist of recrystallized snow accumulated on the surface of the ground, and that move slowly downslope.Glacier Dammed LakeIn hydrologic terms, the lake formed when a glacier flows across the mouth of an adjoining valley and forms an ice dam.Glacier WindA shallow downslope wind above the surface of a glacier, caused by the temperature difference between the air in contact with the glacier and the free air at the same altitude. The glacier wind does not reverse diurnally like slope and along-valley wind systems.GlazeIce formed by freezing precipitation covering the ground or exposed objects. Global Ensemble Forecast System (GEFS)A weather forecast model made up of 21 separate forecasts, or ensemble members. The National Centers for Environmental Prediction (NCEP) started the GEFS to address the nature of uncertainty in weather observations, which are used to initialize weather forecast models.Global Forecast System(GFS)- One of the operational forecast models run at NCEP. The GFS is run four times daily, with forecast output out to 384 hours.Global Temperature ChangeThe net result of four primary factors including the greenhouse effect, changes in incoming solar radiation, altered patterns of ocean
circulations, and changes in continental position, topography and/or vegetation. Three feedback mechanisms which affect global temperature change include cloud
height and amount, snow and ice distribution, and atmospheric water vapor levels.GOESGeostationary Operational Environmental Satellite- Satellites orbiting at 22,370 miles above the Equator with the same rotational velocity as the Earth; therefore, the satellite
remains over the same location on the Earth 24 hours a day. Besides sending back satellite pictures to earth, it also relays the DCPs river and rainfall data back to the groundGradient(abbrev. GRAD) A rate of change with respect to distance of a variable quantity, as temperature or pressure, in the direction of maximum change.Gradient High WindsThese high winds usually cover a large area and are due to synoptic-scale, extra-tropical low pressure systems.Gradual CommencementIn solar-terrestrial terms, the commencement of a geomagnetic storm that has
no well-defined onsetGraupelSame as snow pellets or small hail.Gravity Recovery and Climate Experiment (GRACE)NASA satellites that detect small changes in the Earth’s gravitational field caused by the redistribution of water on and beneath the land surface.Gravity WaveA wave created by the action of gravity on density variations in the stratified atmosphere. A generic classification for lee waves, mountains waves, and many other waves that form in the atmosphere.Great Circle TrackA great-circle track is the shortest distance between two points on a sphere, and when viewed on a 2-dimensional map the track will appear curved. Swell waves travel along routes that mark out great circles. Great Lakes FaxbackDissemination systems housed at Weather Forecast Office (WFO) Cleveland by which Great Lakes customers request and receive hard copies of selected marine products. Great Lakes Freeze-Up/Break-Up Outlook(FBO) - A National Weather Service product to keep mariners informed of the projected freeze-up date or break-up date of ice on the Great Lakes. Great Lakes Marine Forecast (MAFOR)A National Weather Service coded summary appended to each of the Great Lakes Open Lakes forecasts. Great Lakes Storm Summary(GLS) - A National Weather Service forecast product providing updated information whenever a storm warning is in effect on any of the Great Lakes.Great Lakes Weather Broadcast(LAWEB) - A National Weather Service product containing an observation summary prepared to provide Great Lakes mariners with a listing of weather observations along or on the Lakes.Green LineThe green line is one of the strongest (and first-recognized)
visible coronal lines. It identifies moderate temperature regions
of the CORONA.Greenhouse EffectAtmospheric heating caused by solar radiation being readily transmitted inward through the earth's atmosphere but longwave radiation less readily transmitted outward, due to absorption by certain gases in the atmosphere.Greenhouse GasesThe gases that absorb terrestrial radiation and contribute to the greenhouse effect; the main greenhouse gasses are water vapor, methane, carbon dioxide, and ozone.Ground ClutterA pattern of radar echoes from fixed ground targets (buildings, hills, etc.) near
the radar. Ground clutter may hide or confuse precipitation echoes near the radar antenna.Ground Heat FluxThe flux of heat from the ground to the earth's surface; a component of the surface energy budget.Ground receive sitesIn hydrologic terms, a satellite dish and associated computer which receives signals from the GOES satellite, decodes the information, and transmits it
to a another site for further processing. The GOES satellite ground-receive site is located at Wallops Island, VA; and the
information is relayed to a mainframe computer at NWSH for processing.Ground StrokeThe current that propagates along the ground from the point where a direct stroke of lightning hits the ground.Ground WaterIn hydrologic terms, water within the earth that supplies wells and springs; water in the zone of saturation where all openings in rocks and soil are filled,
the upper surface of which forms the water table. Also termed Phreatic water.Ground Water DivideIn hydrologic terms, A line on a water table where on either side of which the water table slopes downward. It is analogous to a drainage divide
between two drainage basins on a land surfaceGround Water FlowIn hydrologic terms, streamflow which results from precipitation that infiltrates into the soil and eventually moves through the soil to the stream channel.
This is also referred to as baseflow, or dry-weather flowGround Water HydrologyThe branch of hydrology that specializes in ground water; its occurrence and movements; its replenishment and depletion; the
properties of rocks that control ground water movement and storage; and the methods of investigation and utilization of ground
waterGround Water OutflowIn hydrologic terms, the part of the discharge from a drainage basin that occurs through the ground water. The term "underflow" is often used to
describe the ground water outflow that takes place in valley alluvium (instead of the surface channel) and thus is not measure at a
gaging station.Ground Water OverdraftPumpage of ground water in excess of safe yield.Ground Water RunoffThat part of the runoff which has passed into the ground, has become ground water, and has been discharged into a stream channel
as spring, or seepage water.Grounded iceIn hydrologic terms, ice that has run aground or is contact with the ground underneath itGroup VelocityThe speed at which a particular wave front or swell train advances.Growing Degree DayThe number of degrees that the average temperature is above a baseline value. For example, 40 degrees for canning purposes; 45 degree for potatoes; and 50
degrees for sweet corn, snap beans, lima beans, tomatoes, grapes, and field corn. Every degree that the average temperature is above the baseline value becomes a growing degree day.
Agricultural related interests use growing degree days to determine planting times. Growing Seasonthe period of time between the last killing frost of spring and the first killing frost of autumn.GrowlerSimilar to a bergy bit, but smaller, extending less than 1 meter above the sea surface and occupying an area of 20 square meters or less. Gulf Stream Warm water current extending from the Gulf of Mexico and Florida up the U.S. east coast then east northeast to Iceland and Norway. GungeSlang for anything in the atmosphere that restricts visibility for storm spotting, such as fog, haze, precipitation (steady rain or drizzle), widespread low clouds (stratus), etc. GyresOceanic current systems of planetary scale driven by the global wind system.H-component of the Geomagnetic Field(Geomagnetic Elements) In solar-terrestrial terms, the components of the geomagnetic field at the surface of the earth. In SESC use, the
northward and eastward components are often called the H and D components, where the D component is
expressed in gammas and is derived from D (the declination angle) using the small angle approximation.Hague LineThe North Atlantic boundary between the U.S. and Canada fishing waters as determined by the World Court in The Hague, Netherlands.Hail IndexAn indication of whether the thunderstorm structure of each storm identified is conducive to the
production of hail.Hail SizeTypically refers to the diameter of the hailstones. Warnings and reports may report hail size through comparisons with real-world objects that correspond to certain diameters:
Description | Diameter (inches) |
Pea | 0.25 |
Marble or Mothball | 0.50 |
Penny or Dime | 0.75 |
Nickel | 0.88 |
Quarter | 1.00 |
Half Dollar | 1.25 |
Walnut or Ping Pong Ball | 1.50 |
Golfball | 1.75 |
Hen's Egg | 2.00 |
Tennis Ball | 2.50 |
Baseball | 2.75 |
Tea Cup | 3.00 |
Grapefruit | 4.00 |
Softball | 4.50 |
Hail SpikeAn area of reflectivity extending away from the radar immediately behind a thunderstorm with extremely large hail. In an area of large hail, radiation from the radar can bounce from hailstone to hailstone before being reflected back to the radar. The time delay between the backscattered radiation from the storm and the bounced and scattered radiation from the large hail causes the reflectivity from the hail to appear to come from a farther range than the actual storm.Haines IndexThis is also called the Lower Atmosphere Stability Index. It is computed from the
morning (12Z) soundings from RAOB stations across North America. The index is composed of a
stability term and a moisture term. The stability term is derived from the temperature difference at
two atmosphere levels. The moisture term is derived from the dew point depression at a single
atmosphere level. This index has been shown to be correlated with large fire growth on initiating
and existing fires where surface winds do not dominate fire behavior. The Haines Indices range
from 2 to 6 for indicating potential for large fire growthHanging (ice) damIn hydrologic terms, a mass of ice composed mainly of frazil or broken ice deposited underneath an ice cover in a region of low flow velocity.Hazardous Seas WarningA warning for wave heights and/or wave steepness values meeting or exceeding locally defined warning criteria. Hazardous Seas WatchA watch for an increased risk of a hazardous seas warning event to meet Hazardous Seas Warning criteria but its occurrence, location, and/or timing is still uncertain. Hazardous Weather OutlookA narrative statement produced by the National Weather Service, frequently issued on a routine basis, to
provide information regarding the potential of significant weather expected during the next 1 to 5 days.Hazards AssessmentCPC's Hazards Assessment provides emergency managers, planners, forecasters and the public advance notice of potential hazards related to climate, weather and hydrological events.Haze(abbrev. HZ)- An aggregation in the atmosphere of very fine, widely dispersed, solid or liquid particles, or both, giving the air an opalescent appearance that subdues colors.HeadIn hydrologic terms, the difference between the pool height and tailwater height. Usually expressed in feet of head, or in lbs./sq. inchHead LossIn hydrologic terms, the decrease in total head caused by frictionHead RaceIn hydrologic terms, a channel which directs water to a water wheel; a forebay.Headward ErosionIn hydrologic terms, erosion which occurs in the upstream end of the valley of a stream, causing it to lengthen its course in such a direction.Headwater BasinIn hydrologic terms, a basin at the headwaters of a river. All discharge of the river at this point is developed within the basin.HeadwatersIn hydrologic terms, streams at the source of a river.Heat AdvisoryIssued within 12 hours of the onset of the following conditions: heat index of at least 105°F but less than 115°F for less than 3 hours per day, or nighttime lows above 80°F for 2 consecutive days.Heat ExhaustionA mild form of heat stroke, characterized by faintness, dizziness, and heavy sweating.Heat IndexThe Heat Index (HI) or the "Apparent Temperature" is an accurate measure of how hot it really feels when the Relative Humidity (RH) is added to the actual air temperature.Heat LightningLightning that occurs at a distance such that thunder is no longer audible.Heat StrokeA condition resulting from excessive exposure to intense heat, characterized by high fever, collapse, and sometimes convulsions or coma.Heat WaveA period of abnormally and uncomfortably hot and unusually humid weather. Typically a heat wave lasts two or more days.Heating Degree Days(abbrev. HDD) A form of degree day used to estimate energy requirements for heating. Typically, heating degree days are calculated as how much colder the mean temperature at a location is than 65°F on a given day. For example, if a location experiences a mean temperature of 55°F on a certain day, there were 10 HDD (Heating Degree Days) that day because 65 - 55 = 10.Heavy Freezing SprayAn accumulation of freezing water droplets on a vessel at a rate of two cm per hour or greater (0.7 cm per hour or greater in Alaska Region) caused by some appropriate combination of cold water, wind, cold air temperature, and vessel movement. Heavy Freezing Spray WarningA warning for an accumulation of freezing water droplets on a vessel at a rate of two cm per hour or greater (0.7 cm per hour or greater in Alaska Region) caused by some appropriate combination of cold water, wind, cold air temperature, and vessel movement. Heavy Freezing Spray WatchA watch for an increased risk of a heavy freezing spray event to meet Heavy Freezing Spray Warning criteria but its occurrence, location, and/or timing is still uncertain. Heavy SnowThis generally means...
- snowfall accumulating to 4" or more in depth in 12 hours or less; or
- snowfall accumulating to 6" or more in depth in 24 hours or less
In forecasts, snowfall amounts are expressed as a range of values, e.g., "8 to 12 inches." However, in heavy snow situations where there is considerable uncertainty concerning the range of values, more appropriate phrases are used, such as "...up to 12 inches..." or alternatively "...8 inches or more...".Heavy Snow WarningIssued by the National Weather Service when snowfall of 6 inches (15 cm) or more in 12 hours or 8 inches (20 cm) or more in 24
hours is imminent or occurring. These criteria are specific for the Midwest and may vary regionally.Heavy Surf AdvisoryAn advisory issued by the National Weather Service for fast moving deep water waves which can result in big breaking waves in shallow water (the surf zone). HectopascalA unit of pressure equal to a millibar (1 hPa = 1 mb). Abbreviated hPa.HeightIn meteorology, usually a reference to Geopotential Height; roughly the height above sea level of a pressure level. For example, if a station reports that the 500 mb height at its location is 5600 m, it means that the level of the atmosphere over that station at which the atmospheric pressure is 500 mb is 5600 meters above sea level. This is an estimated height based on temperature and pressure data.Height Above the Nearest Drainage (HAND)A relative elevation methodology used in inundation mapping, which determines the height of every point on a land surface above the nearest stream reach to which it drains.HelicityA property of a moving fluid which represents the potential for helical flow (i.e. flow which follows the pattern of a corkscrew) to evolve. Helicity is proportional to the strength of the flow, the amount of vertical wind shear, and the amount of turning in the flow (i.e. vorticity).
Atmospheric helicity is computed from the vertical wind profile in the lower part of the atmosphere (usually from the surface up to 3 km), and is measured relative to storm motion. Higher values of helicity (generally, around 150 m2/s2 or more) favor the development of mid-level rotation (i.e. mesocyclones). Extreme values can exceed 600 m2/s2.
Hertz(abbrev. Hz)- An international unit of frequency equal to one cycle per second, and named after a German physicist.High Energy EventIn solar-terrestrial terms, flares (class two or more) with outstanding Centimetric
Bursts and SID. High Energy Protons are reported at the Earth in
case of most of these events occurring on the western part of solar
disk. (Class X flares)High Flow Magnitude (HFM)The magnitude of the maximum forecast streamflow conditions, measured in terms of annual exceedance probabilities.High Frequency (HF)The portion of the radio frequency spectrum between
between 3 and 30 MHzHigh LatitudesWith specific reference to zones of geomagnetic activity,
"high latitudes" refers to 50º to 80º geomagnetic.High Resolution Ensemble Forecast (HREF)An ensemble of products from several different models running at ~3 km horizontal grid spacing.High Resolution Rapid Refresh (HRRR)A real-time 3-km resolution, hourly updated, cloud-resolving, convection-allowing atmospheric model, initialized by 3km grids with 3km radar assimilation.High Risk (of severe thunderstorms)Severe weather is expected to affect more than 10 percent of the
area. A high risk is rare, and implies an unusually dangerous situation and usually the possibility of a major
severe weather outbreak.High Seas Forecast(HSF) - Marine forecasts for the major oceans of the world. In this context, major gulfs or seas (e.g., the Gulf of Mexico or the Bering Sea) are included within these forecast areas. Areas of responsibility for the U.S. are determined by international agreements under the auspices of the World Meteorological Organization (WMO).High Water Arrival TimeThe time at which a stream reach is forecast to achieve its high water flow within the forecast period.High Water FlowThe Office of Water Prediction (OWP) uses the term “high water” as an indicator of elevated flows for a given stream reach. This regionally varied threshold is used in many National Water Model (NWM) data services, such as maximum high flows, peak and high flow arrival times, rapid onset flooding, and flood inundation mapping. Filtering on flows that are greater than or equal to the high water threshold allows these services to focus on areas where flows are significant enough to monitor - where there is potential flooding, for example. High water thresholds are established for eleven clusters that categorize runoff efficiencies across the Continental United States (ConUS). Hawaii, Puerto Rico, and U.S. Virgin Islands domains utilize the 2.0 year recurrence interval for the high water threshold.High Water ProbabilityThe probability (%) that a stream will reach its high water flow within the forecast period.High-Speed StreamIn solar-terrestrial terms, a feature of the solar wind having velocities that are
about double average solar wind values.Hinge CrackIn hydrologic terms, a crack caused by significant changes in water level.Homologous FlaresIn solar-terrestrial terms, solar flares that occur repetitively in the same
active region, with essentially the same position and with a common
pattern of developmentHook EchoA radar reflectivity pattern characterized by a hook-shaped extension of a thunderstorm echo, usually in the right-rear part of the storm (relative to its direction of motion). A hook often is associated with a mesocyclone, and indicates favorable conditions for tornado development.Hourly Precipitation Data (HPD)It contains data on nearly 3,000 hourly precipitation stations (National Weather Service, Federal Aviation Administration, and cooperative observer stations) in inches to tenths or inches to hundredths at local standard time. HPD includes
maximum precipitation for nine (9) time periods from 15 minutes to 24 hours, for selected stations. HSA (Hydrologic Service Area)A geographical area assigned to Weather Service Forecast Office's/Weather Forecast Office's that embraces one or more rivers.Humidity RecoveryThe change in relative humidity over a given period of time; generally between late evening and sunrise. The moisture change in the fine fuels during this period is directly related to the amount of humidity recovery.Hummocked IceIn hydrologic terms, ice piled haphazardly one piece over another to form an uneven surface.Hurricane(abbrev. HURCN) A tropical cyclone in the Atlantic, Caribbean Sea, Gulf of Mexico, or eastern Pacific, which the maximum 1-minute sustained surface wind is 64 knots (74 mph) or greater.Hurricane Force Wind WarningA warning for sustained winds, or frequent gusts, of 64 knots (74 mph) or greater, either predicted or occurring, and not directly associated with a tropical
cyclone. Hurricane Force Wind WatchA watch for an increased risk of a hurricane force wind event for sustained surface winds, or frequent gusts, of 34 knots 64 knots (74 mph) or greater, but its occurrence, location, and/or timing is still uncertain.Hurricane Local StatementA public release prepared by local National Weather Service offices in or near a
threatened area giving specific details for its county/parish warning area on (1)
weather conditions (2) evacuation decisions made by local officials (3) other
precautions necessary to protect life and property.Hurricane SeasonThe part of the year having a relatively high incidence of tropical cyclones. In the Atlantic, Caribbean, and Gulf of Mexico, and central North Pacific, the hurricane season is the period from June through November; in the eastern Pacific, May 15 through November 30. Tropical cyclones can occur year-round in any basin.Hurricane WarningAn announcement that hurricane conditions (sustained winds of 74 mph or higher) are expected somewhere within the specified coastal area. Because hurricane preparedness activities become difficult once winds reach tropical storm force, the hurricane warning is issued 36 hours in advance of the anticipated onset of tropical-storm-force winds. The warning can remain in effect when dangerously high water or a combination of dangerously high water and waves continue, even though winds may be less than hurricane force. Hurricane WatchAn announcement that hurricane conditions (sustained winds of 74 mph or higher) are possible within the specified coastal area. Because hurricane preparedness activities become difficult once winds reach tropical storm force, the hurricane watch is issued 48 hours in advance of the anticipated onset of tropical-storm-force winds.Hyder FlareIn solar-terrestrial terms, a filiment-associated two-ribbon flare, often occurring in
spotless regions. The flare presumably results from the impact
on the chromosphere of infalling filament material.Hydraulic Grade LineIn hydrologic terms, a line whose plotted ordinate position represents the sum of pressure head plus elevation head for the various positions along a
given fluid flow path, such as along a pipeline or a ground water streamline.Hydraulic HeadIn hydrologic terms, (1) The height of the free surface of a body of water above a given point beneath the surface. (2) The height of the water level at
the headworks, or an upstream point, of a waterway, and the water surface at a given point downstream. (3) The height of a
hydraulic grade line above the center line of a pressure pipe, at a given point.Hydraulic PermeabilityIn hydrologic terms, the flow of water through a unit cross-sectional area of soil normal to the direction of flow when the hydraulic gradient is unity.HydrometeorologyAn interdisciplinary science involving the study and analysis of the interrelationships between the atmospheric and land phases of water as it moves through the hydrologic cycle.Hydrograph SeparationIn hydrologic terms, the process where the storm hydrograph is separated into baseflow components and surface runoff components. Hydrographic SurveyIn hydrologic terms, an instrumental survey to measure and determine characteristics of streams and other bodies of water within an area, including such
things as location, areal extent, and depth of water in lakes or the ocean; the width, depth, and course of streams; position and
elevation of high water marks; location and depth of wells, etc.Hydrologic BudgetIn hydrologic terms, an accounting of the inflow to, outflow from, and storage in, a hydrologic unit, such as a drainage basin, aquifer, soil zone, lake,
reservoir, or irrigation project. Hydrologic CycleThe description of the transport of water substance between the earth, the atmosphere, and the seas.
or In hydrologic terms, the natural pathway water follows as it changes between liquid, solid, and gaseous states.Hydrologic Ensemble Forecast System (HEFS)A probabilistic forecast tool with the goals to provide hydrologic forecasts including an analysis of “probable outcomes” and to minimize biases in the atmospheric models and in the hydrologic models.Hydrologic EquationIn hydrologic terms, the water inventory equation (Inflow = Outflow + Change in Storage) which expresses the basic principle that during a given time
interval the total inflow to an area must equal the total outflow plus the net change in storage.Hydrologic ModelIn hydrologic terms, a conceptual or physically-based procedure for numerically simulating a process or processes which occur in a watershed.Hydrologic Service AreaHSA. A geographical area assigned to Weather Service Forecast Office's/Weather Forecast Office's that embraces one or more rivers.HydrometeorA particle of condensed water (liquid, snow, ice, graupel, hail) in the atmosphere.HydrometeorologistsIn hydrologic terms, individuals who have the combined knowledge in the fields of both meteorology and hydrology which enables them to study and
solve hydrologic problems where meteorology is a factor.HydrometeorologyAn interdisciplinary science involving the study and analysis of the interrelationships between the atmospheric and land phases of water as it moves through the hydrologic cycle.
Hydrostatic HeadIn hydrologic terms, a measure of pressure at a given point in a liquid in terms of the vertical height of a column of the same liquid which would produce
the same pressureHyetographA graphical representation of rainfall intensity with respect to time.HygrometerAn instrument which measures the humidity of the air.HypothermiaA rapid, progressive mental and physical collapse that accompanies the lowering of body temperature.Ice AgeA time of widespread glaciation.Ice BoomIn hydrologic terms, a floating structure designed to retain ice.Ice BridgeIn hydrologic terms, a continuous ice cover of limited size extending from shore to shore like a bridge.Ice CrystalsA barely visible crystalline form of ice that has the shape of needles, columns or plates. Ice crystals are so small that they seem to be suspended in air. Ice crystals occur at very low temperatures in a stable atmosphere.Ice Fog(Also called ice-crystal fog, frozen fog, frost fog, frost flakes, air hoar, rime fog, pogonip.) A type of fog, composed of suspended particles of ice; partly ice crystals 20 to 100 micron in
diameter, but chiefly (especially when dense) ice particles about 12–20 micron in diameter, formed by direct freezing of supercooled water
droplets with little growth directly from the vapor. It occurs at very low temperatures, and usually in clear, calm weather in high latitudes.
The sun is usually visible and may cause halo phenomena. Ice fog is rare at temperatures warmer than -30°C, and increases in frequency with decreasing temperature until it is almost always present at air temperatures of -45°C in the vicinity of a source of water vapor. Such
sources are the open water of fast-flowing streams or of the sea, herds of animals, volcanoes, and especially products of combustion for heating or propulsion. At temperatures warmer than -30°C, these sources can cause steam fog of liquid water droplets, which may turn into ice fog when cooled (see frost smoke). See ice-crystal haze, arctic mist.Ice GorgeIn hydrologic terms, the gorge or opening left in a jam after it has broken.Ice JamIn hydrologic terms, a stationary accumulation that restricts or blocks streamflow.Ice NucleusAny particle that serves as a nucleus in the formation of ice crystals in the atmosphere.Ice Pellets(abbrev. IP) Same as Sleet; defined as pellets of ice composed of frozen or mostly frozen raindrops or refrozen partially melted snowflakes. These pellets of ice usually bounce after hitting the ground or other hard surfaces. A Winter Storm Warning is issued for sleet or a combination of sleet and snow based on total accumulation which is locally defined by area.Ice PushIn hydrologic terms, compression of an ice cover particularly at the front of a moving section of ice cover.Ice RunIn hydrologic terms, flow of ice in a river. An ice run may be light or heavy, and may consist of frazil, anchor, slush, or sheet iceIce ShoveIn hydrologic terms, on-shore ice push caused by wind, and currents, changes in temperature, etcetera.Ice StormAn ice storm is used to describe occasions when damaging accumulations of ice are expected during freezing rain situations. Significant accumulations of ice pull down trees and utility lines resulting in loss of power and communication. These accumulations of ice make walking and driving extremely dangerous. Significant ice accumulations are usually accumulations of ¼" or greater.Ice Storm WarningThis product is issued by the National Weather Service when freezing rain produces a significant and possibly damaging accumulation of ice. The criteria for this
warning varies from state to state, but typically will be issued any time more than 1/4" of ice is expected to accumulate in an area.Ice TwitchIn hydrologic terms, downstream movement of a small section of an ice cover. Ice twitches occur suddenly and often appear successively.IcebergA piece of a glacier which has broken off and is floating in the sea.Icelandic LowA semi-permanent, subpolar area of low pressure in the North Atlantic Ocean. Because of its broad area and range of central pressure, it is an area where migratory lows tend to slow down and deepen. It is strongest during a Northern Hemisphere winter and early spring, centered over Iceland and southern Greenland, and is the dominate weather feature in the area. During the summer, it is weaker, less intense, and might divide into two parts, one west of Iceland, the other over the Davis Strait between Greenland and Baffin Island. Then the Azores or Bermuda High becomes the dominate weather feature in the North Atlantic.Ideal Gas LawsThe thermodynamic laws applying to perfect gases.ImpermeableMaterial that does not permit fluids to pass through it.ImperviousIn hydrologic terms, the ability to repel water, or not let water infiltrateImpulse(abbrev. IMPL) Alternate term for Upper Level System and Shortwave; a general term for any large-scale or mesoscale disturbance capable of producing upward motion (lift) in the middle or upper parts of the atmosphere.Inactive Storage CapacityIn hydrologic terms, the portion of capacity below which the reservoir is not normally drawn, and which is provided for
sedimentation, recreation, fish and wildlife, aesthetic reasons, or for the creation of a minimum controlled operational or power head in
compliance with operating agreements or restrictions.Inch-DegreesThe product of rainfall (in inches) multiplied by the temperature (in degrees Fahrenheit) above freezing. Used as a measure of the snowmelting capacity of rainfall.Inches of Mercury(or in Hg) Unit of atmospheric pressure used in the United States. The name comes from the use of mercurial barometers which equate the height of a column of mercury with air pressure. One inch of mercury is equivalent to 33.86 millibars or 25.40 millimeters. See barometric pressure. First divised in 1644 by Evangelista Torricelli (1608-1647), an Italian physicist and mathematician, to explain the fundamental principles of hydromechanics.
To convert millibars (mb) to inches of mercury (in Hg), divide the millibar reading by 33.86:
in Hg = mb / 33.86Inches of RunoffIn hydrologic terms, the volume of water from runoff of a given depth over the entire drainageIndex of WetnessThe ratio of precipitation for a given year over the mean annual precipitation.Indirect HitGenerally refers to locations that do not experience a direct hit from a tropical
cyclone, but do experience hurricane force winds (either sustained or gusts) or tides
of at least 4 feet above normal.Infiltration IndexIn hydrologic terms, an average rate of infiltration, in inches per hour, equal to the average rate of rainfall such as that the volume of rainfall at greater
rates equals the total direct runoff.Infiltration RateIn hydrologic terms, (1) The rate at which infiltration takes place expressed in depth of water per unit time, usually in inches per hour. (2) The rate,
usually expressed in cubic feet per second, or million gallons per day per mile of waterway, at which ground water enters an
infiltration ditch or gallery, drain, sewer, or other underground conduit.Inflow JetsLocal jets of air near the ground flowing inward toward the base of a tornado.Inflow StingerA beaver tail cloud with a stinger-like shape. Influent SeepageIn hydrologic terms, movement of gravity water in the zone of aeration from the ground surface toward the water table.Influent StreamIn hydrologic terms, any watercourse in which all, or a portion of the surface water flows back into the ground namely the, vadose zone, or zone of
aerationInfrared Satellite ImageryThis satellite imagery senses surface and cloud top temperatures by measuring the wavelength of electromagnetic radiation emitted from these objects.
This energy is called "infrared". High clouds are very cold, so they appear white. Mid-level clouds are somewhat warmer, so they will be a light gray shade. Low cloud are warmer still, so
they appear as a dark shade of gray or black. Often, low clouds are the same temperature as the surrounding terrain and cannot be distinguished at all. The satellite picks up this infrared
energy between 10.5 and 12.6 micrometer (um) channels. Initial DetentionIn hydrologic terms, the volume of water on the ground, either in depressions or in transit, at the time active runoff begins.Inland freshwater wetlandsIn hydrologic terms, swamps, marshes, and bogs found inland beyond the coastal saltwater wetlands. Instrument Flight RulesRefers to the general weather conditions pilots can expect at the surface and applies to the weather situations at an airport during which a pilot must use instruments to assist take off and landing. IFR conditions for fixed wing aircraft means the minimum cloud ceiling is greater than 500 feet and less than 1,000 feet and/or visibility is greater than 1 mile and less than 3 miles.Instrument ShelterA boxlike structure designed to protect temperature measuring instruments from exposure to direct sunshine, precipitation, and condensation, while at the same time time providing adequate ventilation. Intangible Flood DamageIn hydrologic terms, estimates of the damage done by disruption of business, danger to health, shock, and loss of life and in general all costs not directly
measurable which require a large element of judgment for estimating.Interbasin TransferIn hydrologic terms, the physical transfer of water from one watershed to another.InterceptionIn hydrologic terms, the process by which precipitation is caught and held by foliage, twigs, and branches of trees, shrubs, and other vegetation, and
lost by evaporation, never reaching the surface of the ground. Interception equals the precipitation on the vegetation minus
streamflow and through fall.Interception Storage RequirementsIn hydrologic terms, water caught by plants at the onset of a rainstorm. This must be met before rainfall reaches the ground.InterflowIn hydrologic terms, the lateral motion of water through the upper layers until it enters a stream channel. This usually takes longer to reach stream
channels than runoff. This also called subsurface storm flow.Intermediate Synoptic TimesThe times of 0300, 0900, 1500, and 2100 UTC.Intermittent StreamIn hydrological terms, a stream that flows periodicallyInternational Date LineThe line of longitude located at 180 degrees East or West (with a few local deviations) where the date changes by a day. West of the line it is one day later than east of the line.Interplanetary Magnetic Field(abbrev. IMF) In solar-terrestrial terms, the magnetic field carried with the
solar wind.InterpolateTo estimate a value within an interval between two known values. This technique is sometimes used with computer models for locations in between the model's "gridpoints."Intertropical Convergence Zone(ITCZ) The region where the northeasterly and southeasterly trade winds converge, forming an often continuous band of clouds or thunderstorms near the equator. Intraseasonal OscillationOscillation with variability on a timescale less than a season. One example is the Madden-Julian Oscillation.Inversion(abbrev. INVRN) Generally, a departure from the usual increase or decrease in an atmospheric property with altitude. Specifically it almost always refers to a temperature inversion, i.e., an increase in temperature with height, or to the layer within which such an increase occurs. An inversion is present in the lower part of a cap.IonosphereA complex atmospheric zone of ionized gases that extends between 50 and 400 miles (80 to 640 kilometers) above the earth's surface. It is located between the mesosphere and the exosphere and is included as part of the thermosphere.Ionospheric StormA disturbance in the F region of the ionosphere, which
occurs in connection with geomagnetic activityIridescenceBrilliant spots or borders of colors in clouds, usually red and green, caused by diffraction of light by small cloud particles. The phenomenon is usually observed in thin cirrus clouds within about 30° of the sun and is characterized by bands of color in the cloud that contour the cloud edges.Iridescent CloudsClouds that exhibit brilliant bright spots, bands, or borders of colors, usually red and green, observed up to about 30 degrees from the sun. The coloration is due to the diffraction with small cloud particles producing the effect. It is usually seen in thin cirrostratus, cirrocumulus, and altocumulus clouds.Irrigation RequirementIn hydrologic terms, the quantity of water, exclusive of precipitation, that is required for crop production. It includes surface evaporation and other
economically unavoidable wastes.Isentropic AnalysisA way in the forecaster can look at the atmosphere in 3-dimensions instead of looking at constant pressure surfaces (such as the 850 mb, 700 mb, 500 mb, etc.) which are in 2-dimensions. In this analysis method, the forecaster looks at constant potential temperature (the temperature that it would take if we compressed or expanded it adiabatically to the pressure of 1000 mb) surfaces. Air parcels move up and down these surfaces; therefore, the forecaster can see where the moisture is located and how much moisture is available.Isentropic LiftLifting of air that is traveling along an upward-sloping isentropic surface.
Isentropic lift often is referred to erroneously as overrunning, but more accurately describes the physical process by which the lifting occurs. Situations involving isentropic lift often are characterized by widespread stratiform clouds and precipitation, but may include elevated convection in the form of embedded thunderstorms.Isentropic SurfaceA two-dimensional surface containing points of equal potential temperature.Isobaric ProcessAny thermodynamic change of state of a system that takes a place at constant pressure.IsochroneA line on a chart connecting equal times of occurrence of an event. In a weather analysis, a sequence plotted on a map of the frontal positions at several different
observation times would constitute a set of isochrones.IsodrosothermA line connecting points of equal dew point temperature.IsoheightSame as a contour depicting vertical height of some surface above a datum plane.IsohelA line on a weather map connecting points receiving equal sunlight.IsohyetA line connected points of equal precipitation amounts.IsolatedA National Weather Service convective precipitation descriptor for a 10 percent chance of measurable precipitation (0.01 inch). Isolated is used interchangeably with few. IsoplethA broad term for any line on a weather map connecting points with equal values of a particular atmospheric variable (temperature, dew point, etc.). Isotherms, isotachs, etc. are all examples of isopleths.IsothermA line connecting points of equal temperature.Issuance TimeThe time the product is transmitted. JetA fast-moving wind current surrounded by slower moving air.Jet Max( or Jet Streak) A point or area of relative maximum wind speeds within a jet stream.Jet StreakSame as Jet Max; a point or area ("streak") of relative maximum wind speeds within a jet stream.Jet Stream(abbrev. JSTR) Relatively strong winds concentrated in a narrow stream in the atmosphere, normally referring to horizontal, high-altitude winds. The position and orientation of jet streams vary from day to day. General weather patterns (hot/cold, wet/dry) are related closely to the position, strength and orientation of the jet stream (or jet streams). A jet stream at low levels is known as a low-level jet.Jet Stream CirrusA loose term for filamentous cirrus that appears to radiate from a point in the sky, and exhibits characteristics associated with strong vertical wind shear, such as twisted or curved filaments.Jet Wind Speed ProfileA vertical wind speed profile characterized by a relatively narrow current of high winds with slower moving air above and below. A large wind (speed) shear occurs above and below the jet axis.JettyIn hydrologic terms, a structure (e.g.; a pier, or mole of wood or stone) extending into a sea, lake, or river to influence the current or tide or to protect
a harborJuvenile WaterIn hydrologic terms, water formed chemically within the earth and brought to the surface in intrusive rockK-IndexA measure of the thunderstorm potential based on vertical temperature lapse rate, moisture content of the lower atmosphere, and the vertical extent of the moist layer. The temperature difference between 850 mb and 500 mb is used to parameterize the vertical temperature lapse rate. The 850 dew point provides information on the moisture content of the lower atmosphere. The vertical extent of the moist layer is represented by the difference of the 700 mb temperature and 700 mb dew point. This is called the 700 mb temperature-dew point depression. The index is derived arithmetically and does not require a plotted sounding.
K-index = (850 mb temperature - 500 mb temperature) + 850 mb dew point - 700 mb dew point depressionKeetch-Byrum Drought IndexAn index used to gage the severity of drought in deep duff and organic soils.Kelvin Temperature ScaleAn absolute temperature scale in which a change of 1 Kelvin equals a change of 1 degree Celsius; 0ºK is the lowest temperature on the Kelvin scale. The freezing point of water is +273ºK (Kelvin) and the boiling point of +373ºK. It is used primarily for scientific purposes. It is also known as the Absolute Temperature Scale.Kelvin WavesFluctuations in wind speed at the ocean surface at the Equator result in eastward propagating waves, known as Kelvin Waves. Kelvin Waves cause variations in the depth of the oceanic thermocline, the boundary between warm waters in the upper ocean and cold waters in the deep ocean. They play an important role in monitoring and predicting El Niño episodes.Kelvin-Helmholtz WavesVertical waves in the air associated with wind shear across statically-stable regions. Can appear as breaking waves and as braided patterns in radar images and cloud photos.Kinetic EnergyEnergy that a body has as a result of its motion. Mathematically, it is defined as one-half the product of a body's mass and the square of its speed (KE = 1/2 * mass * velocity squared).KnucklesSlang for lumpy protrusions on the edges, and sometimes the underside, of a thunderstorm anvil. They usually appear on the upwind side of a back-sheared anvil, and
indicate rapid expansion of the anvil due to the presence of a very strong updraft. They are not mammatus clouds. See also cumuliform anvil and anvil rollover.Kp IndexA 3-hourly planetary geomagnetic index of activity generated in
Gottingen, Germany, based on the K Index from 12 or 13 stations
distributed around the worldLake BreezeA thermally produced wind blowing during the day from the surface of a large lake to the shore, caused by the difference in the rates of heating of the surfaces of the lake and of the land.Lake Effect SnowSnow showers that are created when cold, dry air passes over a large warmer lake, such as one of the Great Lakes, and picks up moisture and heat.Lake Effect Snow AdvisoryThis product is issued by the National Weather Service when pure lake effect snow (this is where the snow is a direct result of lake effect snow and not
because of a low pressure system) may pose a hazard or it is life threatening. The criteria for this advisory varies from area to area.Lake Effect Snow SquallA local, intense, narrow band of moderate to heavy snow squall that can extend long distances inland. It may persist for many hours. It may also be accompanied by strong, gusty, surface winds and possibly lightning. Accumulations can be 6 inches or more in 12 hours.Lake Effect Snow WarningThis product is issued by the National Weather Service when pure lake effect snow (this is where the snow is a direct result of lake effect snow and not because of a synoptic storm or low pressure system) may pose a hazard or it is life threatening.Lake Effect StormA fall or winter storm that produces heavy but localized precipitation as a result of temperature differences between the air over snow-covered ground and the air over the open waters of a lake.Lakeshore Flood AdvisorySee Coastal/Lakeshore Flood Advisory.Lakeshore Flood WatchSee: COASTAL/LAKESHORE FLOOD WATCHLakeshore FloodingSee COASTAL/LAKESHORE FLOODINGLand BreezeA coastal breeze at night blowing from land to sea, caused by the difference in the rates of cooling of their respective surfaces.Lapse RateThe rate of change of an atmospheric variable, usually temperature, with height. A steep lapse rate implies a rapid decrease in temperature with height (a sign of instability) and a steepening lapse rate implies that destabilization is occurring.Large Scale(Synoptic Scale) Size scale referring generally to weather systems with horizontal dimensions of several hundred miles or more. Most high and low
pressure areas seen on weather maps are synoptic-scale systems.Last UpdateThe time and date in which the forecast was issued or updated. The forecast may be updated at any time as weather conditions warrant. Latent HeatHeat absorbed or released during a change of phase at constant temperature and pressure.Latent Heat FluxThe flux of heat from the earth's surface to the atmosphere that is associated with evaporation or condensation of water vapor at the surface; a component of the surface energy budget.
Latitude(abbrev. LAT) The location north or south in reference to the equator, which is designated at zero (0) degrees. Lines of latitude are parallel to the equator and circle the globe. The North and South poles are at 90 degrees North and South latitude.LAWEB(Great Lakes Weather Broadcast) - A National Weather Service product containing an observation summary prepared to provide Great Lakes mariners with a listing of weather observations along or on the Lakes. Layer Composite Reflectivity AverageThis WSR-88D radar product displays the average reflectivities for a layer. Data is taken from all elevation angles contained in a given layer for each grid box. It is available for 3 layers (low, mid, high). It is used to aid in determining storm intensity trends by comparing mid level layer composite products with a low level elevation angle base reflectivity product and aid in routing air traffic.Layer Composite Reflectivity MaximumThis WSR-88D radar product displays the maximum reflectivities for a layer. Data is taken from all elevation angles contained in a given layer for each grid box. It is available for 3 layers (low, mid, high). Currently, the low layer extends from the surface to 24,000 feet, the mid layer extends from 24,000 feet to 33,000 feet, and high layer extends above 33,000 feet. It is used to aid in determining storm intensity trends by comparing mid level layer composite products with a low level elevation angle base reflectivity product and aid in routing air traffic.Layered HazeHaze produced when air pollution from multiple line, area or point sources is transported long distances to form distinguishable layers of discoloration in a stable atmosphere.LeaderThe streamer which initiates the first phase of each stroke of a lightning discharge. The
first stroke is led by a steeped leader, which may be preceded by a pilot streamer. All subsequent
strokes begin with a dart leader.Leader SpotIn solar-terrestrial terms, in a magnetically bipolar or multipolar sunspot group, the
western part precedes and the main spot in that part is called the
leader.LeeThe side or part that is sheltered or turned away from the wind, such as with a mountain.Lee WaveThe wavelike effect, characterized by severe updrafts and downdrafts, that occurs in the lee of a mountain range when rapidly flowing air is lifted up the steep front of a mountain range. Compare mountain wave.Leeside LowExtratropical cyclones that form on the downwind (lee) side of a mountain chain. In the United States, they frequently form on the eastern side of the Rockies and Sierra Nevadas.LeewardThe side away from the wind. Compare windward.Left Exit RegionUsed interchangably with Left Front Quadrant; the area downstream from and to the left of an upper-level jet max (as would be viewed looking along the direction of flow). Upward motion and severe thunderstorm potential sometimes are increased in this area relative to the wind speed maximum. See also entrance region, right rear quadrant. Left Front QuadrantUsed interchangably with Left Exit Region; the area downstream from and to the left of an upper-level jet max (as would be viewed looking along the direction of flow). Upward motion and severe thunderstorm potential sometimes are increased in this area relative to the wind speed maximum. See also entrance region, right rear quadrant.Left MoverA thunderstorm which moves to the left relative to the steering winds, and to other nearby thunderstorms; often the northern part of a splitting storm.LengthIn hydrologic terms, the distance in the direction of flow between two specific points along a river, stream, or channel.Lentic SystemIn hydrologic terms, a nonflowing or standing body of fresh water, such as a lake or pondLenticular CloudA very smooth, round or oval, lens-shaped cloud that is often seen, singly or stacked in groups, near or in the lee of a mountain ridge.Levee(Dike) In hydrologic terms, a long, narrow embankment usually built to protect land from flooding. If built of concrete or masonary the structure is usually
referred to as a flood wall. Levees and floodwalls confine streamflow within a specified area to prevent flooding. The term "dike" is
used to describe an embankment that blocks an area on a reservoir or lake rim that is lower than the top of the dam.Level of Free Convection(LFC) - The level at which a parcel of saturated air becomes warmer than the surrounding air and begins to rise freely. This occurs most readily in a conditionally unstable atmosphere.LEWPLine Echo Wave Pattern. A bulge in a thunderstorm line producing a wave-shaped "kink" in the
line. The potential for strong outflow and damaging straight-line winds increases near the bulge,
which often resembles a bow echo. Severe weather potential also is increased with storms near the crest of
a LEWP. Lifted Index(abbrev. LI)- A common measure of atmospheric instability. Its value is obtained by computing the temperature that air near the ground would have if it were lifted to some higher level (around 18,000 feet, usually) and comparing that temperature to the actual temperature at that level. Negative values indicate instability - the more negative, the more unstable the air is, and the stronger the updrafts are likely to be with any developing thunderstorms. However there are no "magic numbers" or threshold LI values below which severe weather becomes imminent.Lifting Condensation Level(LCL) - The level at which a parcel of moist air becomes saturated when it is lifted dry adiabatically.Light BridgeIn solar-terrestrial terms, it is observed in white light, a bright tongue or streaks penetra-
ting or crossing sunspot umbrae. The appearance of a light bridge
is frequently a sign of impending region division or dissolutionLightning ChannelThe irregular path through the air along which a lightning discharge occurs.
A typical discharge of flash between the ground and the cloud is actually a composite flash
which is composed of several sequential lightning strokes, each of which is initiated by a leader
and terminated by a return streamer.Lightning DischargeThe series of electrical processes by which charge is transferred along a
channel of high ion density between electrical charge centers of opposite sign. This can be
between a cloud and the Earth's surface of a cloud-to-ground discharge.Lightning StrokeAny of a series of repeated electrical discharges comprising a single lightning
discharge (strike). Specifically, in the case of a cloud-to-ground discharge, a leader plus its
subsequent return streamer. Likely(abbrev. LKLY) In probability of precipitation statements, the equivalent of a 60 or 70 percent chance.Limb FlareIn solar-terrestrial terms, a solar flare seen at the edge (Limb) of the sun.Line Echo Wave Pattern(abbrev. LEWP) A radar echo pattern formed when a segment of a line of thunderstorms surges forward at an accelerated rate.Line SourceAn array of pollutant sources along a defined path that can be treated in dispersion models as an aggregate uniform release of pollutants along a line. Example: the sum of emissions from individual cars traveling down a highway can be treated as a line source. Compare area source and point source.Liquid Water EquivalentSame as Water Equivalent; the liquid content of solid precipitation that has accumulated on the ground (snow depth). The accumulation may consist of snow, ice formed by freezing precipitation, freezing liquid precipitation, or ice formed by the refreezing of melted snow. LithometeorAtmospheric phenomena which affect the state of the atmosphere. They constitute dry particles that hang suspended in the atmosphere, such as dust, smoke, sand, and haze. LithosphereIn hydrologic terms, that part of the earth which is composed predominantly of rocks (either coherent or incoherent, and including the disintegrated
rock materials known as soils and subsoils), together with everything in this rocky crust.Littoral ZoneIn hydrologic terms, the area on, or near the shore of a body waterLive CapacityIn hydrologic terms, the total amount of storage capacity available in a reservoir for all purposes, from the dead storage level to the normal water or
normal pool level surface level. Does not include surcharge, or dead storage, but does include inactive storage, active conservation
storage and exclusive flood control storage.Loaded Gun (Sounding)[Slang], a sounding characterized by extreme instability but containing a cap,
such that explosive thunderstorm development can be expected if the cap can be weakened or the air below
it heated sufficiently to overcome it.Local Convective WindIn fire weather terminology, local thermally driven winds arising over a comparatively small area and influenced by local terrain. Examples include sea and land breezes, lake breezes, diurnal mountain wind systems and columnar convective currents.Long Term RetentionRetention of data for 5 years to satisfy requirements for local studies and to support litigation.LongitudeThe location east or west in reference to the Prime Meridian, which is designated as zero (0) degrees longitude. The distance between lines of longitude are greater at the equator and smaller at the higher latitudes, intersecting at the earth's North and South Poles. Time zones are correlated to longitude.Longwave RadiationA term used to describe the infrared energy emitted by the earth and atmosphere at wavelengths between about 5 and 25 micrometers. Compare shortwave radiation.Longwave TroughA trough in the prevailing westerly flow aloft which is characterized by large length and (usually) long duration.Loop Prominence System(abbrev. LPS) In solar-terrestrial terms, a system of loop prominences associated with
major flares.LOPRESlow pressureLotic SystemIn hydrologic terms, a flowing body of fresh water, such as a river or stream.Low Frequency(abbrev. LF) The portion of the radio frequency spectrum from 30 to
300 kHz.Low Level Jet(abbrev. LLJ)- A region of relatively strong winds in the lower part of the atmosphere. Specifically, it often refers to a southerly wind maximum in the boundary layer, common over the Plains states at night during the warm season (spring and summer).
The term also may be used to describe a narrow zone of strong winds above the boundary layer, but in this sense the more proper term would be low-level jet stream.Low Pressure SystemAn area of a relative pressure minimum that has converging winds and rotates in the same direction as the earth. This is counterclockwise in the Northern
Hemisphere and clockwise in the Southern Hemisphere. Also known as an cyclone, it is the opposite of an area of high pressure, or a anticyclone. Low Water AdvisoryAn advisory to describe water levels which are significantly below average levels over the Great Lakes, coastal marine zones, and any tidal marine area, waterway, or river inlet within or adjacent to a marine zone that would potentially be impacted by low water conditions creating a hazard to navigation. LysimeterIn hydrologic terms, a device to measure the quantity or rate of downward water movement through a block of soil usually undisturbed, or to collect
such percolated water for analysis as to qualityMackeral SkyThe name given to cirrocumulus clouds with small vertical extent and composed of ice crystals. The rippled effect gives the appearance of fish scales. MacroscaleLarge scale, characteristic of weather systems several hundred to several thousand kilometers in diameter.Madden-Julian Oscillation(abbrev. MJO)- Tropical rainfall exhibits strong variability on time scales shorter than the seasonal El Niño-Southern Oscillation (ENSO). These fluctuations in tropical rainfall often go through an entire cycle in 30-60 days, and are referred to as the Madden-Julian Oscillation or intraseasonal oscillations. The intraseasonal oscillations are a naturally occurring component of our coupled ocean-atmosphere system. They significantly affect the atmospheric circulation throughout the global Tropics and subtropics, and also strongly affect the wintertime jet stream and atmospheric circulation features over the North Pacific and western North America. As a result, they have an important impact on storminess and temperatures over the United States. During the summer these oscillations have a modulating effect on hurricane activity in both the Pacific and Atlantic basins.MagflareIn solar-terrestrial terms, a geomagnetic and/or cosmic storm has been associated with this
flareMagnetic BayIn solar-terrestrial terms, a relatively smooth excursion of the H (horizontal) component
of the geomagnetic field away from and
returning to quiet levels.MagnetogramIn solar-terrestrial terms, solar magnetograms are a graphic representation of solar magnetic field strengths and polarityMagnetopauseIn solar-terrestrial terms, the boundary layer between the solar wind and the magnetosphere.MagnetosphereIn solar-terrestrial terms, the magnetic cavity surrounding the earth, carved out of the
passing solar wind by virtue of the geomagnetic field, which prevents, or at least impedes, the direct entry of the solar wind
plasma into the cavityMain StemIn hydrologic terms, the reach of a river/stream formed by the tributaries that flow into it.Main Synoptic TimesThe times of 0000, 0600, 1200, and 1800 UTC. Also known as the standard synoptic times. Major FlareIn solar-terrestrial terms, this flare is the basis for the forecast of geomagstorm, cosmic
storm and/or protons in the earth's vicinityMajor HurricaneA hurricane which reaches Category 3 (sustained winds greater than 110 mph) on the Saffir/Simpson Hurricane Scale. MARC Velocity SignatureA Doppler radar-velocity based precursor towards forecasting the initial onset of damaging straight-line winds in a linear Quasi_Linear Convective System (QLCS) or bowing convective system.Mare's TrailThe name given to thin, wispy cirrus clouds composed of ice crystals that appear as veil patches or strands, often resembling a horse's tail.MAREP(MArine REPort) A voluntary marine observation program of the National Weather Service whose goal is to solicit meteorological and oceanographic observations in plain language from recreational and small commercial mariners who are not part of Voluntary Observing Ship program. Marginal Visual Flight Rules(Abbrev. MVFR) - In an aviation product, refers to the general weather conditions pilots can expect at the surface. VFR stands for Visual Flight Rules and MVFR means Minimum or Marginal Visual Flight Rules. MVFR criteria means a ceiling between 1,000 and 3,000 feet and/or 3 to 5 miles visibility.Marine InversionTemperature inversion produced when cold marine air underlies warmer air.Marine PushA replacement of the current air mass with air from off the ocean. Temperatures are much cooler and relative humidities much higher. The air mass is generally much more stable in this situation.Marine Small Craft Thunderstorm AdvisoryA marine warning issued by Environment Canada
Atmospheric Environment Branch when the possibility of thunderstorms is greater than 40
percent.Marine Small Craft Wind WarningA marine warning issued by Environment Canada
Atmospheric Environment Branch for winds which are forecasted to be in the 20-33 knot range
inclusive.Marine Weather StatementA National Weather Service product to provide mariners with details on significant or potentially hazardous conditions not otherwise covered in existing marine warnings and forecasts. Marine weather statements are also used to supplement special marine warnings.Marine ZoneSpecific, defined over-water areas contained in the various NWS marine forecasts. These are the equivalent of "zones" in the public forecast program. Maritime Air MassAn air mass influenced by the sea. It is a secondary characteristic of an air mass classification, signified by the small "m" before the primary characteristic, which is based on source region. For example, mP is an air mass that is maritime polar in nature. Also known as a marine air mass. Maritime Polar Air MassAn air mass characterized by cold, moist air. Abbreviated mP.Maritime Tropical Air MassAn air mass characterized by warm, moist air. Abbreviated mT.Max Parcel Level (MPL)This signifies the highest attainable level that a convective updraft can reach; therefore, it is a good indication of how tall a thunderstorm may reach.Maximum Spillway DischargeIn hydrologic terms, spillway discharge (cfs) when reservoir is at maximum designed water surface elevation.Maximum Sustained Surface WindWhen applied to a particular weather system, refers to the highest one-minute average wind (at an elevation of 10 meters with an unobstructed exposure) associated with that weather system at a particular point in time. Maximum TemperatureThis is the highest temperature recorded during a specified period of time. Common time periods include 6, 12 and 24 hours. The most common reference is to the daily maximum temperature, or "high."Maximum Unambiguous RangeThe range from the radar at which an echo can be known unquestionably as being at that range. As the radar sends out a pulse of energy, the pulse hits a target and part of the energy bounces back to the radar, but part of the energy may continue to travel away from the radar. The distance to the target is computed by knowing the time that has elapsed since the pulse was emitted. Then a second pulse of energy is transmitted. If some of the energy from the first pulse strikes a target at a far range and returns to the radar when radiation from the second pulse arrives, the RDA misinterprets the returned first pulse as arriving from a target near the returned second pulse. The maximum unambiguous range is related to the amount of time that elapses between successive pulses of emitted energy.Maximum Unambiguous VelocityThe highest radial velocity that can be measured unambiguously by a pulsed Doppler radar. The maximum unambiguous velocity is related to the radar's successive pulses of emitted energy. When a target's velocity exceeds the maximum unambiguous velocity, the velocity will be "folded" to appear as a different velocity.MeanThe arithmetic average of a set of data (numbers), or the middle point between its two extremes.Mean Annual TemperatureThe average temperature for the entire year at any given location.Mean Areal Precipitation(abbrev. MAP)- The average rainfall over a given area, generally expressed as an average depth over the area.Mean Daily TemperatureThe average of the highest and lowest temperatures during a 24-hour period.Mean DepthIn hydrologic terms, the average depth of water in a stream channel or conduit. It is equal to the cross-sectional area divided by the surface width.Mean Doppler VelocityReflectivity-weighted average velocity of targets in a given pulse resolution volume. Usually determined from a large number of successive radar pulses. Also called mean radial (towards or away from the antenna) velocity. Doppler velocity refers to spectral density first moment, radial velocity to base data.Mean Low Water(MLW) - A tidal datum. The average of all the low water heights observed over the National Tidal Datum Epoch. For stations with shorter series, simultaneous observational comparisons are made with a control tide station in order to derive the equivalent datum of the National Tidal Datum Epoch.Mean Lower Low Water(MLLW) - A tidal datum. The average of the lower low water height of each tidal day observed over the National Tidal Datum Epoch. For stations with shorter series, simultaneous observational comparisons are made with a control tide station in order to derive the equivalent datum of the National Tidal Datum Epoch. Mean Sea Level(MSL) - The arithmetic mean of hourly water elevations observed over a specific 19-year tidal epoch. MeanderIn hydrologic terms, the winding of a stream channelMeander BeltIn hydrologic terms, the area between lines drawn tangential to the extreme limits of fully developed meandersMeasured CeilingA ceiling classification applied when the ceiling value has been determined by an instrument, such as a ceilometer or ceiling light, or by the known heights of unobscured portions of objects, other than natural landmarks, near the runway. See variable ceiling. Medium Frequency(abbrev. MF)- That portion of the radio frequency spectrum from 0.3
to 3 MHz.Medium RangeIn forecasting, (generally) three to seven days in advance.Medium Range Forecast (MRF)A configuration of the National Water Model (NWM) that runs every 6 hours and produces 3-hourly deterministic forecasts of streamflow and hydrologic states for the contiguous United States (ConUS). This configuration is an ensemble forecast with 7 members; member 1 extends out to 10 days, while members 2-7 extend out to 8.5 days. Meteorological forcing data are drawn from the GFS.MEGGMergingMelting LevelThe altitude which ice crystals and snowflakes melt as they descend through the atmosphere.Melting PointThe temperature at which a solid substance undergoes fusion, changing from a solid to a liquid state. Contrast with freezing point.MeniscusIn hydrologic terms, the curved surface of the liquid at the open end of a capillary columnMercury BarometerAn instrument for measuring atmospheric pressure. The instrument contains an evacuated and graduated glass tube in which mercury rises or falls as the pressure of the atmosphere increases or decreases.MeridianAn imaginary line on the earth's surface passing through both geographic poles and through any given point on the planet, also called a line of longitude.Meridional FlowLarge-scale atmospheric flow in which the north-south component (i.e., longitudinal, or along a meridian) is pronounced. The accompanying zonal (east-west) component often is weaker than normal. Compare with zonal flow.MESOMesocyclone- A storm-scale region of rotation, typically around 2-6 miles in diameter and often found in the right rear flank of a
supercell (or often on the eastern, or front, flank of an HP storm). The circulation of a mesocyclone covers an
area much larger than the tornado that may develop within it. Properly used, mesocyclone is a radar term; it is
defined as a rotation signature appearing on Doppler radar that meets specific criteria for magnitude, vertical
depth, and duration. It will appear as a yellow solid circle on the Doppler velocity products. Therefore, a
mesocyclone should not be considered a visually-observable phenomenon (although visual evidence of rotation,
such as curved inflow bands, may imply the presence of a mesocyclone)MesoclimateThe climate of a small area of the earth's surface which may differ from the general climate of the district.Mesocyclone(abbrev. MESO)- A storm-scale region of rotation, typically around 2-6 miles in diameter and often found in the right rear flank of a supercell (or often on the eastern, or front, flank of an HP storm). The circulation of a mesocyclone covers an area much larger than the tornado that may develop within it. Properly used, mesocyclone is a radar term; it is defined as a rotation signature appearing on Doppler radar that meets specific criteria for magnitude, vertical depth, and duration. It will appear as a yellow solid circle on the Doppler velocity products. Therefore, a mesocyclone should not be considered a visually-observable phenomenon (although visual evidence of rotation, such as curved inflow bands, may imply the presence of a mesocyclone).MesohighA relatively small area of high atmospheric pressure that forms beneath a thunderstorm. It is usually associated with an MCS or its remnants. Mesolow(or Sub-synoptic Low) - A mesoscale low-pressure center. Severe weather potential often increases in the area near and just ahead of a mesolow. Mesolow should not be confused with mesocyclone, which is a storm-scale phenomenon.MesonetA regional network of observing stations (usually surface stations) designed to diagnose mesoscale weather features and their associated processes.MesopauseThe top of the mesosphere, corresponding to the level of minimum temperature in the atmosphere found at 70 to 80 km.MesoscaleSize scale referring to weather systems smaller than synoptic-scale systems but larger than
storm-scale systems. Horizontal dimensions generally range from around 50 miles to several hundred miles.
Squall lines, MCCs, and MCSs are examples of mesoscale weather systemsMesoscale Convective Complex(abbrev. MCC)- MCC - Mesoscale Convective Complex. A large Mesoscale Convective System (MCS), generally round or oval-shaped, which normally reaches peak intensity at night. The formal definition includes specific minimum criteria for size, duration, and eccentricity (i.e., "roundness"), based on the cloud shield as seen on infrared satellite photographs:
* Size: Area of cloud top -32 degrees C or less: 100,000 square kilometers or more (slightly smaller than the state of Ohio), and area of cloud top -52 degrees C or less: 50,000 square kilometers or more.
* Duration: Size criteria must be met for at least 6 hours.
* Eccentricity: Minor/major axis at least 0.7.
MCCs typically form during the afternoon and evening in the form of several isolated thunderstorms, during which time the potential for severe weather is greatest. During peak intensity, the primary threat shifts toward heavy rain and flooding.Mesoscale Convective System(MCS): A complex of thunderstorms which becomes organized on
a scale larger than the individual thunderstorms, and normally persists for several hours or more.
MCSs may be round or linear in shape, and include systems such as tropical cyclones, squall
lines, and MCCs (among others). MCS often is used to describe a cluster of thunderstorms that
does not satisfy the size, shape, or duration criteria of an MCC. Mesoscale DiscussionWhen conditions actually begin to shape up for severe weather, SPC (Storm Prediction Center) often issues a Mesoscale Discussion (MCD) statement anywhere from roughly half an hour to several hours before issuing a weather watch. SPC also puts out MCDs for hazardous winter weather events on the mesoscale, such as locally heavy snow, blizzards and freezing rain (see below). MCDs are also issued on occasion for heavy rainfall, convective trends, and other phenomena, when the forecaster feels he/she can provide useful information that is not readily available or apparent to field forecasters. MCDs are based on mesoscale analysis and interpretation of observations and of short term, high resolution numerical model output.
The MCD basically describes what is currently happening, what is expected in the next few hours, the meteorological reasoning for the forecast, and when/where SPC plans to issue the watch (if dealing with severe thunderstorm potential). Severe thunderstorm MCDs can help you get a little extra lead time on the weather and allow you to begin gearing up operations before a watch is issued. The MCD begins with a numerical string that gives the LAT/LON coordinates of a polygon that loosely describes the area being discussed.Mesoscale High WindsThese high winds usually follow the passage of organized convective systems and are associated with wake depressions or strong mesohighs.MesosphereThe atmospheric shell between about 20 km and about 70 to 80 km, extending from the top of the stratosphere (the stratopause) to the upper temperature minimum that defines the mesopause (the base of the thermosphere).MET1) Meteorological
2) MOS (Model Output Statistics) guidance based on Environmental Modeling Centers ETA modelMETARAn international code (Aviation Routine Weather Report) used for reporting, recording and transmitting weather observations.MeteogramA graphical depiction of trends in meteorological variables such as temperature, dew point, wind speed and direction, pressure, etc. The time series meteogram can be constructed using observed data or forecast data.Meteoric WaterWater derived from precipitation. Meteorological Model Ensemble River Forecast (MMEFS)An automated short-term hydrologic ensemble forecast system which utilizes temperature and precipitation output from the National Centers for Environmental Prediction (NCEP) GEFS and NAEFS meteorological models as inputs to River Forecast Center hydrologic models.MeteorologistA person who studies meteorology. There are many different paths within the field of meteorology. For example, one could be a research meteorologist, radar meteorologist, climatologist, or operational meteorologist. MeteorologyThe science dealing with the atmosphere and its phenomena. A distinction can be drawn between meteorology and climatology, the latter being primarily concerned with average, not actual, weather conditions.METROMetropolitanMeVMega (million) electronvolt. A unit of energy used to describe the
total energy carried by a particle or photonMEX1) GFS Model Guidance
2) MexicoMicroclimateThe climate of a small area such as a cave, house, city or valley that may be different from that in the general region.MicroscalePertaining to meteorological phenomena, such as wind circulations or cloud patterns, that are less than 2 km in horizontal extent.MicrowaveA type of electromagnetic radiation with wavelengths between those of infrared radiation and radio waves.Microwave BurstIn solar-terrestrial terms, a radiowave signal associated with optical and/or X-ray
flaresMid-Flame WindWind measured at the midpoint of the flames, considered to be most representative of the speed of the wind that is affecting fire behavior.Mid-Latitude AreasAreas between 30o and 60o north and south of the Equator.Mid-level CoolingLocal cooling of the air in middle levels of the atmosphere (roughly 8 to 25 thousand feet), which can lead to destabilization of the entire atmosphere if all other factors are equal.Middle Clouds(or Mid-Level Clouds) - A term used to signify clouds with bases between 6,500 and 23,000 feet. At the higher altitudes, they may also have some ice crystals, but they are composed mainly of water droplets. Altocumulus, altostratus, and nimbostratus are the main types of middle clouds. This altitude applies to the temperate zone. In the polar regions, these clouds may be found at lower altitudes. In the tropics, the defining altitudes for cloud types are generally higher. Middle Latitudes1) The latitude belt roughly between 35 and 65 degrees North and South. Also referred to as the temperate region.
or
2) With specific reference to zones of geomagnetic activity, "middle latitudes" refers to 20º to 50º geomagneticMie ScatteringAny scattering produced by spherical particles whose diameters are greater than
1/10 the wavelength of the scattered radiation. This type of scattering causes the clouds to
appear white in the sky. Often, hail exhibits in this type of scattering.Minimum Discernible SignalIn a receiver, it is the smallest input signal that will a produce a detectable signal at the output. In radar terms, it is the minimal amount of back scattered energy that is required to produce a target on the radar screen. In other words, MDS is a measure of the radar's sensitivity. Minimum TemperatureThis is the lowest temperature recorded during a specified period of time. The time period can be 6, 12 or 24 hours. The most common reference is to the daily minimum temperature, or "low."Minor Tidal OverflowMinor flooding caused by high tides that results in little if any damage. MisoscaleThe scale of meteorological phenomena that ranges in size from 40 meters to about 4 kilometers. It includes rotation within a thunderstorm.Mixed LayerAn atmospheric layer, usually the layer immediately above the ground, in which pollutants are well mixed by convective or shear-produced turbulence.Mixed PrecipitationAny of the following combinations of freezing and frozen precipitation: snow and sleet, snow and freezing rain, or sleet alone. Rain may also be present.Mixing DepthVertical distance between the ground and the altitude to which pollutants are mixed by turbulence caused by convective currents or vertical shear in the horizontal wind.Mixing HeightsThe height to which a parcel of air, or a column of smoke, will rise, mix or disperse. A column of smoke will remain trapped below this height.MLCAPEMean Layer CAPE - CAPE calculated using a parcel consisting of Mean Layer values of temperature and moisture from the lowest 100 mb above ground level. See Convective Available Potential Energy (CAPE).Model Output Statistics(abbrev. MOS) - the Hydrometeorological Center (HPC) produces a short range (6 to 60 hours) MOS (Model Output Statistics) guidance package generated from the NGM, GFS, and ETA models for over 300 individual stations in the continental United States. These alphanumeric messages are made available at approximately 0400 and 1600 UTC for the 0000 and 1200 UTC forecast cycles, respectively. Model Output Statistics are a set of statistical equations that use model output to forecast the probability of precipitation, high and low temperature, cloud cover, and precipitation amount for many cities across the USA. The statistical equations were specifically tailored for each location, taking into account factors such as each location's climate.Moderate FloodingThe inundation of secondary roads; transfer to higher elevation necessary to save property -- some evacuation may be required.Moderate Risk (of severe thunderstorms)Severe thunderstorms are expected to affect between 5 and
10 percent of the area. A moderate risk indicates the possibility of a significant severe weather episode. See
high risk, slight risk, convective outlook. Moist Adiabatic Lapse Rate(abbrev. MALR)- The rate at which the temperature of a parcel of saturated air decreases as the parcel is lifted in the atmosphere. The moist adiabatic lapse rate (abbreviated MALR) is not a constant like the dry adiabatic lapse rate but is dependent on parcel temperature and pressure.MoistureRefers to the water vapor content in the atmosphere, or the total water, liquid, solid or vapor, in a given volume of air.Moisture AdvectionTransport of moisture by horizontal winds.Moisture ConvergenceA measure of the degree to which moist air is converging into a given area, taking into account the effect of converging winds and moisture advection. Areas of persistent moisture convergence are favored regions for thunderstorm development, if other factors (e.g., instability) are favorable.Moisture EquivalentIn hydrologic terms, the ratio of the weight of water which the soil, after saturation, will retain against a centrifugal force 1,000 times the force of gravity, to the weight of the soil when dry. The ratio is stated as a percentage.Moisture RidgeAn axis of relatively high dew point values. This axis is sometimes referred to as a 'moist tongue'.MoleculeThe smallest particle of a substance that retains the properties of the substance and is composed of one or more atoms.Monitor StageThe stage which, when reached by a rising stream, represents the level where appropriate officials (e.g., county sheriff, civil defense officials, or bypass gate operators) are notified of the threat of possible flooding. (Used if different from action stage, and at the discretion of the WFO or river forecast center [RFC].) The term "alert stage" is to be used instead of warning stage. Monitor stage or caution stage may be used instead of alert stage in some parts of the country. see/ alert stage/.Monthly Climatological ReportThis climatological product is issued once a month by each
National Weather Service office. It is a mix of tabular and narrative information. It is organized so
that similar items are grouped together (i.e., temperature, precipitation, wind, heating/cooling
degree information, etc.). Mostly ClearWhen the 1/8th to 2/8ths of the sky is covered by with opaque (not transparent) clouds. Sometimes referred to as Mostly Sunny if this condition is present during daylight hours.Mount Wilson Magnetic ClassificationsIn solar-terrestrial terms, a classification system for sunspots:
- Alpha: Denotes a unipolar sunspot group.
- Beta: A sunspot group having both positive and negative magnetic polarities, with a simple and distinct division between the polarities.
- Beta-Gamma: A sunspot group that is bipolar but in which no continuous line can be drawn separating spots of opposite polarities.
- Delta: A complex magnetic configuration of a solar sunspot group consisting of opposite polarity umbrae within the same penumbra.
- Gamma: A complex active region in which the positive and negative polarities are so irregularly distributed as to prevent classification as a bipolar group.
Mountain WaveThe wavelike effect, characterized by updrafts and downdrafts, that occurs above and behind a mountain range when rapidly flowing air encounters the mountain range's steep front.Mountain Wind SystemThe system of diurnal winds that forms in a complex terrain area, consisting of mountain-plain, along-valley, cross-valley and slope wind systems.Mountain-Plain Wind SystemA closed, large-scale, thermally driven circulation between the mountains and the surrounding plain. The mountain-to-plain flow making up the lower branch of the closed circulation usually occurs during nighttime, while the plain-to-mountain flow occurs during daytime.Movable BedIn hydrologic terms, a stream bed made up of materials readily transportable by the stream flowMud SlideFast moving soil, rocks and water that flow down mountain slopes and canyons during a heavy downpour of rain.Multi-Radar Multi-Sensor (MRMS)An automated system that rapidly and intelligently integrates data from multiple radars and radar networks, surface observations, numerical weather prediction (NWP) models, and climatology to generate seamless, high spatio-temporal resolution mosaics.Multicell ThunderstormThese thunderstorms are organized in clusters of at least 2-4 short-lived cells. Each cell generates a cold air outflow and these individual outflows combine to form a large gust front. Convergence along the gust front causes new cells to develop every 5 to 15 minutes. The cells move roughly with the mean wind. However, the area (storm) motion usually deviates significantly from the mean wind due to discrete propagation (new cell development) along the gust front. The multicellular nature of the storm is usually apparent on radar with multiple reflectivity cores and maximum tops.Multiple Doppler AnalysisThe use of more than one radar (and hence more than one look angle) to reconstruct spatial distributions of the 2D or 3D wind field, which cannot be measured from a single radar alone. Includes dual Doppler, triple Doppler, and overdetermined multiple Doppler analysis.Multiple Vortex TornadoA tornado in which two or more condensation funnels or debris clouds are present at the same time, often rotating about a common center or about each other. Multiple-vortex tornadoes can be especially damaging.Multipurpose ReservoirIn hydrologic terms, a reservoir constructed and equipped to provide storage and release of water for two or more purposes such as flood control,
power development, navigation, irrigation, recreation, pollution abatement, domestic water supply, etc.Municipal Use of WaterIn hydrologic terms, the various uses to which water is put to use developed urban areas, including domestic use, industrial use, street sprinkling, fire
protection, etc.Nacreous CloudsClouds of unknown composition that have a soft, pearly luster and that form at altitudes about 25 to 30 km above the Earth's surface. They are also called "mother-of-the-pearl clouds."Nanotesla (nT) A unit of magnetism equal to 10-9 tesla, equivalent to a gamma (10-5 gauss).NAO IndexThis index measures the anomalies in sea level pressure between the Icelandic low pressure system and the Azores high pressure system in the North Atlantic Ocean.
When the NAO is in its is positive phase (+NAO), the northeastern United States sees an increase in temperature and a decrease in snow days; the central US has increased precipitation, the North Sea has an increase in storms; and Norway along with Northern Europe has warmer temperatures and increased precipitation.
When the NAO is in its negative phase (-NAO), the Tropical Atlantic and Gulf coast have increased number of strong hurricanes; northern Europe is drier, and Turkey along with other Mediterranean countries has increased precipitation.National Ambient Air Quality StandardsIn the United States, national standards for the ambient concentrations in air of different air pollutants designed to protect human health and welfare.National Blend of Models (NBM)The National Blend of Models (NBM) is a nationally consistent and skillful suite of calibrated forecast guidance based on a blend of both NWS and non-NWS numerical weather prediction model data and post-processed model guidance.National Climatic Data CenterThe agency that archives climatic data from the National Oceanic and Atmospheric Administration, as well as other climatological organizations.National Digital Forecast Database(NDFD)- The National Weather Service's NDFD provides access to gridded forecasts of sensible weather elements (e.g., wind, wave height) through the National Digital Forecast Database (NDFD). NDFD contains a seamless mosaic of digital forecasts from NWS field offices working in collaboration with the National Centers for Environmental Prediction (NCEP). The database is made available to all customers and partners from the public, private and academic sectors. Those customers and partners may use this data to create a wide range of text, graphic, gridded and image products of their own. National Fire Danger Rating SystemA uniform fire danger rating system used in the United States that focuses on the environmental factors that impact the moisture content of fuels. Fire danger is rated daily over large administrative areas, such as national forests.National Hurricane Center One of three branches of the Tropical Prediction Center (TPC). This center maintains a continuous watch on tropical cyclones over the Atlantic, Caribbean, Gulf of Mexico, and the Eastern Pacific from 15 May through November 30. The Center prepares and distributes hurricane watches and warnings for the general public, and also prepares and distributes marine and military advisories for other users. During the "off-season" NHC provides training for U.S. emergency managers and representatives from many other countries that are affected by tropical cyclones. NHC also conducts applied research to evaluate and improve hurricane forecasting techniques, and is involved in public awareness programs.National Hurricane Operations Plan(NHOP) - The NHOP is issued annually by the Federal Coordinator for Meteorological Services and Supporting Research. It documents interdepartmental agreements relating to tropical cyclone observing, warning, and forecasting services. National Hurricane Center (NHC), Central Pacific Hurricane Center (CPHC), and the JTWC serve as the principal offices in coordinating the day-to-day activities of the NWS in support of the Plan in their region of responsibility. National Severe Storms LaboratoryThis is one of NOAA's internationally known Environmental Research Laboratories, leading the way in investigations of all aspects of severe weather. Headquartered in Norman OK with staff in Colorado, Nevada, Washington, Utah, and Wisconsin, the people of NSSL, in partnership with the National Weather Service, are dedicated to improving severe weather warnings and forecasts in order to save lives and reduce property damage.National Water Model (NWM)A hydrologic modelling framework that simulates observed and forecast streamflow over the entire continental United States and Hawaii.National Water Model Medium-Range Forecast (NWM MRF)A 10-day streamflow forecast for the over 3.6 million waterway miles across the Nation, forced by the GFS and updated every 6 hours.National Water Model Short-Range Forecast (NWM SRF)An 18-hour streamflow forecast for the over 3.4 million waterway miles across the Nation, forced by the HRRR and updated hourly.National Weather and Crop SummaryA product of the National Agricultural Statistics Service,
Agricultural Statistics Board, and U.S. Department of Agriculture. It contains weekly national
agricultural weather summaries, including the weather's effect on crops; summaries and farm
progress for 44 states and New England area.Nautical MileA unit of distance used in marine navigation and marine forecasts. It is equal to 1.15 statue miles or 1852 meters. It is also the length of 1 minute of latitude.Navigation MethodsIn hydrologic terms, there are three basic methods of providing and managing inland waterways - 1) Run-of-the-River: no provision of upstream storage; 2)
Slack-Water: locks and dams provide slack water or pools with adequate depth for the draft of heavy barges and area to prevent
excessive velocities; 3) Canalization: in lieu of a series of dams on the river a canal with locks adjoins the river.NAVTEXAn international automated medium frequency (518 kHz) direct-printing service for delivery of navigational and meteorological warnings and forecasts, as well as urgent marine safety information to ships. It was developed to provide a low-cost, simple, and automated means of receiving this information aboard ships at sea within approximately 200 nautical miles of shore and is an element of the Global Maritime Distress and Safety System (GMDSS). NAVTEX stations in the U.S. are operated by the U.S. Coast Guard. There are no user fees associated with receiving NAVTEX broadcasts. The International System uses the frequency of 518 kHz worldwide and optional National Systems use frequencies of 490 and 4205.5 kHz. Safety messages on 518 kHz have to be in the English language worldwide but broadcasts on the 490 and 4209.5 kHz frequency can be in the local language. NAVTEX Forecast(NAV) - A National Weather Service marine forecast combining various Coastal Waters and Offshore forecasts, optimized to accommodate transmission via NAVTEX. NCEPNational Centers for Environmental Prediction. A part of the National Weather Service which provides nationwide computerized and manual guidance to
Warning and Forecast Offices concerning the forecast of basic weather elements.NENortheastNeap TideA minimum tide occurring at the first and third quarters of the moon.Nearshore Forecast(NSH) - National Weather Service seasonal marine forecasts for an areas of the Great Lakes extending from a line approximating mean low water datum along the coast or an island, including bays, harbors, and sounds, out to 5 nm. These forecasts are normally issued from Daylight Savings Time ~April 7 through December 31, though the dates may be shortened or extended based on local/regional requirements. NECNecessaryNEGNegativeNegative Vorticity Advection(Abbrev. NVA) - the advection of lower values of vorticity into an area.Negative-tilt TroughAn upper level system which is tilted to the west with increasing latitude (i.e., with an axis from southeast to northwest). A negative-tilt trough often is a sign of a developing or intensifying system.NELYNortheasterlyNERNNortheasternNESDISNational Environmental Satellite, Data, and Information Service. NESDIS collects, processes, stores, analyzes, and disseminates various types of hydrologic, meteorologic, and oceanic data.
NESDIS is also responsible for the development of analytical and descriptive products so as to meet the needs of it’s users. Net All-Wave RadiationThe net or resultant value of the upward and downward longwave and shortwave radiative fluxes through a plane at the earth-atmosphere interface; a component of the surface energy budget.Net RainfallIn hydrologic terms, the portion of rainfall which reaches a stream channel or the concentration point as direct surface flow.Neutral LineThe line that separates longitudinal magnetic fields of opposite polarity.Neutral StabilityAn atmospheric condition that exists in unsaturated air when the environmental lapse rate equals the dry adiabatic rate, or in saturated air when the environmental lapse rate equals the moist adiabatic rate.NEWDNortheastwardNEXRADNEXt Generation RADar. A NWS network of about 140 Doppler radars operating nationwide.Nieve PenitenteA spike or pillar of compacted snow, firn or glacier ice, caused by differential melting and evaporation. The pillars form most frequently on low-latitude mountains where air temperatures are near freezing, dew points are much below freezing and insolation is strong. Penitents are oriented individually toward the noon-day sun, and usually occur in east-west lines.NOAA Weather Radio"The voice of the National Weather Service" - NOAA Weather Radio broadcasts National Weather Service warnings, watches, forecasts and other hazard information 24 hours a day. It is provided as a public service by NOAA. The NOAA Weather Radio network has more than 480 stations in the 50 states and near adjacent coastal waters, Puerto Rico, the U.S. Virgin Islands and U.S. Pacific Territories.NOAA Weather WireMass dissemination via satellite of National Weather Service products to the media and public.Noctilucent CloudsWavy, thin, bluish-white clouds that are best seen at twilight in polar latitudes. They form at altitudes about 80 to 90 km above the Earth's surface.Nocturnal InversionUsed interchangably with Radiational Inversion; a temperature inversion that develops during the night as a result of radiational cooling of the surface. Because the immediate surface (lower Boundary Layer) cools much more rapidly during radiational cooling conditions than the air just above (upper Boundary Layer), a temperature inversion can be created overnight, but typically erodes quickly after sunrise.Nocturnal JetThis wind speed maximum occurs just above the nocturnal inversion at night. It is typically found in the south central United States during the late spring and summer months. It is important in the development of Mesoscale Convective Complexes (MCC) or Mesoscale Convective Systems (MCS).Nocturnal ThunderstormsThunderstorms which develop after sunset. They are often associated with the strengthening of the low level jet and are most common over the Plains states. They also occur over warm water and may be associated with the seaward extent of the overnight land breeze.Nonattainment AreaAn area that does not meet ambient air quality standards.Normal Water Surface ElevationIn hydrologic terms, the lowest crest level of overflow on a reservoir with a fixed overflow level (spillway crest elevation). For a reservoir whose
outflow is controlled wholly or partly by movable gates, siphons, or other means, it is the maximum level to which water may rise
under normal operating conditions, exclusive of any provision for flood surcharge.Normal YearA year during which the precipitation or stream flow approximates the average for a long period of record.North American Ensemble Forecast System (NAEFS)An atmospheric ensemble of 20 members each from the NCEP GEFS and CMC EPS ensemble systems.North American Mesoscale Forecast System (NAM)One of the major weather models run by the National Centers for Environmental Prediction (NCEP) for producing weather forecasts.Northern LightsCommon name for Aurora Borealis; the luminous, radiant emission from the upper atmosphere over middle and high latitudes, and centred around the earth's magnetic poles. These silent fireworks are often seen on clear winter nights in a variety of shapes and colours. Nor\'easterA strong low pressure system that affects the Mid Atlantic and New England States. It can form over land or over the coastal waters. These winter weather events are notorious for producing heavy snow, rain, and tremendous waves that crash onto Atlantic beaches, often causing beach erosion and structural damage. Wind gusts associated with these storms can exceed hurricane force in intensity. A nor'easter gets its name from the continuously strong northeasterly winds blowing in from the ocean ahead of the storm and over the coastal areas.Numerical ForecastingA computer forecast or prediction based on equations governing the motions and the forces affecting motion of fluids. The equations are based, or initialized, on specified weather or climate conditions at a certain place and time.Numerical Weather PredictionSame as Numerical Forecasting; a computer forecast or prediction based on equations governing the motions and the forces affecting motion of fluids. The equations are based, or initialized, on specified weather or climate conditions at a certain place and time. NumerousA National Weather Service convective precipitation descriptor for a 60 or 70 percent chance of measurable precipitation (0.01 inch).Obscuring PhenomenaAny atmospheric phenomenon, except clouds, that restricts vertical visibility (e.g., dust, rain, snow, etc.).Observation WellIn hydrologic terms, a non-pumping well used for observing the elevation of the water table or piezometric surfaceOccluded FrontA composite of two fronts, formed as a cold front overtakes a warm or quasi-stationary front. Two types of occlusions can form depending on the relative coldness of the air behind the cold front to the air ahead of the warm or stationary front. A cold occlusion results when the coldest air is behind the cold front and a warm occlusion results when the coldest air is ahead of the warm front.Occluded MesocycloneA mesocyclone in which air from the rear-flank downdraft has completely enveloped the circulation at low levels, cutting off the inflow of warm unstable low-level air.OceanographyThe study of the ocean, embracing and integrating all knowledge pertaining to the ocean's physical boundaries, the chemistry and physics of sea water, and marine biology.OEODMThe Office of Equal Opportunity and Diversity Management. Provides a duel role. Advises and assists the Assistant Administrator in carrying out the National Weather Service's (NWS) responsibilities relative to Civil Rights laws, Executive Orders, regulatory guidelines, and other nondiscrimination laws within the Federal Government. Advises and assists the Assistant Administrator in carrying out the NWS policy of diversity management by fostering an inclusive workforce, building an environment that respects the individual and offering opportunities for all employees to develop to their full potential. Cultural Diversity is the mixture of differences and similarities each employee brings to the workplace to accomplish the goals of the NWS.Office of Global ProgramsThe Office of Global Programs (OGP) sponsors focused scientific research, within approximately eleven research elements, aimed at understanding climate variability and its predictability. Through studies in these areas, researchers coordinate activities that jointly contribute to improved predictions and assessments of climate variability over a continuum of timescales from season to season, year to year, and over the course of a decade and beyond.Offshore BreezeA wind that blows from the land towards a body of water. Also known as a land breeze.Offshore FlowOccurs when air moves from land to sea, and is usually associated with dry weather.Offshore WatersThat portion of the oceans, gulfs, and seas beyond the coastal waters extending to a specified distance from the coastline, to a specified depth contour, or covering an area defined by specific latitude and longitude points. Offshore Waters Forecast(OFF) - A National Weather Service marine forecast product for that portion of the oceans, gulfs, and seas beyond the coastal waters extending to a specified distance from the coastline, to a specified depth contour, or covering an area defined by specific latitude and longitude points. OmegaA term used to describe vertical motion in the atmosphere. The "omega equation" used in numerical weather models is composed of two terms, the "differential vorticity advection" term and the "thickness advection" term. Put more simply, omega is determined by the amount of spin (or large scale rotation) and warm (or cold) advection present in the atmosphere. On a weather forecast chart, high values of omega (or a strong omega field) relate to upward vertical motion (UVV) in the atmosphere. If this upward vertical motion is strong enough and in a sufficiently moist airmass, precipitation results.Omega HighA warm high aloft which has become displaced and is on the polarward side of the jet stream. It frequently occurs in the late winter and early spring in the Northern Hemisphere. The name comes from its resemblance to the Greek letter, Omega, when analyzed on upper air charts. It is an example of a blocking high.Onshore BreezeA wind that blows from a body of water towards the land. Also known as a seabreezeOnshore FlowOccurs when air moves from sea to land, and is usually associated wtih increased moisture.OpaqueA condition where a material, such as a cloud, blocks the passage of radiant energy, especially light. Opaque sky cover refers to the amount of sky cover that completely hides all that might be above it.Open Lakes Forecast(GLF) - A National Weather Service marine forecast product for the U.S. waters within a Great Lake not including the waters covered by an existing Nearshore Waters Forecast (NSH). When the seasonal Nearshore forecast is not issued, the Open Lake forecast includes a forecast of nearshore waters. Operational ProductsA product that has been fully tested and evaluated and is produced on a regular and ongoing basis.OrificeIn hydrologic terms, (1) An opening with closed perimeter, usually sharp edged, and of regular form in a plate, wall, or partition through which water
may flow, generally used for the purpose of measurement or control of water. (2) The end of a small tube, such as a Pitot tube,
piezometer, etc. Orographic PrecipitationPrecipitation which is caused by hills or mountain ranges deflecting the moisture-laden air masses upward, causing them to cool and precipitate their moisture.Orographic WavesA wavelike airflow produced over and in the lee of a mountain barrier.Outer Convective BandBands in a hurricane that occur in advance of main rain shield and up to 300 miles from the eye of the hurricane. The typical hurricane has two or three bands (and sometimes more) which are comprised of cells resembling ordinary thunderstorms. Wind gusts are usually higher in these bands than in the Pre-Hurricane Squall Line.Outflow ChannelIn hydrologic terms, a natural stream channel which transports reservoir releases.Outgoing Longwave RadiationOutgoing Longwave Radiation is a polar satellite derived measurement of the radiative character of energy radiated from the warmer earth surface to cooler space. This measurement provides information on cloud-top temperature which can be used to estimate tropical precipitation amounts which is important in forecasting weather and climate.OutletIn hydrologic terms, an opening through which water can be freely discharged from a reservoir.Outlet Discharge StructureIn hydrologic terms, protects the downstream end of the outlet pipe from erosion and is often designed to slow down the velocity of released water to
prevent erosion of the stream channelOvercast(Abbrev. OVC)- An official sky cover classification for aviation weather observations, when the sky is completely covered by an obscuring phenomenon. This is applied only when obscuring phenomenon aloft are present--that is, not when obscuring phenomenon are surface-based, such as fog.OverhangRadar term indicating a region of high reflectivity at middle and upper levels above an area of
weak reflectivity at low levels. (The latter area is known as a weak-echo region, or WER.) The overhang is
found on the inflow side of a thunderstorm (normally the south or southeast side).Overland FlowIn hydrologic terms, the flow of rainwater or snowmelt over the land surface toward stream channels. After it enters a watercourse it becomes runoff.OverrunningA weather pattern in which a relatively warm air mass is in motion above another air mass of greater density at the surface. Embedded thunderstorms sometimes develop in such a pattern; severe thunderstorms (mainly with large hail) can occur, but tornadoes are unlikely.
Overrunning often is applied to the case of warm air riding up over a retreating layer of colder air, as along the sloping surface of a warm front. Such use of the term technically is incorrect, but in general it refers to a pattern characterized by widespread clouds and steady precipitation on the cool side of a front or other boundary.Overshooting Top(or Penetrating Top) - A dome-like protrusion above a thunderstorm anvil, representing a very strong updraft and hence a higher potential for severe weather with that storm. A persistent and/or large overshooting top (anvil dome) often is present on a supercell.
A short-lived overshooting top, or one that forms and dissipates in cycles, may indicate the presence of a pulse storm.OzoneA form of oxygen, O3. A powerful oxidizing agent that is considered a pollutant in the lower troposphere but an essential chemical in the stratosphere where it protects the earth from high-energy ultraviolet radiation from the sun.Ozone Action DayA "heads-up" message issued by the Department of Natural Resources (DNR) through the National Weather Service when ozone levels may reach dangerous levels the next day. This message encourages residents to prevent air pollution by postponing the use of lawn mowing, motor vehicles, boats, as well as filling their vehicle gas tanks.Ozone AdvisoryIt is issued by the Department of Natural Resources (DNR) through the National Weather Service when ozone levels reach 100. Ozone levels above 100 are unhealthy for people with heat and/or respiratory ailments.Ozone HoleA severe depletion of stratospheric ozone over Antarctica that occurs each spring. The possibility exists that a hole could form over the Arctic as well. The depletion is caused by a chemical reaction involving ozone and chlorine, primarily from human produced sources, cloud particles, and low temperatures.Ozone LayerAn atmospheric layer that contains a high proportion of oxygen that exists as ozone. It acts as a filtering mechanism against incoming ultraviolet radiation. It is located between the troposphere and the stratosphere, around 9.5 to 12.5 miles (15 to 20 kilometers) above the earth's surface.Pacific Decadal Oscillation(Abbrev. PDO) - a recently described pattern of climate variation similar to ENSO though on a timescale of decades and not seasons. It is characterized by SST anomalies of one sign in the north-central Pacific and SST anomalies of another sign to the north and east near the Aleutians and the Gulf of Alaska. It primarily affects weather patterns and sea surface temperatures in the Pacific Northwest, Alaska, and northern Pacific Islands.Palmer Drought Severity Index(Abbrev. PDSI) - an index used to gage the severity of drought conditions by using a water balance equation to track water supply and demand. This index is calculated weekly by the National Weather Service.Pancake IceIn hydrologic terms, circular flat pieces of ice with a raised rim; the shape and rim are due to repeated collisionsPanhandle HookLow pressure systems that originate in the panhandle region of Texas and Oklahoma which initially move east and then "hook" or recurve more northeast toward the upper Midwest or Great Lakes region. In winter, these systems usually deposit heavy snows north of their surface track. Thunderstorms may be found south of the track. ParameterA subset of the group of evaluations that constitute each element of an observation.Parapet WallIn hydrologic terms, a solid wall built along the top of the dam for ornament, safety, or to prevent overtoppingParcelA volume of air small enough to contain uniform distribution of its meteorological properties and large enough to remain relatively self-contained and respond to all meteorological processes.ParhelionThe scientific name for sun dogs. Either of two colored luminous spots that appear at roughly 22 degrees on both sides of the sun at the same elevation. They are caused by the refraction of sunlight passing through ice crystals. They are most commonly seen during winter in the middle latitudes and are exclusively associated with cirriform clouds. They are also known as mock suns. Partial Beam FillingA limitation of the rainfall estimation techniques used by NEXRAD. At far ranges from the radar, a storm may occupy only a portion of the radar beam (which may be several miles across). However, the radiation received by the radar antenna consists of the average reflectivity across the entire beam, so the reflectivity and associated rainfall rates are underestimated.Partial-Duration Flood SeriesIn hydrologic terms, a list of all flood peaks that exceed a chosen base stage or discharge, regardless of the number of peaks occurring in a year.Particle Trajectory ModelA computer sub-model that tracks the trajectories of multiple particles that are released into an atmospheric flow model.Peak DischargeIn hydrologic terms, the rate of discharge of a volume of water passing a given locationPeak Flow Arrival Time (PFAT)The time at which a stream reach is forecast to achieve its maximum flow within the forecast period.Peak GustThe highest instantaneous wind speed observed or recorded.Peak PulseThe amount of power transmitted by a radar during a given pulse. Note that because these pulses are widely spaced, the average power will be much smaller.Peak Wind SpeedThe maximum instantaneous wind speed since the last observation that exceeded 25 knots.Pendant EchoRadar signature generally similar to a hook echo, except that the hook shape is not as well defined.Penetrating TopSame as Overshooting Top; a dome-like protrusion above a thunderstorm anvil, representing a very strong updraft and hence a higher potential for severe weather with that storm. A persistent and/or large overshooting top (anvil dome) often is present on a supercell.
A short-lived overshooting top, or one that forms and dissipates in cycles, may indicate the presence of a pulse storm. PenumbraIn solar-terrestrial terms, the sunspot area that may surround the darker umbra or umbrae. It consists of linear bright and dark elements radial from the sunspot umbra.Perched GroundwaterIn hydrologic terms, local saturated zones above the water table which exist above an impervious layer of limited extent.PercolationIn hydrologic terms, the movement of water, under hydrostatic pressure, through the interstices of a rock or soil, except the movement through large
openings such as cavesPercolation PathIn hydrologic terms, the course followed by water moving or percolating through any other permeable material, or under a dam which rests upon a
permeable foundation.Perennial StreamIn hydrologic terms, a stream that flows all year round.PerigeeThe closest distance between moon and earth or the earth and sun.PerihelionThe point on the annual orbit of a body (about the sun) that is closest to the sun; at present, the earth reaches this point on about 5 January. Opposite of aphelion.PermafrostA layer of soil at varying depths below the surface in which the temperature has remained below freezing continuously from a few to several thousands of years.PermeabilityIn hydrologic terms, the ability of a material to transmit fluid through its pores when subjected to a difference in head.Permeability CoefficientIn hydrologic terms, the rate of flow of a fluid through a cross section of a porous mass under a unit hydraulic gradient, at a temperature of 60 degrees
Fahrenheit.PermeameterIn hydrologic terms, a laboratory instrument for determining permeability by measuring the discharge through a sample of the material when a known
hydraulic head is applied.PersistenceContinuation of existing conditions. When a physical parameter
varies slowly, the best prediction is often persistencePersistence ForecastA forecast that the current weather condition will persist and that future weather will be the same as the present (e.g., if it is raining today, a forecast predicting rain tonight).Perturbation ModelA computer model used to calculate air pollution concentrations. A perturbation model produces a wind field from solutions to a simplified set of equations that describe atmospheric motions.Pervious ZoneIn hydrologic terms, a part of the cross section of an embankment dam comprising material of high permeabilityPhenomenological ModelA computer model used to calculate air pollution concentrations. A phenomenological model focuses on an individual phenomenon, such as plume impingement or fumigation.PhiezometerIn hydrologic terms, an instrument used to measure pressure head in a conduit, tank, soil, etc. They are used in dams to measure the level of saturation.Photochemical SmogAir pollution containing ozone and other reactive chemical compounds formed by the reaction of nitrogen oxides and hydrocarbons in the presence of sunlight.PhotosphereThe intensely bright portion of the sun visible to the unaided eye; the "surface" of the sun. Reaching temperatures estimated at about 11,000 degrees Fahrenheit, it is the portion of the sun's atmosphere which emits continuous electromagnetic radiation.Phreatic waterIn hydrologic terms, water within the earth that supplies wells and springs; water in the zone of saturation where all openings in rocks and soil are filled,
the upper surface of which forms the water table. Also termed Groundwater.Pilot Report(Abbrev. PIREP)- A report of inflight weather by an aircraft pilot or crew member. A complete coded report includes the following information in this order: location and/or extent of reported weather phenomenon: type of aircraft (only with reports turbulence or icing).PIREPPilot Report. A report of inflight weather by an aircraft pilot or crew member. A complete coded report includes the following information in this order: location and/or extent of reported weather phenomenon: type of aircraft (only with reports turbulence or icing).Pitot TubeIn hydrologic terms, a device for measuring the velocity of flowing water using the velocity head of the stream as an index of velocity. It consists
essentially of an orifice held to a point upstream in the water, connected with a tube in which the rise of water due to velocity head
may be observed and measured. It also may be constructed with an upstream and downstream orifice, with two water columns, in
which case the difference in height of water column in the tubes is the index of velocity. PlageIn solar-terrestrial terms, an extended emission feature of an active region that exists from the emergence of the first magnetic flux until the widely scattered remnant magnetic fields merge with the background.Plage CorridorIn solar-terrestrial terms, a space in chromospheric plage lacking plage intensity, coinciding with polarity inversion line.PlagenilIn solar-terrestrial terms, spotless disc free of calcium plage.Planetary Boundary LayerThe layer within the atmosphere between 1 km and the earth's surface where friction affects wind speed and wind direction.Plume BlightVisibility impairment caused by air pollution plumes aggregated from individual sources.Plume ImpingementThe collision of a plume with topography that rises above the plume altitude; often a temporary condition that occurs as the plume sweeps by the face of a hill as the wind shifts.Plume-dominated FireA fire whose behavior is governed primarily by the local wind circulation produced in response to the strong convection above the fire rather than by the general wind.Point PrecipitationPrecipitation at a particular site, in contrast to the mean precipitation over an area.Point SourceA pollutant source that can be treated in a dispersion model as though pollutants were emitted from a single point that is fixed in space. Example: the mouth of a smokestack. Compare area source and line source.Polar JetMarked by a concentration of isotherms and strong vertical shear, this jet is the boundary between the polar air and the subtropical air. It often divides into two branches, the north and the south, and marks the high speed core of the prevailing westerlies. It is associated with the location and motion of the high and low pressure areas of the middle latitudes, and therefore, is variable in position, elevation, and wind speed. Its position tends to migrate south in the Northern Hemispheric winter and north in the summer, and its core winds increase during the winter and become less strong in the summer.Polar Jet StreamUsed interchangably with Polar Jet; a jet stream marked by a concentration of isotherms and strong vertical shear, this jet is the boundary between the polar air and the subtropical air. It often divides into two branches, the north and the south, and marks the high speed core of the prevailing westerlies. It is associated with the location and motion of the high and low pressure areas of the middle latitudes, and therefore, is variable in position, elevation, and wind speed. Its position tends to migrate south in the Northern Hemispheric winter and north in the summer, and its core winds increase during the winter and become less strong in the summer.Polar Orbiting SatelliteA weather satellite which travels over both poles each time it orbits the Earth. It orbits about 530 miles (850 km) above the Earth's surface. A satellite with an orbit nearly parallel to the earth's meridian lines which crosses the polar regions on each orbit.PondageIn hydrologic terms, (1) The holding back of water for later release for power development above the dam of a hydroelectric plant to (a) equalize daily
or weekly fluctuations of streamflow or (b) to permit irregular hourly use of water by the wheels to care for fluctuations in the load
demand. (2) In general the holding back of water for later releases. (3) The storage capacity available for the use of such water.Pool HeightIn hydrologic terms, the height of the water behind a dam. (Various datums may be used and various pool height may be used, e.g., conservation pool,
flood control pool, etc.) Popcorn ConvectionSlang for showers and thunderstorms that form on a scattered basis with little or no apparent organization, usually during the afternoon in response to diurnal heating. Individual thunderstorms typically are of the type sometimes referred to as air-mass thunderstorms: they are small, short-lived, very rarely severe, and they almost always dissipate near or just after sunset.Positive AreaThe area on a sounding representing the layer in which a lifted parcel would be warmer than the environment; thus, the area between the environmental temperature profile and the path of the lifted parcel. Positive area is a measure of the energy available for convection; see CAPE.Positive Cloud to Ground LightningA CG flash that delivers positive charge to the ground, as opposed to the more common negative charge. Positive CGs have been found to occur more frequently in some severe thunderstorms. Their occurrence is detectable by most lightning detection networks, but visually it is not considered possible to distinguish between a positive CG and a negative CG. (Some claim to have observed a relationship between staccato lightning and positive CGs, but this relationship is as yet unproven.)Positive Vorticity Advection(Abbrev. PVA) - Advection of higher values of vorticity into an area, which often
is associated with upward motion (lifting) of the air. PVA typically is found in advance of
disturbances aloft (i.e., shortwaves), and is a property which often enhances the potential for
thunderstorm development.Positive-tilt TroughAn upper level system which is tilted to the east with increasing latitude (i.e., from southwest to northeast). A positive-tilt trough often is a sign of a weakening weather system, and generally is less likely to result in severe weather than a negative-tilt trough if all other factors are equal.Post-Flare LoopsIn solar-terrestrial terms, a loop prominence system often seen after a major two-ribbon flare, which bridges the ribbons.Post-storm ReportA report issued by a local National Weather Service office summarizing the impact of a tropical cyclone on it's forecast area. These reports include information on observed winds, pressures, storm surges, rainfall, tornadoes, damage and casualties.Potential TemperatureThe temperature a parcel of dry air would have if brought adiabatically (i.e.,
without transfer of heat or mass) to a standard pressure level of 1000 mb.Potential VorticityThis plays an important role in the generation of vorticity in cyclogenesis, especially along the polar front. It is also very useful in tracing intrusions of stratospheric air deep into the troposphere in the vicinity of jet streaks.Powder SnowDry, loose, unconsolidated snow.PowerA radar equation to describe the amount of power that a radar emits.
P = I * V (or)
P = V2 / R (or)
P = I2 / R
where I is current (amps), V is voltage (volts), R is resistance (ohms), P is power (watts).PPINEPlan Position Indicates No Echoes, referring to the fact that a radar detects no precipitation within its range. An intensity-modulated display on which echo signals are shown in plain view with range and azimuth angle displayed in polar coordinates, forming a map-like display. Each PPI is taken at a single, fixed elevation angle, and thus forms a cone of coverage in space. PPIs may be run in sequence, creating a "volume scan".Pre-Frontal Squall LineA line of thunderstorms that precedes an advancing cold front.Pre-Frontal TroughAn elongated area of relatively low pressure preceding a cold front that is usually associated with a shift in wind direction.Pre-Hurricane Squall LineIt is often the first serious indication that a hurricane is approaching. It is a generally a straight line and resembles a squall-line that occurs with a mid-latitude cold front. It is as much as 50 miles or even more before the first ragged rain echoes of the hurricane's bands and is usually about 100 to 200 miles ahead of the eye, but it has been observed to be as much as 500 miles ahead of the eye in the largest hurricanes.PRECDPrecedePrecipitable WaterMeasure of the depth of liquid water at the surface that would result after precipitating all of the water vapor in a vertical column over a given location, usually extending from the surface to 300 mb.PrecipitationThe process where water vapor condenses in the atmosphere to form water droplets that fall to the Earth as rain, sleet, snow, hail, etc.Precipitation AttenuationThe loss of energy that radar beam experiences as it passes through an area of precipitation.Precipitation ModeThe standard, or default, operational mode of the WSR-88D. The radar automatically switches into precipitation mode from clear-air mode if the measured reflectivity exceeds a specific threshold value. The precipitation mode of NEXRAD is more sensitive than previous weather radars. The minimum detectable reflectivity in NEXRAD's precipitation mode is 5 dBZ, compared to 28 dBZ with the old WSR-57.Precipitation Processing SystemThe WSR-88D system that generates 1-hour running, 3-hourly, and running storm total precipitation accumulations. Five functional steps are performed to calculate the best estimate of precipitation: 1) development of a sectorized hybrid scan, 2) conversion to precipitation rate, 3) precipitation accumulation, 4) adjustment using rain gages, 5) product update.PrecisionThe accuracy with which a number can be represented, i.e., the number of digits used to represent a number.Predominant WindThe wind that prevails and generates the local component of the significant sea conditions across the forecast area. This is the wind included in all marine forecast products and is defined as a 10-meter wind, except over the nearshore marine zones where it is defined to be the wind at a 3-meter height.Preliminary ReportNow known as the "Tropical Cyclone Report". A report summarizing the life history
and effects of an Atlantic or eastern Pacific tropical cyclone. It contains a summary of
the cyclone life cycle and pertinent meteorological data, including the post-analysis
best track (six-hourly positions and intensities) and other meteorological statistics. It
also contains a description of damage and casualties the system produced, as well
as information on forecasts and warnings associated with the cyclone. NHC writes
a report on every tropical cyclone in its area of responsibility.PRESPressurePrescribed FireA management ignited or natural wildland fire that burns under specified conditions where the fire is confined to a predetermined area and produces the fire behavior and fire characteristics required to attain planned fire treatment and resource management objectives.Present MovementThe best estimate of the movement of the center of a tropical cyclone at a given time and given position. This estimate does not reflect the short-period, small scale oscillations of the cyclone center.Present WeatherThe type of weather observed at the reporting time. These conditions may include types and intensity of precipitation such as light rain or heavy snow, as well as the condition of the air environment such as foggy, hazy or blowing dust.PressureThe exertion of force upon a surface by a fluid (e.g., the atmosphere) in contact with it.Pressure AltimeterAn aneroid barometer calibrated to indicate altitude in feet instead of units of pressure. It is read accurately only in a standard atmosphere and when the correct altimeter setting is used.Pressure AltitudeThe altitude in standard atmosphere at which a given pressure will be observed. It is the indicated altitude of a pressure altimeter at an altitude setting of 29.92 inches of mercury, and is therefore the indicated altitude above the 29.92 constant pressure surface.Pressure ChangeThe net difference between the barometric pressure at the beginning and ending of a specified interval of time, usually the three hour period preceding an observation.Pressure CharacteristicThe pattern of the pressure change during the specified period of time, usually the three hour period preceding an observation. This is recorded in three categories: falling, rising, or steady.Pressure CoupletIt is an area where you have a high pressure area located adjacent to a low pressure area.Pressure Falling RapidlyA decrease in station pressure at a rate of 0.06 inch of mercury or more
per hour which totals 0.02 inch or more.Pressure GageA device for registering the pressure of solids, liquids, or gases. It may be graduated to register pressure in any units desired.Pressure GradientThe amount of pressure change occurring over a given distance.Pressure Gradient ForceA three-dimensional force vector operating in the atmosphere that accelerates air parcels away from regions of high pressure and toward regions of low pressure in response to an air pressure gradient. Usually resolved into vertical and horizontal components.Pressure HeadEnergy contained by fluid because of its pressure, usually expressed in feet of fluid (foot pounds per pound).Pressure IceFloating sea, river, or lake ice that has been deformed, altered, or forced upward in pressure ridges by the lateral stresses of any combination of wind, water currents, tides, waves, and surf.Pressure Induced WaveA rare type of wave that does not develop from wind or seismic activity. Instead, these waves develop as a pressure perturbation moves over the water surface. The water surface adjusts to account for the atmospheric pressure change. As atmospheric pressure decreases, the force exerted upward by the water increases, creating a pressure induced wave. Pressure JumpA sudden, sharp increase in atmospheric pressure, typically occurring along an active front and preceding a storm.Pressure Rising RapidlyAn increase in station pressure at a rate of 0.06 inch of mercury or more per hour which totals 0.02 inch or more.Pressure TendencyThe character and amount of atmospheric pressure change during a specified period of time, usually 3-hour period preceding an observation.Pressure UnsteadyA pressure that fluctuates by 0.03 inch of mercury or more from the mean pressure during the period of measurement.Pressure-driven ChannelingChanneling of wind in a valley by synoptic-scale pressure gradients superimposed along the valley's axis. Compare forced channeling.Prevailing VisibilityThe visibility that is considered representative of conditions at the station; the greatest distance that can be seen throughout at least half the horizon circle, not necessarily continuous.Prevailing WesterliesThe westerly winds that dominant in middle latitudes.Prevailing WindsA wind that consistently blows from one direction more than from any other.Prevention of Significant DeteriorationA program, specified in the Clean Air Act, whose goal is to prevent air quality from deteriorating significantly in areas of the country that are presently meeting the ambient air quality standards.Primary Ambient Air Quality StandardsAir quality standards designed to protect human health.Primary Control Tide Station A tide station where continuous observations have been made for a minimum of 19 years. Its purpose is to provide data for computing accepted values essential to tide predictions and for determining tidal datums for coastal and marine boundaries. The data series from primary control tide stations serves as a primary control for the reduction of tidal datum for subordinate tide stations with a shorter period of record. The 19 year period is the official tidal epoch for calculating tidal datums.Primary Swell DirectionPrevailing direction of swell propagation.Probability ForecastA forecast of the probability that one or more of a mutually exclusive set of weather conditions will occur.Probability of Precipitation(Abbrev. PoP)- The probability that precipitation will be reported at a certain location during a specified period of time.
Probability of ThunderstormsThe probability based on climatology that a thunderstorm will be reported at that location during a specified period of time.Probability of Tropical Cyclone ConditioThe probability, in percent, that the cyclone center will pass within 50 miles to the
right or 75 miles to the left of the listed location within the indicated time period
when looking at the coast in the direction of the cyclone's movement.Product ResolutionThe smallest spatial increment or data element that is distinguishable in a given Doppler radar product.ProfilerAn instrument designed to measure horizontal winds directly above its location, and thus measure the vertical wind profile. Profilers operate on the same principles as Doppler radar.PROGGEDForecastedProgressive DerechoDerecho characterized by a short curved squall line oriented nearly perpendicular to the mean wind direction with a bulge in the general direction of the mean flow. Downburst activity occurs along the bulging portion of the line. This type of derecho typically occurs in the warm season (May through August) and is most frequent in a zone extending from eastern South Dakota to the upper Ohio Valley. The severe wind storms typically begin during the afternoon and continue into the evening hours. Several hours typically pass between initial convection and the first wind damage report.Prominence A term identifying cloud-like features in the solar atmosphere.
The features appear as bright structures in the CORONA above the
solar LIMB and as dark FILAMENTs when seen projected against the
solar DISKProperty ProtectionMeasures that are undertaken usually by property owners in order to prevent, or reduce flood damage. Property protection measures are often inexpensive for the community because they are implemented by or cost-shared with property owners. In many cases the buildings' appearance or use is unaffected, so these measurements are particularity appropriate for historical sites and landmarks. These measures include relocation and acquisition, flood proofing, and buying flood insurance.Proton EventIn solar-terrestrial terms, the measurement of at least 10 protons/cm2/sec/steradian at energies greater than 10 MeV.Proton FlareIn solar-terrestrial terms, any flare producing significant fluxes of greater-than-10 MeV protons in the vicinity of the earth.Pseudo-Cold FrontA boundary between a supercell's inflow region and the rear-flank downdraft (or RFD). It extends outward from the mesocyclone center, usually toward the south or southwest (but occasionally bows outward to the east or southeast in the case of an occluded mesocyclone), and is characterized by advancing of the downdraft air toward the inflow region. It is a particular form of gust front.Pseudo-Warm FrontA boundary between a supercell's inflow region and the forward-flank downdraft (or FFD). It extends outward from at or near the mesocyclone center, usually toward the east or southeast, and normally is either nearly stationary or moves northward or northeastward ahead of the mesocyclone.PsychrometerAn instrument used to measure the water vapor content of the air; a hygrometer consisting essentially of two similar thermometers with the bulb of one being kept wet so that the cooling that results from evaporation makes it register a lower temperature than the dry one and with the difference between the readings constituting a measure of the dryness of the atmosphere PTYPEprecipitation typePublic Information StatementA narrative statement issued by a National Weather Service Forecast Office that can be used for:
1) A current or expected nonhazardous event of general interest to the public that can usually be covered with a single message (e.g., unusual atmospheric phenomena such as sun dogs, halos, rainbows, aurora borealis, lenticular clouds, and stories about a long-term dry/cold/wet/warm spell).
2) Public educational information and activities, such as storm safety rules, awareness activities, storm drills, etc.
3) Information regarding service changes, service limitations, interruptions due to reduced or lost power or equipment outages, or special information clarifying interpretation of NWS data. For example, this product may be used to inform users of radar equipment outages or special information clarifying interpretation of radar data originating from an unusual source which may be mistaken for precipitation (such as chaff drops, smoke plumes, etc., that produces echoes on the radar display.Public Severe Weather OutlookThese are issued when the Storm Prediction Center (SPC) in Norman, Oklahoma anticipates an especially significant and/or widespread outbreak of severe weather. This outlook will stress the seriousness of the situation, defines the threat area, and provides information on the timing of the outbreak. The lead time on this outlook is normally less than 36 hours prior to the severe weather event.PuddleIn hydrologic terms, (1) The act of compacting earth, soil clay, etc., by mixing them with water and rolling or tamping the mixture. (2) A compact mass
of earth, soil, clay, or a mixture of material, which has been compacted through the addition of water, rolling and tamping. This
makes the material less permeable. (3) A small pool of water, usually a few inches in depth and from several inches to several feet
in it greatest dimension.Puget Sound Convergence ZoneA situation where wind forced around the Olympic Mountains converges over the Puget Sound. Causes extreme variability in weather conditions around Seattle, Washington with some areas of sunshine and others in clouds and rain.PulseA short burst of electromagnetic energy that a radar sends out in a straight line to detect a precipitation target. The straight line that this pulse travels along is called a radar beam.Pulse DurationThe time over which a radar pulse lasts. The pulse duration can be multiplied by the speed of light to determine the pulse length or pulse width.Pulse LengthThe linear distance in range occupied by an individual pulse from a radar. h = c * t , where t is the duration of the transmitted pulse, c is the speed of light, h is the length of the pulse in space. Note, in the radar equation, the length h/2 is actually used for calculating pulse volume because we are only interested in signals that arrive back at the radar simultaneously. This is also called a pulse width.Pulse RadarA type of radar, designed to facilitate range (distance) measurements, in which are transmitted energy emitted in periodic, brief transmission.Pulse Repetition Frequency (PRF)The amount of time between successive pulses, or bursts, of electromagnetic energy that is transmitted by a radar. The PRF determines the maximum range at which echoes can be detected and also the maximum radial velocity that can be detected by a Doppler radar.Pulse Repetition Time (PRT)The time elapsed between pulses by the radar. This is also called the pulse interval.Pulse Resolution VolumeA discrete radar sampling volume, of dimensions (horizontal beamwidth * vertical beamwidth * 1 range gate).Pulse Severe ThunderstormsSingle cell thunderstorms which produce brief periods of severe weather (3/4 inch hail, wind gusts in the excess of 58 miles an hour, or a tornado). Pulse StormA thunderstorm within which a brief period (pulse) of strong updraft occurs, during and immediately after which the storm produces a short episode of severe weather. These storms generally are not tornado producers, but often produce large hail and/or damaging winds. See also overshooting top.Pulse WidthSame as Pulse Length; the linear distance in range occupied by an individual pulse from a radar. h = c * t , where t is the duration of the transmitted pulse, c is the speed of light, h is the length of the pulse in space. Note, in the radar equation, the length h/2 is actually used for calculating pulse volume because we are only interested in signals that arrive back at the radar simultaneously.Pulse-Pair ProcessingNickname for the technique of mean velocity estimation by calculation of the signal complex covariance argument. The calculation requires two consecutive pulses, hence "pulse-pair".QPFERDNCEP Excessive Rainfall DiscussionQuadratureThe component of the complex signal that is 90 degrees out of phase with the inphase component. This component lies along the imaginary axis the complex plane.Quantitative Precipitation Estimate (QPE)A spatial and temporal analysis estimating the amount of precipitation that has occurred using a variety of techniques including observational and remote sensing data.Quantitative Precipitation ForecastA spatial and temporal precipitation forecast that will predict the potential amount of future precipitation for a specified region, or area.Quiescent Prominence (Filament) Long, sheet-like prominences nearly vertical
to the solar surfaceQuietSolar activity levels with less than one chromospheric event per day.Radar BeamThe straight line that a radar pulse travels along. As the radar beam gets further away from the radar, it gets wider and wider. In order for a precipitation target to be detected by the radar, it must fill the entire radar beam; therefore, the radar will have a difficult time detecting small showers and thunderstorms at a great distance from the radar.Radar Coded MessageThis is an alphanumeric coded message which will be used in preparation of a national radar summary chart. It is automatically produced by the WSR-88D's Radar Product Generator (RPG) in 3 parts (reflectivities, storm motion, and echo tops).Radar Cross SectionThe area of a fictitious, perfect reflector of electromagnetic waves (e.g., metal sphere) that would reflect the same amount of energy back to the radar as the actual target (e.g., lumpy snowflake).Radar MeteorologyBranch of meteorology that uses radars for weather observations and forecasts.Radar Product Generator (RPG)The RPG is the computer in the NEXRAD system that receives polar-coordinate base radar data from the RDA and processes these data into end-user products. Algorithms are utilized for pattern-recognition, rainfall estimation, computation of VIL and other products. The RPG communicates these products to end-users. A specific subset of available products is always generated for the NIDS vendors for distribution outside of the NWS, DoD, and FAA. Other products are generated by the RPG upon request from a PUP.Radar RangeDistance from the radar antenna. The WSR-88D radar has a range for velocity products out to 124 nautical miles and reflectivity products out to 248 nautical miles.Radar ReflectivityThe sum of all backscattering cross-sections (e.g., precipitation particles) in a pulse resolution volume divided by that volume. The radar reflectivity can be related to the radar reflectivity factor through the dielectric constant term |K|^2, and the radar wavelength.Radar Reflectivity Factor (z)z = the sum (over i) of (N_i * D_i^6), where N_i is the number of drops of diameter D_i in a pulse resolution volume. Note that z may be expressed in linear or logarithmic units. The radar reflectivity factor is simply a more meteorologically meaningful way of expressing the radar reflectivity.Radial VelocityComponent of motion toward or away from a given location. As "seen" by Doppler
radar, it is the component of motion parallel to the radar beam. (The component of motion perpendicular to
the beam cannot be seen by the radar. Therefore, strong winds blowing strictly from left to right or from right
to left, relative to the radar, can not be detected.)RadianceA measure of the intensity of the radiant energy flux emitted by a body in a given direction.Radiational InversionUsed interchangably with Nocturnal Inversion; a temperature inversion that develops during the night as a result of radiational cooling of the surface. Because the immediate surface (lower Boundary Layer) cools much more rapidly during these conditions than the air just above (upper Boundary Layer), a temperature inversion can be created overnight, but typically erodes quickly after sunrise.Radio EmissionEmissions of the sun in radio wavelengths from centimeters
to dekameters, under both quiet and disturbed conditions.
Type I. A noise storm composed of many short, narrow-band bursts
in the metric range (300 - 50 MHz).
Type II. Narrow-band emission that begins in the meter range
(300 MHz) and sweeps slowly (tens of minutes) toward deka-
meter wavelengths (10 MHz). Type II emissions occur in
loose association with major FLAREs and are indicative of
a shock wave moving through the solar atmosphere.
Type III. Narrow-band bursts that sweep rapidly (seconds) from
decimeter to dekameter wavelengths (500 - 0.5 MHz). They
often occur in groups and are an occasional feature of complex
solar ACTIVE REGIONs.
Type IV. A smooth continuum of broad-band bursts primarily in the
meter range (300 - 30 MHz). These bursts are associated with
some major flare events beginning 10 to 20 minutes after the
flare maximum, and can last for hoursRadio EventFlares with Centimetric Bursts and/or definite Ionospheric Event
(SID)RadiofacsimileAlso known as HF FAX, radiofax or weatherfax, is a means of broadcasting graphic weather maps and other graphic images via HF radio. HF radiofax is also known as WEFAX, although this term is generally used to refer to the reception of weather charts and imagery via satellite. Maps are received using a dedicated radiofax receiver or a single sideband shortwave receiver connected to an external facsimile recorder or PC equipped with a radiofax interface and application software. Radioisotope Snow GageA snow water equivalent gage based on the absorption of gamma radiation by snow; this gage can measure up to 55 inches water equivalent with a 2 to 5 percent error.RadiosondeAn instrument that is carried aloft by a balloon to send back information on atmospheric temperature, pressure and humidity by means of a small, expendable radio transmitter. Radiosondes can be tracked by radar, radio direction finding, or navigation systems (such as the satellite Global Positioning System) to obtain wind data. See also rawinsonde.Rain ForestA forest which grows in a region of heavy annual precipitation. There are two major types, tropical and temperate.Rain GaugeAn instrument for measuring the quantity of rain that has fallen.Rain Induced FogWhen warm rain falls through cooler air, water evaporates from the warm rain. It subsequently condenses in the cool air forming fog. Such fog can be quite dense. It generally will persist as long as the rain continues. Since temperature rises little during the day, there is little diurnal variation in rain induced fog. Improvement in visibility cannot be expected until the rain stops or moves out of the affected area.Rain ShieldIn a hurricane, a solid or nearly solid area of rain that typically becomes heavier as one approaches the eye. The outer edge is well defined and its distance from the eye varies greatly from storm to storm. The wind, both sustained and peak gusts, keeps increasing as much as one moves through the rain shield toward the storm's eye.Rain-free BaseA dark, horizontal cloud base with no visible precipitation beneath it. It typically marks the location of the thunderstorm updraft. Tornadoes may develop from wall clouds attached to the rain-free base, or from the rain-free base itself - especially when the rain-free base is on the south or southwest side of the main precipitation area. Note that the rain-free base may not actually be rain free; hail or large rain drops may be falling. For this reason, updraft base is more accurate.Rainfall EstimatesA series of NEXRAD products that employ a Z-R relationship to produce accumulations of surface rainfall from observed reflectivity.RangeDistance from the radar antenna. The WSR-88D radar has a range for velocity products
out to 124 nm and reflectivity products out to 248 nm. Range FoldingThis occurs when the radar receives a signal return from a pulse other than the most recent pulse. In this case, the radar sends out a pulse (a short burst of energy). This pulse will continue to go in a straight line until it strikes a target. When it strikes the target, a portion of the pulse will be back scattered towards the radar. If the target it strikes is well beyond the normal range of the radar, it will take longer for the back scattered energy to arrive back at the radar. As a result, the radar will most likely have sent out another pulse in the same direction before the back scattered energy arrives back at the radar. Therefore, when the radar receives the back scattered energy, it will assume that it came from an object much closer to the radar and it will improperly locate the echo. A multiple-trip return appears at the difference of the true range and a multiple of the unambiguous range, i.e., R_displayed = R_true - n * R_max, where n = 0,1,2,...Range GateThe discrete point in range along a single radial of radar data at which the received signal is sampled. Range gates are typically spaced at 100-1000 meter intervals. A "radial" of radar data is composed of successive range gates, out to the maximum unambiguous range.Range Height IndicatorThe RHI is a radar display in which the radar scans vertically, with the antenna pointing at a specific azimuth or radial. NEXRAD does not support RHI, but the PUP software allows the NEXRAD operator to construct a vertical cross-section using data from multiple scans of the radar.Range NormalizationA receiver gain function in the radar which compensates for the effect of
range (distance) on the received power for an equivalent reflectivity.Range ResolutionThe ability of the radar to distinguish two targets along the same radial but at different ranges. it is approximately ½ the pulse length.Range UnfoldingProcess of removing range ambiguity in apparent range of a multitrip target on the radar.Rankine VortexVelocity profile for a symmetric circulation in which the inner core is in solid rotation and tangential winds outside the core vary inversely with radial distance from the center.Rapid DeepeningA decrease in the minimum sea-level pressure of a tropical cyclone of 1.75 mb/hr or
42 mb for 24 hoursRapid Onset Flooding (ROF)In the context of the National Water Model (NWM), rapid onset flooding refers to stream reaches that are forecast to at least double their flow within an hour, and meet or exceed their high water flow within six hours of this flow increase.Rapidly IntensifyingAny maritime cyclone whose central pressure is dropping, or is expected to drop, at a rate of 1 MB per hour for 24 hours. RawinsondeA radiosonde that is tracked to measure winds.Rawinsonde ObservationA radiosonde observation which includes wind data.Rayleigh ScatteringChanges in directions of electromagnetic energy by particles whose diameters are 1/16 wavelength or less. This type of scattering is responsible for the sky being blue.ReachIn hydrologic terms, the distance between two specific points outlining that portion of the stream, or river for which the forecast applies. This generally
applies to the distance above and below the forecast point for which the forecast is valid. ReachA section of river or stream between an upstream and downstream location, for which the stage or flow measured at a point somewhere along the section (e.g., gaging station or forecast point) is representative of conditions in that section of river or stream. Real-TimeRefers to the rapid retrieval, processing and transmission of data.Rear Flank DowndraftA region of dry air subsiding on the back side of, and wrapping around, a mesocyclone. It often is visible as a clear slot wrapping around the wall cloud. Scattered large precipitation particles (rain and hail) at the interface between the clear slot and wall cloud may show up on radar as a hook or pendant; thus the presence of a hook or pendant may indicate the presence of an RFD.ReceiverThe electronic device which detects the backscattered radiation, amplifies it and converts it to a low-frequency signal which is related to the properties of the target.RechargeThe replenishment of groundwater through infiltration of precipitation or snowmelt into the soil and gravity flow of streams into valley alluvium, sinkholes, or other large openings. Reconnaissance CodeAn aircraft weather reconnaissance code that has come to refer primarily to in-flight tropical weather observations, but actually signifies any detailed weather observation or investigation from an aircraft in flight.Record Event ReportThis non-routine narrative product is issued by the National Weather Service to report meteorological and hydrological events that equal or exceed existing records.Recreation ReportThis National Weather Service product is used to relay reports on conditions for resorts and recreational areas and/or events. This report may also contain forecast information. Reports for recreational areas and resorts are often routine products, typically for a season, but possibly year-round.RecurrenceUsed especially in reference to the recurrence of physical
parameters every 27 days (the rotation period of the sun)Red FlagThis a fire weather program which highlights the onset of critical weather conditions conducive to extensive wildfire occurrences.Red Flag WarningA term used by fire-weather forecasters to call attention to limited weather conditions of particular importance that may result in extreme burning conditions. It is issued when it is an on-going event or the fire weather forecaster has a high degree of confidence that Red Flag criteria will occur within 24 hours of issuance. Red Flag criteria occurs whenever a geographical area has been in a dry spell for a week or two, or for a shorter period , if before spring green-up or after fall color, and the National Fire Danger Rating System (NFDRS) is high to extreme and the following forecast weather parameters are forecasted to be met: 1) a sustained wind average 15 mph or greater 2) relative humidity less than or equal to 25 percent and 3) a temperature of greater than 75 degrees F. In some states, dry lightning and unstable air are criteria. A Fire Weather Watch may be issued prior to the Red Flag Warning.Red Watch or Red BoxSlang for Tornado Watch.REFReferenceReference MarkA relatively permanent point of known elevation which is tied to a benchmark.ReflectionThe process whereby radiation (or other waves) incident upon a surface is directed back into the medium through which it traveled.ReflectivityUsually a reference to Radar Reflectivity; the sum of all backscattering cross-sections (e.g., precipitation particles) in a pulse resolution volume divided by that volume. The radar reflectivity can be related to the radar reflectivity factor through the dielectric constant term |K|^2, and the radar wavelength.Reflectivity Cross SectionThis WSR-88D radar product displays a vertical cross section of reflectivity on a grid with heights up to 70,000 feet on the vertical axis and distance up to 124 nm on the horizontal axis. Cross Section is similar to the Range Height Indicator (RHI) slices observed on conventional radar, but it is not limited to alignments along the radar radials. Instead the 2 end points are operator selected anywhere within 124 nm of the radar that are less than 124 nm apart. It is used to: 1) Examine storm structure features such as overhang, tilt, Weak Echo Regions (WER), and Bounded Weak Echo Regions (BWER); 2) Estimate height of higher dBZ's and echo tops; and 3) Locate the bright band (where snow is melting and becoming rain).Reflectivity FactorThe result of a mathematical equation (called the Weather Radar Equation) that converts the analog power (in Watts) received by the radar antenna into a more usable quantity. The reflectivity factor (denoted by Z) takes into account several factors, including the distance of a target from the radar, the wavelength of the transmitted radiation, and certain assumptions about the kind and size of targets detected by the radar. The reflectivity factor ranges over several orders of magnitudes, so it is usually expressed on a logarithmic scale called dBZ (decibels of reflectivity).RefractionChanges in the direction of energy propagation as a result of density changes within the propagating medium. In weather terms, this is important on determining how a radar beam reacts in the atmosphere.Refractive IndexA measure of the amount of refraction. Numerically equal to the ratio of wave velocity in a vacuum to a wave speed in the medium, i.e., n = c / v
where: v is actual speed, and c is speed of light in a vacuum.RefractivityExpressed as N; N = (n-1)*106, where n is refractive index and N is a function of temperature, pressure and vapor pressure (in the atmosphere).Regional HazeHaze that is mixed uniformly between the surface and the top of a convective boundary layer.Relative HumidityA dimensionless ratio, expressed in percent, of the amount of atmospheric moisture present relative to the amount that would be present if the air were saturated. Since the latter amount is dependent on temperature, relative humidity is a function of both moisture content and temperature. As such, relative humidity by itself does not directly indicate the actual amount of atmospheric moisture present. See dew point.Relative VorticityThe sum of the rotation of an air parcel about the axis of the pressure system and the rotation of the parcel about its own axis.Relative WindThe wind with reference to a moving point. Sometimes called APPARENT WIND. See also APPARENT WIND, TRUE WIND.RELBLReliableRelocatedA term used in an advisory to indicate that a vector drawn from the preceding advisory position to the latest known position is not necessarily a reasonable representation of the cyclone's movement.Remote Observing System AutomationA type of automated data transmitter used by NWS Cooperative Program observers.REPRepresent/RepresentativeReplace and Route (R&R)A methodology that ingests the official streamflow forecasts issued by the NWS RFCs at AHPS gauge locations, and utilizes the National Water Model (NWM) to route these forecasts downstream. This method is used to delineate the River Forecast Center Flood Inundation Map (RFC FIM).ReportA weather report is a statement of the actual weather conditions observed at a specific time at a specific site.ReservoirIn hydrologic terms, a manmade facility for the storage, regulation and controlled release of water.Residual Layerthe elevated portion of a convective boundary layer that remains after a stable boundary layer develops at the ground (usually in late afternoon or early evening) and cuts off convection.Residual MoistureAtmospheric moisture which lingers over an area after the main weather system has departed.ResonanceThe state of a system in which an abnormally large vibration is produced in response to an external stimulus, occurring when the frequency of the stimulus is the same, or nearly the same, as the natural vibration frequency of the system.Response TimeIn hydrologic terms, the amount of time in which it will take a watershed to react to a given rainfall eventRetrogression(or Retrograde Motion) - Movement of a weather system in a direction opposite to that of the basic flow in which it is embedded, usually referring to a closed low or a longwave trough which moves westward.Return FlowSouth winds on the back (west) side of an eastward-moving surface high pressure system. Return flow over the central and eastern United States typically results in a return of moist air from the Gulf of Mexico (or the Atlantic Ocean).Return StrokeAn electrical discharge that propagates upward along a lightning channel from the ground to the cloud.Rex BlockA blocking pattern where there is an upper level high located directly north of a closed low.Ridge1) An elongated area of relatively high atmospheric pressure; the opposite of trough.
or
2) In hydrologic terms, a line or wall of broken ice forced up by pressure. May be fresh or weatheredRidge IceIn hydrologic terms, ice piled haphazardly one piece over another in the form of ridges or walls.Right AscensionThe celestial longitude of the sun. This value is 0 at the vernal equinox, 90 at the summer solstice, 180 at the autumnal equinox and 270 at the winter solstice.Right Entrance RegionUsed interchangably with Right Rear Quadrant; the area upstream from and to the right of an upper-level jet max (as would be viewed looking along the direction of flow). Upward motion and severe thunderstorm potential sometimes are increased in this area relative to the wind speed maximum. See also exit region, left front quadrant.Right MoverA thunderstorm that moves appreciably to the right relative to the main steering winds and to other nearby thunderstorms. Right movers typically are associated with a high potential for severe weather. (Supercells often are right movers).Right Rear Quadrant(Abbrev. RRQ) - Used interchangably with Right Entrance Region; the area upstream from and to the right of an upper-level jet max (as would be viewed looking along the direction of flow). Upward motion and severe thunderstorm potential sometimes are increased in this area relative to the wind speed maximum. See also exit region, left front quadrant. Rime IceAn opaque coating of tiny, white, granular ice particles caused by the rapid freezing of supercooled water droplets on impact with an object. See also clear ice.Riometer(Relative Ionospheric Opacity meter). A specially designed radio
receiver for continuous monitoring of cosmic noise. The
absorption of cosmic noise in the polar regions is very
sensitive to the solar low-energy cosmic ray fluxRip CurrentA relatively small-scale surf-zone current moving away from the beach. Rip currents form as waves disperse along the beach causing water to become trapped between the beach and a sandbar or other underwater feature. The water converges into a narrow, river-like channel moving away from the shore at high speed.Rip TideSee RIP CURRENTSRiparian ZoneIn hydrologic terms, a stream and all the vegetation on its banks.River BasinIn hydrologic terms, drainage area of a river and its tributaries.River Flood StatementThis product is used by the local National Weather Service Forecast Office (NWFO) to update and expand the information in the River Flood Warning. This statement may be used in lieu of a warning if flooding is forecasted, imminent, or existing and it presents no threat to life or property. The statement will also be used to terminate a River Flood Warning.River Flood WarningThis is product is issued by the local National Weather Service Forecast Office (NWFO) when forecast points (those that have formal gaging sites and established flood stages) at specific communities or areas along rivers where flooding has been forecasted, is imminent, or is in progress. Flooding is defined as the inundation of normally dry areas as a result of increased water levels in an established water course. The flood warning is based on the RVF product from the River Forecast Center (RFC) in Minneapolis, Minnesota. The flood warning normally specifies crest information. It usually occurs 6 hours or later after the causative event and it is usually associated with widespread heavy rain and/or snow melt or ice jams.
It will contain the forecast point covered by the warning, the current stage (if it is available), and the established flood stage. It will also contain the forecasted crest from the River Forecast Center (RFC) in Minneapolis, Minnesota. From this forecasted crest, the NWFO will be able to determine which areas will be affected by the river flooding. This information will be included in the warning. Finally, the statement will include a site/event specific call to action.River FloodingThe rise of a river to an elevation such that the river overflows its natural banks causing or threatening damage.River ForecastAn internal product issued by RFCs to other NWS offices. An RVF contains stage and/ or flow forecasts for specific locations based on existing, and forecasted hydrometeorologic conditions. The contents of these products are used by the HSA office to prepare Flood Warnings (FLW), Flood Statements (FLS), River Statements (RVS), as well as other products available to the public.River Forecast CenterCenters that serve groups of Weather Service Forecast offices and Weather Forecast offices, in providing hydrologic guidance and is the first echelon office for the preparation of river and flood forecasts and warnings.River GageA device for measuring the river stage.River Gage DatumThe arbitrary zero datum elevation which all stage measurements are made from.River Ice StatementA public product issued by the RFC containing narrative and numeric information on river ice conditions.River Observing StationAn established location along a river designated for observing and measuring properties of the river.River Recreation StatementA statement released by the NWS to inform river users of current and forecast river and lake conditions. These statements are especially useful for planning purposes.River StatementA NWS product issued to communicate notable hydrologic conditions which do not involve flooding, i.e., within river bank rises, minor ice jams, etc.River SystemIn hydrologic terms, all of the streams and channels draining a river basin.RocketsondeA type of radiosonde that is shot into the atmosphere by a rocket, allowing it to collect data during its parachute descent from a higher position in the atmosphere than a balloon could reach.Rogue WaveCommonly used term by mariners of a wave of an unexpected wave of much greater height or steepness than other waves in the prevailing sea or swell system. Rogue waves have been part of marine folklore for centuries. They are generally considered to be unexpectedly high waves which in some instances come from a direction different from the predominant waves in the local area. A single rogue wave has certainly been known to spell disaster for the mariner. They have, over the past twenty or thirty years, come to be recognized as unique phenomena albeit with several possible causes.
(1) Constructive interference. Several different wave trains of differing speeds and directions meet at the same time. The heights of the crests are additive so that an extreme wave may result when very high waves are included in the wave trains. The effect is normally short lived since the wave trains continue to separate and move on.
(2) Focusing of wave energy. When storm forced waves are developed in a water current counter to the wave direction an interaction can take place which results in a shortening of the wave frequency. The result is the superimposing of the wave trains and the generation of extreme waves. Examples of currents where these are sometimes seen are the Gulf Stream and Agulhas current. Extreme wave developed in this regime tend to be longer lived.
(3) Normal part of the wave spectrum. The generation of waves on water results not in a single wave height but in a spectrum of waves distributed from the smallest capillary waves to large waves indeed. Within this spectrum there is a finite possibility of each of the wave heights to occur with the largest waves being the least likely. The wave height most commonly observed and forecast is the significant wave height. This is defined as the average of the one third highest waves. The probability of encountering such a wave is about 1 in 10 while 1 in 1000 waves will be nearly double the significant wave height or higher. This is thought to be the source of at least some reports of rogue waves.
Rolled Filled DamIn hydrologic terms, an embankment dam of earth or rock in which the material is placed in layers and compacted by using rollers or rolling equipmentRope(Also "Rope Funnel") - a narrow, often contorted condensation funnel usually associated with the decaying stage of a tornado. See rope stage.Rope CloudIn satellite meteorology, a narrow, rope-like band of clouds sometimes seen on satellite images along a front or other boundary. The term sometimes is used synonymously with rope or rope funnel.Rope StageThe dissipating stage of a tornado, characterized by thinning and shrinking of the condensation funnel into a rope (or rope funnel). Damage still is possible during this stage.Rossby WavesA series of troughs and ridges on quasi-horizontal surfaces in the major belt of upper tropospheric westerlies. The waves are thousands of kilometers long and have significant latitudinal amplitude.Rotten IceIn hydrologic terms, ice in an advanced stage of disintegration.Rough SeasSea conditions associated with regionally defined wind thresholds over bays, inlets, harbors, inland waters, and estuaries where larger waves are forming with whitecaps and spray everywhere.RTERouteRunway Visual RangeThe maximum distance at which the runway, or the specified lights or markers delineating it, can be seen from a position above a specified point on its center line. This value is normally determined by visibility sensors located alongside and higher than the center line of the runway. RVR is calculated from visibility, ambient light level, and runway light intensity.Sacramento Soil Moisture Accounting Model (SAC-SMA)A continuous soil moisture accounting model with spatially lumped parameters that simulates runoff within a basin.SafetyNET Inmarsat-C SafetyNET is an internationally adopted, automated satellite system for promulgating weather forecasts and warnings, marine navigational warnings and other safety related information to all types vessels and is part of the Global Maritime Distress and Safety System (GMDSS).Saffir-Simpson Hurricane Wind ScaleThe Saffir-Simpson Hurricane Wind Scale is a 1 to 5 categorization based on the hurricane's intensity at the indicated time. The scale provides examples of the type of damage and impacts in the United States associated with winds of the indicated intensity. In general, damage rises by about a factor of four for every category increase. The maximum sustained surface wind speed (peak 1-minute wind at the standard meteorological observation height of 10 m [33 ft] over unobstructed exposure) associated with the cyclone is the determining factor in the scale. The scale does not address the potential for other hurricane-related impacts, such as storm surge, rainfall-induced floods, and tornadoes.
SAME(Specific Area Message Encoding) - A tone alert system which allows NOAA Weather Radio receivers equipped with the SAME feature to sound an alert for only certain weather conditions or within a limited geographic area such as a county. SAMEXStorm and Mesoscale Ensemble ExperimentSampling FrequencyThe rate at which sensor data is read or sampled.Satellite Hydrology ProgramA NOHRSC program that uses satellite data to generate areal extent of snow cover data over large areas of the western United States.Saturation Vapor PressureThe vapor pressure of a system, at a given temperature, wherein the vapor of a substance is in equilibrium with a plane surface of that substance's pure liquid or solid phase.SBCAPESurface Based CAPE; CAPE calculated using a Surface based parcel.ScatteredWhen used to describe precipitation (for example: "scattered showers") - Area coverage of convective weather affecting 30 percent to 50 percent of a forecast zone (s).
When used to describe sky cover: 3/8th to 4/8th (sky cover is measured in eighths or oktas) of the sky covered by clouds. In U.S. weather observing procedures, this is reported with the contraction “SCT.”ScatteringThe process in which a beam of light is diffused or deflected by collisions with particles suspended in the atmosphere.SESoutheastSea BreezeA thermally produced wind blowing during the day from a cool ocean surface onto the adjoining warm land, caused by the difference in the rates of heating of the surfaces of the ocean and of the land.Sea Breeze Convergence ZoneThe zone at the leading edge of a sea breeze where winds converge. The incoming air rises in this zone, often producing convective clouds.Sea Breeze FrontThe leading edge of a sea breeze, whose passage is often accompanied by showers, a wind shift, or a sudden drop in temperature.Sea FogCommon advection fog caused by transport of moist air over a cold body of water. Sea IceAny form of ice found at sea which has originated from the freezing of sea water (sea ice does NOT include superstructure icing). Ice formed from the freezing of the waters of the Great Lakes will be considered the same as sea ice. Sea Level PressureThe sea level pressure is the atmospheric pressure at sea level at a given location. When observed at a reporting station that is not at sea level (nearly all stations), it is a correction of the station pressure to sea level. This correction takes into account the standard variation of pressure with height and the influence of temperature variations with height on the pressure. The temperature used in the sea level correction is a twelve hour mean, eliminating diurnal effects. Once calculated, horizontal variations of sea level pressure may be compared for location of high and low pressure areas and fronts.Sea Surface TemperaturesThe term refers to the mean temperature of the ocean in the upper few meters.SeasThe combination of both wind waves and swell. Used to describe the combination or interaction of wind waves and swell in which the separate components are not distinguished. This includes the case when swell is negligible or is not considered in describing sea state. Specifically, Seas2 = S2+W2 where S is the height of the swell and W is the height of the wind wave. When used, Seas should be considered as being the same as the Significant Wave HeightSecond-Day FeetIn hydrologic terms, the volume of water represented by a flow of one cubic foot per second for 24 hours; equal to 86,400 cubic feet. This is used
extensively as a unit of runoff volume. Often abbreviated as SDF.Secondary Ambient Air Quality StandardsAir quality standards designed to protect human welfare, including the effects on vegetation and fauna, visibility and structures.Secondary PollutantPollutants generated by chemical reactions occurring within the atmosphere. Compare primary pollutant.Sector BoundaryIn solar-terrestrial terms, in the solar wind, the area of demarcation between sectors, which are large-scale features distinguished by the predominant direction of the interplanetary magnetic field, toward or away from the sun.Sector VisibilityThe visibility in a specific direction that represents at least a 45º arc of a horizontal circle.Sectorized Hybrid ScanA single reflectivity scan composed of data from the lowest four elevation scans. Close to the radar, higher tilts are used to reduce clutter. At further ranges, either the maximum values from the lowest two scans are used or the second scan values are used alone.SecuriteA headline within National Weather Service high seas forecasts transmitted via the GMDSS to indicate that no hurricane or hurricane force winds are forecast.Sediment Storage CapacityIn hydrologic terms, the volume of a reservoir planned for the deposition of sediment.SeepageIn hydrologic terms, the interstitial movement of water that may take place through a dam, its foundation, or abutments.SeicheA standing wave oscillation of water in large lakes usually created by strong winds and/or a large barometric pressure gradient.SELA watch cancellation statement issued to terminate a watch before its original expiration time.SELSSevere Local StormSELYSoutheasterlySensible Heat FluxThe flux of heat from the earth's surface to the atmosphere that is not associated with phase changes of water; a component of the surface energy budget.Sentinel-1 (S1)A European Space Agency satellite remote sensing mission which images with a 5cm wavelength Synthetic Aperture Radar (SAR).Sentinel-2 (S2)A European Space Agency satellite remote sensing mission which images with a multi-spectral scanner.Separation EddyAn eddy that forms near the ground on the windward or leeward side of a bluff object or steeply rising hillside; streamlines above this eddy go over the object.Serial DerechoType of derecho that consists of an extensive squall line which is oriented such that the angle between the mean wind direction and the squall line axis is small. A series of LEWPs and bow echoes move along the line. The downburst activity is associated with the LEWPs and bows. A Serial Derecho tends to be more frequent toward the north end of the line during the late winter and spring months. It occurs less frequently than its cousin the "progressive derecho."
It is associated with a linear type mesoscale convective system that moves along and in advance of a cold front or dry line. These boundaries are often associated with a strong, migratory surface low pressure system and strong short wave trough at 500 mb (strong dynamic forcing). Lifted Indices are typically -6 or lower and the advection of dry air in the mid-troposphere (3-7 km above ground) by relatively strong winds leads to high convective instability and increased downdraft potential. The bow echoes move along the line in the direction of the mean flow, often southwest to northeast. These storms move at speeds exceeding 35 knots. Squall line movement is often less than 30 knots.SERNSoutheasternService HydrologistThe designated expert of the hydrology program at a WFO.Servo LoopIn radar meteorology, a generic description of hardware needed to remotely control the motion of the antenna dish.SetThe direction towards which a current is headed. For example, a current moving from west to
east is said to be set to east.Set-upThe process whereby strong winds blowing down the length of a lake cause water to "pile
up" at the downwind end, raising water levels there and lowering them at the upwind end of the
lake.Severe IcingThe rate of ice accumulation on an aircraft is such that de-icing/anti-icing equipment
fails to reduce or control the hazard. Immediate diversion is necessary.Severe Local StormA convective storm that usually covers a relatively small geographic area, or moves in a narrow path, and is sufficiently intense to threaten life and/or property. Examples include severe thunderstorms with large hail, damaging wind, or tornadoes. Although cloud-to-ground lightning is not a criteria for severe local storms, it is acknowledged to be highly dangerous and a leading cause of deaths, injuries, and damage from thunderstorms. A thunderstorm need not be severe to generate frequent cloud-to-ground lightning. Additionally, excessive localized convective rains are not classified as severe storms but often are the product of severe local storms. Such rainfall may result in related phenomena (flash floods) that threaten life and property.Severe Local Storm WatchAn alert issued by the National Weather Service for the contiguous U.S. and its adjacent waters of the potential for severe thunderstorms or tornadoes. Severe ThunderstormA thunderstorm that produces a tornado, winds of at least 58 mph (50knots), and/or hail at least 1" in diameter. Structural wind damage mayimply the occurrence of a severe thunderstorm. A thunderstorm wind equalto or greater than 40 mph (35 knots) and/or hail of at least 1" isdefined as approaching severe.Severe Thunderstorm WarningThis is issued when either a severe thunderstorm is indicated by the WSR-88D radar or a spotter reports a thunderstorm producing hail one inch or larger in diameter and/or winds equal or exceed 58 miles an hour; therefore, people in the affected area should seek safe shelter immediately. Severe thunderstorms can produce tornadoes with little or no advance warning. Lightning frequency is not a criteria for issuing a severe thunderstorm warning. They are usually issued for a duration of one hour. They can be issued without a Severe Thunderstorm Watch being already in effect.
Like a Tornado Warning, the Severe Thunderstorm Warning is issued by your National Weather Service Forecast Office (NWFO). Severe Thunderstorm Warnings will include where the storm was located, what towns will be affected by the severe thunderstorm, and the primary threat associated with the severe thunderstorm warning. If the severe thunderstorm will affect the nearshore or coastal waters, it will be issued as the combined product--Severe Thunderstorm Warning and Special Marine Warning. If the severe thunderstorm is also causing torrential rains, this warning may also be combined with a Flash Flood Warning. If there is an ampersand (&) symbol at the bottom of the warning, it indicates that the warning was issued as a result of a severe weather report.
After it has been issued, the affected NWFO will follow it up periodically with Severe Weather Statements. These statements will contain updated information on the severe thunderstorm and they will also let the public know when the warning is no longer in effect.Severe Thunderstorm WatchThis is issued by the National Weather Service when conditions are favorable for the development of severe thunderstorms in and close to the watch area. A severe thunderstorm by definition is a thunderstorm that produces one inch hail or larger in diameter and/or winds equal or exceed 58 miles an hour. The size of the watch can vary depending on the weather situation. They are usually issued for a duration of 4 to 8 hours. They are normally issued well in advance of the actual occurrence of severe weather. During the watch, people should review severe thunderstorm safety rules and be prepared to move a place of safety if threatening weather approaches.
A Severe Thunderstorm Watch is issued by the Storm Prediction Center in Norman, Oklahoma. Prior to the issuance of a Severe Thunderstorm Watch, SPC will usually contact the affected local National Weather Service Forecast Office (NWFO) and they will discuss what their current thinking is on the weather situation. Afterwards, SPC will issue a preliminary Severe Thunderstorm Watch and then the affected NWFO will then adjust the watch (adding or eliminating counties/parishes) and then issue it to the public by way of a Watch Redefining Statement. During the watch, the NWFO will keep the public informed on what is happening in the watch area and also let the public know when the watch has expired or been cancelled.Severe Weather AnalysisThis WSR-88D radar product provides 3 base products (reflectivity (SWR), radial velocity (SWV), and spectrum width (SWW)) at the highest resolution available along with radial shear (SWS). These products are mapped into a 27 nm by 27 nm region centered on a point which the operator can specify anywhere within a 124 nm radius of the radar. It is most effective when employed as an alert paired product with the product centered on alert at height that caused the alert. It is used to examine 3 base products simultaneously in a 4 quadrant display; and analyze reflectivity and velocity products at various heights to gain a comprehensive vertical analysis of the thunderstorm.Severe Weather Potential StatementThis statement is designed to alert the public and state/local agencies to the potential for severe weather up to 24 hours in advance. It is issued by the local National Weather Service office.Severe Weather ProbabilityThis WSR-88D radar product algorithm displays numerical values proportional to the probability that a storm will produce severe weather within 30 minutes. Values determined using a statistical regression equation which analyzes output from the VIL algorithm. It is used to quickly identify the most significant thunderstorms.Severe Weather StatementA National Weather Service product which provides follow up information on severe weather conditions (severe thunderstorm or tornadoes) which have occurred or are currently occurring.SEWDSoutheastwardSfericIn solar-terrestrial terms, a transient electric or magnetic field generated by any feature of lightning discharge (entire flash).ShearVariation in wind speed (speed shear) and/or direction (directional shear) over a short distance within the atmosphere. Shear usually refers to vertical wind shear, i.e., the change in wind with height, but the term also is used in Doppler radar to describe changes in radial velocity over short horizontal distances.Sheet FlowIn hydrologic terms, flow that occurs overland in places where there are no defined channels, the flood water spreads out over a large area at a uniform
depth. This also referred to as overland flow. Sheet iceIce formed by the freezing of liquid precipitation or the freezing of melted solid precipitation (see snow depth)Shelf CloudA low, horizontal wedge-shaped arcus cloud, associated with a thunderstorm gust front (or occasionally with a cold front, even in the absence of thunderstorms). Unlike the roll cloud, the shelf cloud is attached to the base of the parent cloud above it (usually a thunderstorm). Rising cloud motion often can be seen in the leading (outer) part of the shelf cloud, while the underside often appears turbulent, boiling, and wind-torn.Shore iceIn hydrologic terms, an ice sheet in the form of a long border attached to the bank or shore.; border ice.Short Range Forecast (SRF)A configuration of the National Water Model (NWM) that runs hourly and produces hourly deterministic forecasts of streamflow and hydrologic states out to 18 hours for the contiguous United States (ConUS), and out 48 hours for Hawaii and Puerto Rico/U.S. Virgin Islands. For ConUS, meteorological forcing data are drawn from the HRRR and RAP. For Hawaii and Puerto Rico/U.S. Virgin Islands, meteorological forcing data are drawn from the NAM-Nest with HIRESW WRF-ARW.Short Term ForecastA product used to convey information regarding weather or hydrologic events in the next few hours.Short Wave Fade (SWF)In solar-terrestrial terms, a particular ionospheric solar flare effect under the broad category of sudden ionospheric disturbances (SIDs)
whereby short-wavelength radio transmissions, VLF, through HF, are absorbed for a period of minutes to hours.Short-Fuse WarningA warning issued by the NWS for a local weather hazard of relatively short duration. Short-fuse warnings include tornado warnings, severe thunderstorm warnings, and flash flood warnings. Tornado and severe thunderstorm warnings typically are issued for periods of an hour or less, flash flood warnings typically for three hours or less.Short-Term Prediction Research and Transition (SPoRT)A NASA- and NOAA-funded activity to transition experimental/quasi-operational satellite observations and research capabilities to the operational weather community to improve short-term weather forecasts on a regional and local scale.Short-Term Prediction Research and Transition Center - Land Information System (SPoRT-LIS)Provides high-resolution (~3 km) gridded soil moisture products in real-time to support regional and local modeling and improve situational awareness.ShortwaveAlso known as Shortwave Trough; a disturbance in the mid or upper part of the atmosphere which induces upward motion ahead of it. If other conditions are favorable, the upward motion can contribute to thunderstorm development ahead of a shortwave.Shortwave RadiationIn solar-terrestrial terms, shortwave radiation is a term used to describe the radiant energy emitted by the sun in the visible and near-ultraviolet wavelengths (between about 0.1 and 2 micrometers).Shortwave TroughAlso called Shortwave; A disturbance in the mid or upper part of the atmosphere which induces upward motion ahead of it. If other conditions are favorable, the upward motion can contribute to thunderstorm development ahead of a shortwave trough.Showalter Index(Abbrev. SWI) - a stability index used to determine thunderstorm potential. The SWI is calculated by lifting an air parcel adiabatically from 850 mb to 500 mb. The algebraic difference between the air parcel and the environmental temperature at 500 mb represents the SWI. It is especially useful when you have a shallow cool airmass below 850 mb concealing greater convective potential aloft. However, the SWI will underestimate the convective potential for cool layers extending above 850 mb. It also does not take in account diurnal heating or moisture below 850 mb. As a result, one must be very careful when using this index.ShowerA descriptor, SH, used to qualify precipitation characterized by the suddenness with which they start and stop, by the rapid changes of intensity, and usually by rapid changes in the appearance of the sky.SidelobeA secondary energy maximum located outside the main radar beam. Typically, it contains a small percentage of energy compared to the main lobe, but it may produce erroneous echoes.SIGMETSignificant Meteorological AdvisorySignal-to-Noise RatioA ratio that measures the comprehensibility of data, usually expressed as the signal power divided by the noise power, usually expressed in decibels (dB).Significant Wave HeightThe mean or average height of the highest one third of all waves in a swell train or in a wave generating region. It approximates the value an experienced observer would report if visually estimating sea height. When expressed as a range (e.g. Seas 2-4 ft) , indicates a degree of uncertainty in the forecast and/or expected changing conditions (not that all waves are between 2-4 ft). Generally, it is assumed that individual wave heights can be described using a Rayleigh distribution.
Example: Significant Wave Height = 10 ft
1 in 10 waves will be larger than 11 ft
1 in 100 waves will be larger than 16 ft
1 in 1000 waves will larger than 19 ft
Therefore, assuming a wave period of 8 seconds, for a significant wave height of 10 feet, a wave 19 feet or higher will occur every 8,000 seconds (2.2 hours).
Significant Weather OutlookA narrative statement produced by the National Weather Service, frequently issued on a routine basis, to
provide information regarding the potential of significant weather expected during the next 1 to 5 days.Single Cell ThunderstormThis type of thunderstorm develops in weak vertical wind shear environments. On a hodograph, this would appear as a closely grouped set of random dots around the center of the graph. They are characterized by a single updraft core and a single downdraft that descends into the same area as the updraft. The downdraft and its outflow boundary then cut off the thunderstorm inflow. This causes the updraft and the thunderstorm to dissipate. Single cell thunderstorms are short-lived. They only last about 1/2 hour to an hour. These thunderstorms will occasionally become severe (3/4 inch hail, wind gusts in the excess of 58 miles an hour, or a tornado), but only briefly. In this case, they are called Pulse Severe Thunderstorms. Sleet(PL) - Sleet is defined as pellets of ice composed of frozen or mostly frozen raindrops or refrozen partially melted snowflakes. These pellets of ice usually bounce after hitting the ground or other hard surfaces. Heavy sleet is a relatively rare event defined as an accumulation of ice pellets covering the ground to a depth of ½" or more.Sleet WarningIssued when accumulation of sleet in excess of 1/2" is expected; this is a relatively rare scenario. Usually issued as a winter storm warning for heavy sleet.Slight ChanceIn probability of precipitation statements, usually equivalent to a 20 percent chance.Sling PsychrometerAn instrument used to measure the water vapor content of the atmosphere in which wet and dry bulb thermometers are mounted on a frame connected to a handle at one end by means of a bearing or a length of chain. The psychrometer is whirled by hand to provide the necessary ventilation to evaporate water from the wet bulb.Small Craft Should Exercise CautionPrecautionary statement issued to alert mariners with small, weather sensitive boats. Small Stream FloodingIn hydrologic terms, flooding of small creeks, streams, or runs.Smoke(abbrev. K) Smoke in various concentrations can cause significant problems for people with respiratory ailments. It becomes a more universal hazard when visibilities are reduced to ¼ mile or less.Smoke DispersalDescribes the ability of the atmosphere to ventilate smoke. Depends on the stability and winds in the lower layers of the atmosphere, i.e., a combination of mixing heights and transport winds.Smoke ManagementThe use of meteorology, fuel moisture, fuel loading, fire suppression and burn techniques to keep smoke impacts from prescribed fires within acceptable limits.Smoothed Sunspot NumberAn average of 13 monthly RI numbers, centered on
the month of concern.SnotelSNOw TELemetry - An automated network of snowpack data collection sites. The Natural Resources Conservation Service (NRCS), formerly the Soil Conservation Service (SCS), has operated the Federal-State-Private Cooperative Snow Survey Program in the western United States since 1935. A standard SNOTEL site consists of a snow pillow, a storage type precipitation gage, air temperature sensor and a small shelter for housing electronics. Snow Accumulation and Ablation ModelIn hydrologic terms, a model which simulates snow pack accumulation, heat exchange at the air-snow interface, areal extent of snow cover, heat
storage within the snow pack, liquid water retention, and transmission and heat exchange at the ground-snow interface.Snow CoreA sample of either freshly fallen snow, or the combined old and new snow on the ground. This is obtained by pushing a cylinder
down through the snow layer and extracting it.Snow CorniceA mass of snow or ice projecting over a mountain ridge.Snow Data Assimilation System (SNODAS)A physically based, mass conserving snow water equivalent (SWE) model.Snow DensityThe mass of snow per unit volume which is equal to the water content of the snow divided by its depth.Snow DepthThe combined total depth of both the old and new snow on the ground.Snow FlurriesSnow flurries are an intermittent light snowfall of short duration (generally light snow showers) with no measurable accumulation (trace category).Snow PelletsPrecipitation, usually of brief duration, consisting of crisp, white, opaque ice particles, round or conical in shape and about 2 to 5 mm in diameter. Same as graupel or small hail.Snow ShowerA snow shower is a short duration of moderate snowfall. Some accumulation is possible.Snow StakeA 1-3/4 inch square, semi-permanent stake, marked in inch increments to measure snow depth.Snow Water EquivalentThe water content obtained from melting accumulated snow.SnowcoverAlso known as Snow Pack; the combined layers of snow and ice on the ground at any one time.SnowflakeAn agglomeration of snow crystals falling as a unit.Snowmelt FloodingIn hydrologic terms, flooding caused primarily by the melting of snow.Soil MoistureWater contained in the upper part of the soil mantle. This moisture evaporates from the soil and is the used and transpired by vegetation.Solar CoordinatesIn solar-terrestrial terms, Central Meridian Distance (CMD). The angular distance in solar
longitude measured from the central meridianSolar CycleIn solar-terrestrial terms, the approximately 11-year quasi-periodic variation in frequency or number of solar active events.Solar Sector Boundary (SSB)In solar-terrestrial terms, the apparent solar origin, or base, of the interplanetary sector boundary marked by the larger-scale polarity inversion lines.SolsticeEither of the two times per year when the sun is at its greatest angular distance from the celestial equator: about June 21 (the Northern Hemisphere summer solstice), when the sun reaches its northernmost point on the celestial sphere, or about December 22 (the Northern Hemisphere winter solstice), when it reaches its southernmost point.Southern Oscillation(SO) - a "see-saw" in surface pressure in the tropical Pacific characterized by simultaneously opposite sea level pressure anomalies at Tahiti, in the eastern tropical Pacific and Darwin, on the northwest coast of Australia. The SO was discovered by Sir Gilbert Walker in the early 1920's. Walker was among the first meteorologists to use the statistical techniques to analyze and predict meteorological phenomena. Later, the three-dimensional east-west circulation related to the SO was discovered and named the "Walker Circulation". The SO oscillates with a period of 2-5 years. During one phase, when the sea level pressure is low at Tahiti and High at Darwin, the El Nino occurs. The cold phase of the SO, called "La Nina" by some, is characterized by high pressure in the eastern equatorial Pacific, low in the west, and by anomalously cold sea surface temperature (SST) in the central and eastern Pacific. This is called El Nino Southern Oscillation or ENSO.Southern Oscillation IndexA numerical index measuring the state of the Southern Oscillation. The SOI is based on the (atmospheric) pressure difference between Tahiti and Darwin, Australia. It is highly correlated with tropical sea surface temperature anomaly indices recorded in Niño3.Space Environment Center(SEC) - This center provides real-time monitoring and forecasting of solar and geophysical events, conducts research in solar-terrestrial physics, and develops techniques for forecasting solar and geophysical disturbances. SEC's parent organization is the National Oceanic and Atmospheric Administration (NOAA). SEC is one of NOAA's 12 Environmental Research Laboratories (ERL) and one of NOAA's 9 National Centers for Environmental Prediction (NCEP). SEC's Space Weather Operations is jointly operated by NOAA and the U.S. Air Force and is the national and world warning center for disturbances that can affect people and equipment working in the space environment.Spearhead EchoA radar echo associated with a downburst with a pointed appendage extending toward the direction of the echo motion. The appendage moves much faster than the parent echo, which is drawn into the appendage. During it's mature stage, the appendage turns into a major echo and the parent echo loses its identity.Special Avalanche WarningIssued by the National Weather Service when avalanches are imminent or occurring in the mountains. It is usually issued for a 24 hour period.Special Fire WeatherMeteorological services uniquely required by user agencies which cannot be provided at an NWS office during normal working hours. Examples are on-site support, weather observer training, and participation in user agency training activities.Special Marine Warning(SMW) A warning product issued for potentially hazardous weather conditions usually of short duration (up to 2 hours) producing sustained marine thunderstorm winds or associated gusts of 34 knots or greater; and/or hail 3/4 inch or more in diameter; and/or waterspouts affecting areas included in a Coastal Waters Forecast, a Nearshore Marine Forecast, or an Great Lakes Open Lakes Forecast that is not adequately covered by existing marine warnings. Also used for short duration mesoscale events such as a strong cold front, gravity wave, squall line, etc., lasting less than 2 hours and producing winds or gusts of 34 knots or greater. Special Tropical Disturbance StatementThis statement issued by the National Hurricane Center furnishes information on strong and formative non-depression systems. This statement focuses on the major threat(s) of the disturbance, such as the potential for torrential rainfall on an island or inland area. The statement is coordinated with the appropriate forecast office(s).Specific GravityThe ratio of the density of any substance to the density of water.Specific HumidityIn a system of moist air, the ratio of the mass of water vapor to the total mass of the system.Specific YieldIn hydrologic terms, the ratio of the water which will drain freely from the material to the total volume of the aquifer formation. This value will always be
less than the porosity.Spectral DensityA radar term for the distribution of power by frequency.Spectral Wave DensityOn a buoy report, energy in (meter*meter)/Hz, for each frequency bin (typically from 0.03 Hz to 0.40 Hz).Spectral Wave DirectionOn a buoy report, mean wave direction, in degrees from true North, for each frequency bin.Spectrum WidthThis WSR-88D radar product depicts a full 360 degree sweep of spectrum width data indicating a measure of velocity dispersion within the radar sample volume. It is available for every elevation angle sampled, it provides a measure of the variability of the mean radial velocity estimates due to wind shear, turbulence, and/or the quality of the velocity samples. It is used to estimate turbulence associated with boundaries, thunderstorms, and mesocyclones; check the reliability of the velocity estimates; and locate boundaries (cold front, outflow, lake breeze, etc.).Spectrum Width Cross SectionThis WSR-88D radar product displays a vertical cross section of spectrum width on a grid with heights up to 70,000 feet on the vertical axis and distance up to 124 nm on the horizontal axis. Two end points to create cross section are radar operator selected along a radial or from one AZRAN to another AZRAN within 124 nm of the radar that are less than 124 nm apart. It is used to: 1) Verify features on the Reflectivity Cross Section (RCS) and Velocity Cross Section (VCS) and to evaluate the quality of the velocity data 2) Estimate vertical extent of turbulence (aviation use).Speed ShearThe component of wind shear which is due to a change in wind speed with height, e.g., southwesterly winds of 20 mph at 10,000 feet increasing to 50 mph at 20,000 feet. Speed shear is an important factor in severe weather development, especially in the middle and upper levels of the atmosphere.SPENESNESDIS Satellite Precipitation EstimatesSphere CalibrationReflectivity calibration of a radar by pointing the dish at a metal sphere of (theoretically) known reflectivity. The sphere is often tethered to a balloon.Spillway CrestIn hydrologic terms, the elevation of the highest point of a spillway.Sporadic EIn solar-terrestrial terms, a phenomenon occurring in the E region of the
ionosphere, which significantly affects HF radiowave
propagation. Sporadic E can occur during daytime or nighttime
and it varies markedly with latitude.Spring TideA tide higher than normal which occurs around the time of the new and full moon.SprinkleVery light rain showers. Precipitation measurement is a trace.Squall LineA line of active thunderstorms, either continuous or with breaks, including contiguous precipitation areas resulting from the existence of the thunderstorms.St Lawrence Freeze-Up Outlook A National Weather Service forecast product to keep mariners informed of the projected freeze-up date of ice the St. Lawrence River.St. Elmo's FireThe glow on a masthead produced by an extreme buildup of electrical charge. Unprotected mariners should immediately move to shelter when this phenomena occurs. Lightning may strike the mast within five minutes after it begins to glow.Stability IndexThe overall stability or instability of a sounding is sometimes conveniently expressed in the form of a single numerical value. Used alone, it can be quite misleading, and at times, is apt to be worthless. The greatest value of an index lies in alerting the forecaster to those soundings which should be examined more closely. StableAn atmospheric state with warm air above cold air which inhibits the vertical movement of
air.Stable Boundary LayerThe stably-stratified layer that forms at the surface and grows upward, usually at night or in winter, as heat is extracted from the atmosphere's base in response to longwave radiative heat loss from the ground. Stable boundary layers can also form when warm air is advected over a cold surface or over melting ice.Stable CorePost-sunrise, elevated remnant of the temperature inversion that has built up overnight within a valley.StageThe level of the water surface of a river or stream above an established datum at a given location. Stair SteppingIn hydrologic terms, the process of continually updating river forecasts for the purpose of incorporating the effects rain that has fallen since the previous
forecast was prepared.Standard AtmosphereA hypothetical vertical distribution of atmospheric temperature, pressure, and density that, by international agreement, is taken to be representative of the atmosphere for purposes of pressure altimeter calibrations, aircraft performance calculations, aircraft and missile design, ballistic tables, etc.Standard Synoptic TimesThe times of 0000, 0600, 1200, and 1800 UTC. Also known as the main synoptic times. State Forecast ProductThis National Weather Service product is intended to give a good general picture of what weather may be expected in the state during the next 5 days. The first 2 days of the forecast is much more specific than the last 3 days. In comparison with the Zone Forecast Product, this product will be much more general.State Weather RoundupThis is a National Weather Service tabular product which provides routine hourly observations within the state through the National Weather Wire Service (NWWS). It gives the current weather condition in one word (cloudy, rain, snow, fog, etc.), the temperature and dew point in Fahrenheit, the relative humidity, wind speed and direction, and finally additional information (wind chill, heat index, a secondary weather condition). These reports are broken up regionally. When the complementary satellite product is not available, reports from unaugmented ASOS stations will report "fair" in the sky/weather column when there are few or no clouds (i.e., scattered or less) below 12,000 feet with no significant weather and/or obstructions to visibility.Station ModelA specified pattern for plotting, on a weather map, the meteorological symbols that represent the state of the weather at a particular observing station.Station PressureThe absolute air pressure at a given reporting station. The air pressure is directly proportional to the combined weight of all air in the atmosphere located in a column directly above the reporting site. Consequently, the station pressure may vary tremendously from one location to another in mountainous regions due to the strong variation of atmospheric pressure with height. Vertical variations of pressure range up to 150 mb per mile whereas horizontal variations are usually less than .1 mb per mile.Steam FogFog formed when water vapor is added to air which is much colder than the source of the vapor. It may be formed when very cold air drifts across relatively warm water. At temperatures below
about-20°F, ice particles or droxtals may be formed in the air producing a type of ice fog known as frost smoke.
SteepnessIn marine terms, on a buoy report, wave steepness is the ratio of wave height to wave length and is an indicator of wave stability. When wave steepness exceeds a 1/7 ratio, the wave becomes unstable and begins to break.Steering CurrentsSame as Steering Winds; a prevailing synoptic scale flow which governs the movement of smaller features embedded within it.Steering WindsSame as Steering Currents; A prevailing synoptic scale flow which governs the movement of smaller features embedded within it.Stepped LeaderA faint, negatively charged channel that emerges from the base of a thunderstorm and propagates toward the ground in a series of steps of about 1 microsecond duration and 50-100 meters in length, initiating a lightning stroke.Storm RelativeMeasured relative to a moving thunderstorm, usually referring to winds, wind shear, or helicity.Storm Relative Mean Radial Velocity Map(SRM): This WSR-88D radar product depicts a full 360º sweep of radial velocity data with the average motion of all identified storms subtracted out. It is available for every elevation angle sampled. It is used to aid in displaying shear and rotation in storms and storm top divergence that might otherwise be obscured by the storm's motion, investigate the 3-D velocity structure of a storm, and help with determining rotational features in fast and uniform moving storms. Storm Relative Mean Radial Velocity Regi(SRR): This WSR-88D radar product depicts a 27
nm by 27 nm region of storm relative mean radial velocity centered on a point which the operator
can specify anywhere within a 124 nm radius of the radar. The storm motion subtracted defaults to
the motion of the storm closest to the product center, or can be input by the operator. It is used to
examine the 3-dimensional storm relative flow of a specific thunderstorm (radar operator centers
product on a specific thunderstorm; aid in displaying shear and rotation in thunderstorms and
storm top divergence that might otherwise be obscured by storm motion; and gain higher
resolution velocity productStorm ScaleReferring to weather systems with sizes on the order of individual thunderstorms. See synoptic scale and mesoscale.Storm SurgeAn abnormal rise in sea level accompanying a hurricane or other intense storm, whose height is the difference between the observed level of the sea surface and the level that would have occurred in the absence of the cyclone. Storm surge is usually estimated by subtracting the normal or astronomic tide from the observed storm tide.Storm TideThe actual level of sea water resulting from the astronomic tide combined with the storm surge. Most NWS flood statements, watches, or warnings quantifying above-normal tides will report the Storm Tide.Storm Total PrecipitationThis radar image is an estimate of accumulated rainfall since the last time there was a one-hour, or more, break in precipitation. It is used to locate flood potential over urban or rural areas, estimate total basin runoff and provide rainfall accumulations for the duration of the event and is available only for the short range (out to 124 nm). To determine accumulated precipitation at greater distances you should link to an adjacent radar. Stormwater DischargeIn hydrologic terms, precipitation that does not infiltrate into the ground or evaporate due to impervious land surfaces but instead flows onto adjacent
land or water areas and is routed into drain/sewer systems.Straight-Line HodographThe name pretty well describes what it looks like on the hodograph. What causes this shape is a steady increase of winds with height (vertical wind shear). This shape of hodograph favors multicell thunderstorms.Straight-line WindsGenerally, any wind that is not associated with rotation, used mainly to differentiate them from tornadic winds.StratopauseThe boundary between the stratosphere and mesosphere.StratosphereThe region of the atmosphere extending from the top of the troposphere to the base of the mesosphere, an important area for monitoring stratospheric ozone.Stratospheric OzoneIn the stratosphere, ozone has beneficial properties where it forms an ozone shield that prevents dangerous radiation from reaching the Earth's surface. Recently, it was discovered that in certain parts of the world, especially over the poles, stratospheric ozone was disappearing creating an ozone hole.Stream lineArrows on a weather chart showing wind speed and direction. The head of the arrow points toward where the wind is blowing and the length of the arrow is proportional to the wind speed. Sometimes shows wind direction and trajectory only. Stream ReachA significant segment of surface water that has similar hydrologic characteristics, such as a stretch of stream/river between two confluences.StreamflowIn hydrologic terms, water flowing in the stream channel. It is often used interchangeably with discharge. StrikeFor any particular location, a hurricane strike occurs if that
location passes within the hurricane's strike circle, a circle of
125 n mi diameter, centered 12.5 n mi to the right of the
hurricane center (looking in the direction of motion). This circle is
meant to depict the typical extent of hurricane force winds, which
are approximately 75 n mi to the right of the center and 50 n mi to
the left. Sublimation of iceThe transition of water from solid to gas without passing through the liquid phase.SubrefractionThe bending of the radar beam in the vertical which is less than under standard
refractive conditions. This causes the beam to be higher than indicated, and lead to the
underestimation of cloud heights. Subsidence1. A descending motion of air in the atmosphere occurring over a rather broad area.
2. In hydrologic terms, sinking down of part of the earth's crust due to underground excavation, such as the removal of groundwater.Subsidence InversionA temperature inversion that develops aloft as a result of air gradually sinking over a wide area and being warmed by adiabatic compression, usually associated with subtropical high pressure areas.Subsurface Storm FlowIn hydrologic terms, the lateral motion of water through the upper layers until it enters a stream channel. This usually takes longer to reach stream
channels than runoff. This also called interflow.Subtle Heavy Rainfall SignatureThis heavy rain signature is often difficult to detect on satellite. These warm top thunderstorms are often embedded in a synoptic-scale cyclonic circulation. Normally, they occur when the 500 mb cyclonic circulation is quasi-stationary or moves slowly to the east or northeast (about 2 degrees per 12 hours). The average surface temperature is 68ºF with northeasterly winds. The average precipitable water (P) value is equal to or greater than 1.34 inches and the winds veer with height, but they are relatively light. The heavy rain often occurs north and east of the vorticity maximum across the lower portion of the comma head about 2 to 3 degrees north or northeast of the 850 mb low.Subtropical CycloneA non-frontal low pressure system that has characteristics of both tropical and extratropical cyclones. This system is typically an upper-level cold low with circulation extending to the surface layer and maximum sustained winds generally occurring at a radius of about 100 miles or more from the center. In comparison to tropical cyclones, such systems have a relatively broad zone of maximum winds that is located farther from the center, and typically have a less symmetric wind field and distribution of convection.Subtropical DepressionA subtropical cyclone in which the maximum 1-minute sustained surface wind is 33 knots (38 mph) or less. Subtropical Jet(Abbrev. STJ) - this jet stream is usually found between 20° and 30° latitude at altitudes between 12 and 14 km.Suction VortexA small but very intense vortex within a tornado circulation. Several suction vortices typically are present in a multiple-vortex tornado. Much of the extreme damage associated with violent tornadoes (F4 and F5 on the Fujita scale) is attributed to suction vortices.Sudden Commencement (SC)In solar-terrestrial terms, an abrupt
increase or decrease in the northward component of the geomagnetic
field, which marks the beginning of a geomagnetic storm.Sudden Impulse (SI+ or SI-)In solar-terrestrial terms, a sudden perturbation of several gammas
in the northward component of the low-latitude geomagnetic field,
not associated with a following geomagnetic storm. (An SI becomes
an SC if a storm follows.)Sudden Ionospheric Disturbance (SID)In solar-terrestrial terms, HF propagation anomalies due to ionospheric changes resulting from solar flares, proton events and geomagnetic storms.Summation PrincipleThis principle states that the sky cover at any level is equal to the summation of the sky cover of the lowest layer plus the additional sky cover
provided at all successively higher layers up to and including the layer in question.SummerTypically the warmest season of the year during which the sun is most nearly overhead. In the Northern Hemisphere, summer customarily includes the
months of June, July, and August.Summer SolsticeThe time at which the sun is farthest north in the Northern Hemisphere, on or around June 21.SunriseThe phenomenon of the sun's daily appearance on the eastern horizon as a result of the earth's rotation. The word is often used to refer to the time at which the first part of the sun becomes visible in the morning at a given location.SunsetThe phenomenon of the sun's daily disappearance below the western horizon as a result of the earth's rotation. The word is often used to refer to the time at which the last part of the sun disappears below the horizon in the evening at a given location.Sunspot NumberIn solar-terrestrial terms, a daily index of sunspot activity (R), defined as
R = k (10 g + s) where S = number of individual spots, g =
number of sunspot groups, and k is an observatory factor.Super TyphoonTyphoon having maximum sustained winds of 130 knots (150 mph) or greater.SupercellShort reference to Supercell Thunderstorm; potentially the most dangerous of the convective storm types. Storms possessing this structure have been observed to generate the vast majority of long-lived strong and violent (F2-F5) tornadoes, as well as downburst damage and large hail. It is defined as a thunderstorm consisting of one quasi-steady to rotating updraft which may exist for several hours.Supercell ThunderstormPotentially the most dangerous of the convective storm types. Storms possessing this structure have been observed to generate the vast majority of long-lived strong and violent (F2-F5) tornadoes, as well as downburst damage and large hail. It is defined as a thunderstorm consisting of one quasi-steady to rotating updraft which may exist for several hours. Supercells usually move to the right of the mean wind. These are called "Right Movers" and they are favored with veering winds. Occasionally, these thunderstorms will move to the left of the mean wind. These thunderstorms are called "Left Movers". These supercells typically don't last as long as their "Right Mover" cousins and they usually only produce large hail (greater than 3/4 inch in diameter) and severe wind gusts in the excess of 58 miles an hour. Left Movers are favored when you have backing winds.
Radar will observe essentially one long-lived cell, but small perturbations to the cell structure may be evident. The stronger the updraft, the better the chance that the supercell will produce severe (hail greater than 3/4 inch in diameter, wind gusts greater than 58 miles an hour, and possibly a tornado) weather.
Severe supercell development is most likely in an environment possessing great buoyancy (CAPE) and large vertical wind shear. A Bulk Richardson Number of between 15 and 35 favor supercell development. Typically, the hodograph will look like a horse shoe. This is due to the wind speed increasing rapidly with height and the wind direction either veering or backing rapidly with height.SupercoolTo cool a liquid below its freezing point without solidification or crystallization.Supercooled Liquid WaterIn the atmosphere, liquid water can survive at temperatures colder than 0 degrees Celsius; many vigorous storms contain large amounts of supercooled liquid water at cold temperatures. Important in the formation of graupel and hail.SuperrefractionBending of the radar beam in the vertical which is greater than sub-standard refractive conditions. This causes the beam to be lower than indicated, and often results in extensive ground clutter as well as an overestimation of cloud top heights.Surcharge CapacityIn hydrologic terms, the volume of a reservoir between the maximum water surface elevation for which the dam is designed and the crest of an
uncontrolled spillway, or the normal full-pool elevation of the reservoir with the crest gates in the normal closed position.Surf ZoneArea of water between the high tide level on the beach and the seaward side of the breaking waves. Surf Zone Forecast(SRF) - A National Weather Service routine or event driven forecast product geared toward non-boating marine users issued for an area extending from the area of water between the high tide level on the beach and the seaward side of the breaking waves. Surface Energy BudgetThe energy or heat budget at the earth's surface, considered in terms of the fluxes through a plane at the earth-atmosphere interface. The energy budget includes radiative, sensible, latent and ground heat fluxes.Surface impoundmentIn hydrologic terms, an indented area in the land's surface, such as a pit, pond, or lagoon.Surface RunoffIn hydrologic terms, the runoff that travels overland to the stream channel. Rain that falls on the stream channel is often lumped with this quantity.Surface WaterWater that flows in streams and rivers and in natural lakes, in wetlands, and in reservoirs constructed by humans.Surface Weather ChartAn analyzed synoptic chart of surface weather observations. A surface chart shows the distribution of sea-level pressure (therefore, the position of highs, lows, ridges and troughs) and the location and nature of fronts and air masses. Often added to this are symbols for occurring weather phenomena. Although the pressure is referred to mean sea level, all other elements on this chart are presented as they occur at the surface point of observation.Surface-based ConvectionConvection occurring within a surface-based layer, i.e., a layer in which the lowest portion is based at or very near the earth's surface. Compare with elevated convection.SurgeIn solar-terrestrial terms, a jet of material from active regions that reaches coronal heights and then either fades or returns into the chromosphere along the trajectory of ascent.Sustained OverdraftIn hydrologic terms, long-term withdrawal from the aquifer of more water than is being recharged.Sustained WindWind speed determined by averaging observed values over a two-minute period.SWESnow Water Equivalent (the amount of water content in a snowpack or snowfall).SWEATSevere Weather ThrEAT index; a stability index developed by the Air Force which incorporates instability, wind shear, and wind speeds as follows:
SWEAT=(12 Td 850 ) + (20 [TT-49]) +( 2 f 850) + f 500 + (125 [s+0.2]) where
- Td 850 is the dew point temperature at 850 mb,
- TT is the total-totals index,
- f 850 is the 850-mb wind speed (in knots),
- f 500 is the 500-mb wind speed (in knots), and
- s is the sine of the angle between the wind directions at 500 mb and 850 mb (thus representing the directional shear in this layer).
SWEAT values of about 250-300 or more indicate a greater potential for severe weather, but as with all stability indices, there are no magic numbers.
The SWEAT index has the advantage (and disadvantage) of using only mandatory-level data (i.e., 500 mb and 850 mb), but has fallen into relative disuse with the advent of more detailed upper air sounding analysis programs.SwellWind-generated waves that have travelled out of their generating area. Swells characteristically exhibit smoother, more regular and uniform crests and a longer period than wind waves.Swell DirectionThe direction from which the swells are propagating.Symmetric Double EyeA concentrated ring of convection that develops outside the eye wall in symmetric, mature hurricanes. The ring then propagates inward and leads to a double-eye. Eventually, the inner eye wall dissipates while the outer intensifies and moves inward. Synchronous DetectionRadar processing that retains the received signal amplitude and phase but that removes the intermediate frequency carrier.Synoptic CodeRules and procedures established by the World Meteorological Organization(WMO) for encoding weather observations. Synoptic ScaleThe spatial scale of the migratory high and low pressure systems of the lower troposphere, with wavelengths of 1000 to 2500 km.Synoptic WeatherWeather occurring over a wide region on time scales exceeding 12 hours.Synthetic Aperture Radar (SAR)A radar mounted on a moving platform (aircraft or satellite) used for imaging. Since the radar antenna moves a significant distance between transmission and receiving the signal back, the radar antenna acts as if it is larger than its physical dimensions, providing better range and azimuth resolution.Synthetic Aperture Radar River Ice Surveillance (SARRIS)An experimental river ice mapping experiment using Synthetic Aperture Radar (SAR).T -NumberA system used to subjectively estimate tropical cyclone intensity based solely on visible and infrared satellite images. Also called the Dvorak technique.Tail-End CharlieSlang for the thunderstorm at the southernmost end of a squall line or other line or band of thunderstorms. Since low-level southerly inflow of warm, moist air into this storm is relatively unimpeded, such a storm often has a higher probability of strengthening to severe levels than the other storms in the line.Tailwater HeightIn hydrologic terms, height of water immediately downstream of the dam.TargetPrecipitation or other phenomena which produces echoes on a radar display.TeleconnectionLinkage between changes in atmospheric circulation occurring in widely separated parts of the globe.TEMPTemperature- A measure of the internal energy that a substance contains. This is the most measured quantity in the atmosphere.Temperature(Abbrev. TEMP)- The temperature is a measure of the internal energy that a substance contains. This is the most measured quantity in the atmosphere.Temperature Inversion(surface-based or elevated) : a layer of the atmosphere in which air temperature increases with height. When the layer's base is at the surface, the layer is called a surface-based temperature inversion; when the base of the layer is above the surface, the layer is called an elevated temperature inversion.Temperature RecoveryThe change in temperature over a given period of time. Generally, the period between late evening and sunrise. Windy or cloudy conditions will tend to produce slow temperature recovery, while clear, calm weather can cause rapid recovery.TEMPStemperaturesTerminal Aerodrome ForecastThis NWS aviation product is a concise statement of the expected meteorological conditions at an airport during a specified period (usually 24 hours). Each country is allowed to make modifications or exceptions to the code for use in each particular country. TAFs use the same weather code found in METAR weather reports.Terrain Forced FlowAn airflow that is modified or channeled as it passes over or around mountains or through gaps in a mountain barrier.Texas HookerSame as Panhandle Hook - low pressure systems that originate in the panhandle region of Texas and Oklahoma which initially move east and then "hook" or recurve more northeast toward the upper Midwest or Great Lakes region. In winter, these systems usually deposit heavy snows north of their surface track. Thunderstorms may be found south of the track. ThalwegIn hydrologic terms, the line of maximum depth in a stream. The thalweg is the part that has the maximum velocity and causes cutbanks and channel
migration.TheodoliteAn instrument used in surveying to measure horizontal and vertical angles with a small telescope that can move in the horizontal and vertical planes. It used to track the movements of either a ceiling balloon or a radiosonde.ThermalA relatively small-scale, rising air current produced when the Earth's surface is heated. Thermals are a common source of low level turbulence for aircraft. Thermal BeltA zone of high nighttime temperatures (and relatively low humidities) that is often experienced within a narrow altitude range on valley sidewalls, especially evident during clear weather with light winds.Thermal HighArea of high pressure that is shallow in vertical extent and produced primarily by cold surface temperatures.Thermal LowArea of low pressure that is shallow in vertical extent and produced primarily by warm surface temperatures.Thermal WindIt is a theoretical wind that blows parallel to the thickness lines, for the layer considered, analogous to how the geostrophic wind blows parallel to the height contours. The closer the thickness isopleths, the stronger the thermal wind. Cold air is always located to the left of the thermal wind (as you face downstream) and the warm air is located on the right. Since thickness contours are tighter on the cold side of thermal wind, your lower thickness values will be found on the left side of the thermal wind. The speed and direction of the thermal wind are determined by vector geometry where the geostrophic wind at the upper level is subtracted from the geostrophic wind at the lower level.Thermally Driven CirculationA diurnally reversing closed cellular wind current resulting from horizontal temperature contrasts caused by different rates of heating or cooling over adjacent surfaces; includes along-slope, cross-valley, along-valley, mountain-plain and sea breeze circulations.ThermistorA resistor whose resistance changes with temperature. Because of the known dependence of resistance on temperature, the resistor can be used as a temperature sensor.ThermoclineAs one descends from the surface of the ocean, the temperature remains nearly the same as it was at the surface, but at a certain depth temperature starts decreasing rapidly with depth. This boundary is called the thermocline. In studying the tropical Pacific Ocean, the depth of 20ºC water ("the 20ºC isotherm") is often used as a proxy for the depth of the thermocline. Along the equator, the 20ºC isotherm is typically located at about 50 m depth in the eastern Pacific, sloping downwards to about 150 m in the western Pacific.Thermodynamic ChartA chart containing contours of pressure, temperature, moisture, and potential temperature, all drawn relative to each other such that basic thermodynamic laws are satisfied. Such a chart typically is used to plot atmospheric soundings, and to estimate potential changes in temperature, moisture, etc. if air were displaced vertically from a given level. A thermodynamic chart thus is a useful tool in diagnosing atmospheric instability.Thermodynamic DiagramUsed interchangably with Thermodynamic Chart; a chart containing contours of pressure, temperature, moisture, and potential temperature, all drawn relative to each other such that basic thermodynamic laws are satisfied. Such a chart typically is used to plot atmospheric soundings, and to estimate potential changes in temperature, moisture, etc. if air were displaced vertically from a given level. A thermodynamic chart thus is a useful tool in diagnosing atmospheric instability.ThermodynamicsIn general, the relationships between heat and other properties (such as temperature, pressure, density, etc.) In forecast discussions, thermodynamics usually refers to the distribution of temperature and moisture (both vertical and horizontal) as related to the diagnosis of atmospheric instability.ThermographAn instrument that measures and records air temperature.ThermometerAn instrument for measuring air temperature.ThermosphereThe atmospheric shell extending from the top of the mesosphere to outer space. It is a region of more or less steadily increasing temperature with height, starting at 70 or 80 km.Theta-e(or Equivalent Potential Temperature) - The temperature a parcel of air would have if a) it was lifted until it
became saturated, b) all water vapor was condensed out, and c) it was returned adiabatically (i.e., without
transfer of heat or mass) to a pressure of 1000 millibars. Theta-e, which typically is expressed in degrees
Kelvin, is directly related to the amount of heat present in an air parcel. Thus, it is useful in diagnosing
atmospheric instability. Theta-e RidgeAn axis of relatively high values of theta-e. Severe weather and excessive rainfall often occur near or just upstream from a theta-e ridge.THETAEAbbreviation for Theta-e; the temperature a parcel of air would have if a) it was lifted until it became saturated, b) all water vapor was condensed out, and c) it was returned adiabatically (i.e., without transfer of heat or mass) to a pressure of 1000 millibars. Theta-e, which typically is expressed in degrees Kelvin, is directly related to the amount of heat present in an air parcel. Thus, it is useful in diagnosing atmospheric instability.Thin Line EchoA narrow, elongated, non-precipitating echo. It is usually associated with
thunderstorm outflows, fronts, or other density discontinuities. It is also known as a Fine Line.Three-Hour Rainfall RateThis WSR-88D Radar product displays precipitation total (in inches) of the current and past two clock hours as a graphical image. It displays hourly precipitation total (in inches) as a graphical image (polar format with resolution 1.1 nm by 1 degree). It is updated once an hour. It is used to: 1) Assess rainfall intensities and amounts over a longer viewing interval; and 2) Possibly adjust flash flood guidance values since the product corresponds to the timing of Flash Flood Guidance values.Threshold RunoffIn hydrologic terms, the runoff in inches from a rain of specified duration that causes a small stream to slightly exceed bankfull. When available, flood
stage is used instead of slightly over bankfull. ThunderThe sound caused by rapidly expanding gases in a lightning discharge.ThunderstormA local storm produced by a cumulonimbus cloud and accompanied by lightning and thunder.Tidal CycleThe periodic changes in the intensity of tides caused primarily by the varying relations between the earth, moon, and sun.Tidal WaveSee TSUNAMITIDEOn a buoy report, the water level in feet above or below Mean Lower Low Water (MLLW).Tide AnomalyActual water level minus the prediction. Tide PredictionThe computation of tidal highs and lows at a given location resulting from the gravitational interactions between the earth and primarily the moon and sun.TidesThe periodic (occurring at regular intervals) variations in the surface water level of the oceans, bays, gulfs, and inlets. Tides are the result of the gravitiational attraction of the sun and the moon on the earth. The attraction of the moon is far greater than the attraction of the sun due to the close proximity of the earth and the moon. The sun is 360 times further from the earth than the moon. Therefore, the moon plays a larger role than the sun in producing tides. Every 27.3 days, the earth and the moon revolve around a common point. This means that the oceans and other water bodies which are affected by the earth-moon system experience a new tidal cycle every 27.3 days. Because of the physical processes which occur to produce the tidal system, there are two high tides and two low tides each day. Because of the angle of the moon with respect to the earth, the two high tides each day do not have to be of equal height. The same holds true for the two low tides each day. Tides also differ in height on a daily basis. The daily differences between tidal heights is due to the changing distance between the earth and the moon. Scientists use measurements of the height of the water level to examine tides and the various phenomena which influence tides, such as hurricanes and winter storms.Tilt SequenceRadar term indicating that the radar antenna is scanning through a series of antenna elevations in order to obtain a volume scan.Tilted StormA thunderstorm or cloud tower which is not purely vertical but instead exhibits a slanted or tilted character. It is a sign of vertical wind shear, a favorable condition for severe storm development.Tilted UpdraftA thunderstorm updraft which is not purely vertical but instead exhibits a slanted or tilted character. It is a sign of vertical wind shear, a favorable condition for severe storm development.Tipping-Bucket Rain GageA precipitation gage where collected water is funneled into a two compartment bucket; 0.01, 0.1 mm, or some other designed quantity of rain will fill one compartment and overbalance the bucket so that it tips, emptying into a reservoir and moving the second compartment into place beneath the funnel. As the bucket is tipped, it actuates an electric circuit.TKETurbulent Kinetic EnergyToe of Dam (Upstream and Downstream) :
The junction of the face of a dam with the ground surfaceTornado EmergencyAn exceedingly rare tornado warning issued when there is a severe threat to human life and catastrophic damage from an imminent or ongoing tornado. This tornado warning is reserved for situations when a reliable source confirms a tornado, or there is clear radar evidence of the existence of a damaging tornado, such as the observation of debris.Tornado Vortex SignatureAn image of a tornado on the Doppler radar screen that shows up as a small region of rapidly changing wind speeds inside a mesocyclone. The following velocity criteria is normally required for recognition: velocity difference between maximum inbound and outbound (shear) is greater than or equal to 90 knots at less than 30 nmi and is greater than or equal to 70 knots between 30 and 55 nmi. It shows up as a red upside down triangle on the Storm Relative Velocity Display. Existence of a TVS strongly increases the probability of tornado occurrence, but does not guarantee it; therefore, the feature triggering it must be examined closely by the radar operator. A TVS is not a visually observable feature.Total-Totals IndexA stability index and severe weather forecast tool, equal to the temperature at 850 mb plus the dew point at 850 mb, minus twice the temperature at 500 mb. The total-totals index is the arithmetic sum of two other indices: the Vertical Totals Index (temperature at 850 mb minus temperature at 500 mb) and the Cross Totals Index (dew point at 850 mb minus temperature at 500 mb). As with all stability indices there are no magic threshold values, but in general, values of less than 50 or greater than 55 are considered weak and strong indicators, respectively, of potential severe storm development.Towering CumulusA large cumulus cloud with great vertical development, usually with a cauliflower-like appearance, but lacking the characteristic anvil of a Cb. (Often shortened to "towering cu," and abbreviated TCU.)TraceIn hydrologic terms, a hydrograph or similar plot for an extended-range time horizon showing one of many scenarios generated through an ensemble
forecast process.Trade WindsPersistent tropical winds that blow from the subtropical high pressure centers towards the equatorial low.TransmitterThe radar equipment used for generating and amplifying a radio frequency (RF) carrier signal, modulating the carrier signal with intelligence, and feeding the modulated carrier to an antenna for radiation into space as electromagnetic waves. Weather radar transmitters are usually magnetrons or klystrons.Transverse BandsBands of clouds oriented perpendicular to the flow in which they are embedded. They often are seen best on satellite photographs. When observed at high levels (i.e., in cirrus formations), they may indicate severe or extreme turbulence. Transverse bands observed at low levels (called transverse rolls or T rolls) often indicate the presence of a temperature inversion (or cap) as well as directional shear in the low- to mid-level winds. These conditions often favor the development of strong to severe thunderstorms.Transverse RollsElongated low-level clouds, arranged in parallel bands and aligned parallel to the low-level winds but perpendicular to the mid-level flow. Transverse rolls are one type of transverse band, and often indicate an environment favorable for the subsequent development of supercells. Since they are aligned parallel to the low-level inflow, they may point toward the region most likely for later storm development.TrapperA valley or basin in which cold air becomes trapped or pooled.Travel TimeIn hydrologic terms, the time required for a flood wave to travel from one location to a subsequent location downstream.Triple DopplerSince any wind has three components (say, in the x, y and z directions), and a single radar measures in only one direction (radial), a single radar cannot give the 3D winds everywhere it samples. However, if three different radars view a storm from three different locations, the 3 measured radial velocities can be transformed into the actual 3D wind field.Triple PointThe intersection point between two boundaries (dry line, outflow boundary, cold front, etc.), often a focus for thunderstorm development. Triple point also may refer to a point on the gust front of a supercell, where the warm moist inflow, the rain-cooled outflow from the forward flank downdraft, and the rear flank downdraft all intersect; this point is a favored location for tornado development (or redevelopment).Tropical Analysis and Forecast BranchOne of three branches of the Tropical Prediction Center (TPC). It provides year-round products involving marine forecasting, aviation forecasts and warnings (SIGMETs), and surface analyses. The unit also provides satellite interpretation and satellite rainfall estimates for the international community. In addition, TAFB provides support to NHC through manpower and tropical cyclone intensity estimates from the Dvorak technique.Tropical CycloneA warm-core, non-frontal synoptic-scale cyclone, originating over tropical or subtropical waters with organized deep convection and a closed surface wind circulation about a well-defined center.Tropical Cyclone Plan of the DayA coordinated mission plan that tasks operational weather reconnaissance requirements during the next 1100 to 1100 UTC day or as required, describes reconnaissance flights committed to satisfy both operational and research requirements, and identifies possible reconnaissance requirements for the succeeding 24-hour period.Tropical Cyclone Position EstimateThe National Hurricane Center issues a position estimate between scheduled advisories whenever the storm center is within 200 nautical miles of U.S. land-based weather radar and if sufficient and regular radar reports are available to the hurricane center. As far as is possible, the position estimate is issued hourly near the beginning of the hour. The location of the eye or storm center is given in map coordinates and distance and direction from a well-known point. Tropical Cyclone UpdateThis brief statement is issued by the National Hurricane Center in lieu of or preceding special advisories to inform of significant changes in a tropical cyclone or the posting or cancellation of watches and warnings.Tropical DepressionA tropical cyclone in which the maximum 1-minute sustained surface wind is 33 knots (38 mph) or less.Tropical DisturbanceA discrete tropical weather system of apparently organized convection--generally 100 to 300 mi in diameter--originating in the tropics or subtropics, having a nonfrontal migratory character and maintaining its identity for 24 hours or more. It may or may not be associated with a detectable perturbation of the wind field.Tropical Wave(formerly known as inverted trough) - A trough or cyclonic curvature maximum in the trade wind easterlies. The wave may reach maximum amplitude in the lower middle troposphere or may be the reflection of an upper tropospheric cold low or an equatorward extension of a mid-latitude trough.Tropical Weather DiscussionThese messages are issued 4 times daily by the Tropical Analysis and Forecast Branch (TAFB) to describe significant synoptic weather features in the tropics. One message will cover the Gulf of Mexico, the Caribbean, and the Atlantic between the equator and 32 degrees North and east of 140 degrees West. Plain language is used in these discussions.Tropical Weather OutlookThis outlook normally covers the tropical and subtropical waters, discussing the weather conditions, emphasizing any disturbed and suspicious areas which may become favorable for tropical cyclone development within the next day to two. In the Atlantic, the outlook is transmitted daily at 0530, 1130, 1730, and 2230 Eastern local time. In the eastern Pacific, it is transmitted daily at 0100, 0700, 1300, and 1900 Eastern local time. For the Central Pacific, transmission times are 1000 and 2200 UTC. Existing tropical and subtropical cyclones are mentioned, as are depressions not threatening land. Given for each system are its location, size, intensity, and movement. For the first 24 hours of a depression or tropical cyclone, the outlook includes a statement identifying the AFOS and World Meteorological Organization (WMO) headers for the advisory on it.Tropical Weather SummaryThe National Hurricane Center issues a monthly summary of tropical weather is included at the end of the month or as soon as feasible thereafter, to describe briefly the past activity or lack thereof and the reasons why.TropopauseThe upper boundary of the troposphere, usually characterized by an abrupt change in lapse rate from positive (decreasing temperature with height) to neutral or negative (temperature constant or increasing with height).Tropopause JetType of jet stream found near the tropopause. Examples of this type of jet are the subtropical and polar fronts.TroposphereThe layer of the atmosphere from the earth's surface up to the tropopause, characterized by decreasing temperature with height (except, perhaps, in thin layers - see inversion, cap), vertical wind motion, appreciable water vapor content, and sensible weather (clouds, rain, etc.).True WindWind relative to a fixed point on the earth. Wind relative to a moving point is called APPARENT or RELATIVE WIND.Tsunami Information StatementA tsunami information statement is issued to inform emergency management officials and the public that an earthquake has occurred, or that a tsunami warning, watch or advisory has been issued for another section of the ocean. In most cases, information statements are issued to indicate there is no threat of a destructive tsunami and to prevent unnecessary evacuations as the earthquake may have been felt in coastal areas. An information statement may, in appropriate situations, caution about the possibility of destructive local tsunamis. Information statements may be re-issued with additional information, though normally these messages are not updated. However, a watch, advisory or warning may be issued for the area, if necessary, after analysis and/or updated information becomes available. Tule FogRadiation fog in the Central Valley of California. It forms during night and morning hours in late fall and winter months following the first significant rainfall. A leading cause of weather related casualties in California.TurbulenceIrregular motion of the atmosphere, as indicated by gusts and lulls in the wind.Turkey TowerSlang for a narrow, individual cloud tower that develops and falls apart rapidly. The sudden development of turkey towers from small cumulus clouds may signify the breaking of a cap.TWEBTranscribed Weather Broadcasts - This NWS aviation product is similar to the Area Forecast (AF) except information is contained in a route format. Forecast sky cover (height and amount of cloud bases), cloud tops, visibility (including vertical visibility), weather, and obstructions to vision are described for a corridor 25 miles either side of the route. Cloud bases and tops are always Mean Sea Level (MSL) unless noted. Ceilings are always above ground level.TwisterIn the United States, a colloquial term for a tornado.Two-Ribbon FlareIn solar-terrestrial terms, a flare that has developed as a pair of bright strands
(ribbons) on both sides of the main inversion ("neutral") line
of the magnetic field of the active region.Type I, II, III, IV(In solar-terrestrial terms) -Emissions of the sun in radio wavelengths from centimeters to dekameters, under both quiet and disturbed conditions. Type I. A noise storm composed of many short, narrow-band bursts in the metric range (300 - 50 MHz). Type II. Narrow-band emission that begins in the meter range (300 MHz) and sweeps slowly (tens of minutes) toward deka- meter wavelengths (10 MHz). Type II emissions occur in loose association with major FLAREs and are indicative of a shock wave moving through the solar atmosphere. Type III. Narrow-band bursts that sweep rapidly (seconds) from decimeter to dekameter wavelengths (500 - 0.5 MHz). They often occur in groups and are an occasional feature of complex solar ACTIVE REGIONs. Type IV. A smooth continuum of broad-band bursts primarily in the meter range (300 - 30 MHz). These bursts are associated with some major flare events beginning 10 to 20 minutes after the flare maximum, and can last for hoursTyphoon SeasonThe part of the year having a relatively high incidence of tropical cyclones. In the western North Pacific, the typhoon season is from July 1 to December 15. Tropical cyclones can occur year-round in any basin.U.S. Geological Survey(Abbrev. USGS)- The Federal Agency chartered in 1879 by congress to classify public lands, and to examine the geologic structure, mineral
resources, and products of the national domain. As part of its mission, the USGS provides information and data on the Nation’s
rivers and streams that are useful for mitigation of hazards associated with floods and droughts.UKMETA medium-range (3 to 7 day) numerical weather prediction model operated by the United Kingdom METeorological Agency. It has a resolution of 75 kilometers and covers the entire northern hemisphere. Forecasters use this model along with the ECMWF and GFS in making their extended forecasts (3 to 7 days).Ultra High Frequency (UHF)Those radio frequencies exceeding 300 MHzUltraviolet IndexThis index provides important information to help you plan your outdoor activities in ways that prevent overexposure to the sun's rays. It was designed by the National Weather Service (NWS) and the Environmental Protection Agency (EPA). Unlike some countries' indices, the United States UV Index is not based upon surface observations. Rather, it is computed using forecasted ozone levels, a computer model that relates ozone levels to UV incidence on the ground, forecasted cloud amounts, and the elevation of the forecast cities. The calculation starts with measurements of current total ozone amounts for the entire globe, obtained via two satellites operated by the National Oceanic and Atmospheric Administration (NOAA). These data are then used to produce a forecast of ozone levels for the next day at various points around the country.
Category | UV Index | Time to Burn | Actions to Take |
Minimal | 0 - 2 | 60 min. + | Apply SPF sunscreen. |
Low | 3 - 4 | 45 min. | Apply SPF sunscreen, wear a hat. |
Moderate | 5 - 6 | 30 min. | Apply SPF 15, wear a hat. |
High | 7 - 9 | 15 - 24 min. | Apply SPF 15 to 30, wear a hat and sunglasses. Limit midday exposure. |
Very High | 10+ | 10 min. | Apply SPF 30; wear a hat, sunglasses, and protective clothing; limit midday exposure. |
Ultraviolet RadiationElectromagnetic radiation of shorter wavelength than visible radiation but longer than x-rays.UndercurrentIn hydrologic terms, a current below the upper currents or surface of a fluid body.UnderflowThe lateral motion of water through the upper layers until it enters a stream channel. This usually takes longer to reach stream
channels than runoff. This also called subsurface storm flow.UndersunAn optical effect seen by an observer above a cloud deck when looking toward the sun, as sunlight is reflected upwards off the faces of ice crystals in the cloud deck. [Also known as subsun.]UndertowA relatively small-scale surf-zone current moving away from the beach. Rip currents form as waves disperse along the beach causing water to become trapped between the beach and a sandbar or other underwater feature. The water converges into a narrow, river-like channel moving away from the shore at high speed.United States Drought Monitor (USDM)A multi-agency product that defines and highlights the severity of drought across the ConUS and OConUS based on expert interpretation of a multitude of indices.Universal Geographic Code(UGC) - UGC's, (e.g. ANZ300 for Western Long Island Sound) are used in many National Weather Service text products to provide geographical information. This allows users easy automated processing and redistribution of the information. More specifically, the purpose of the UGC are to specify the affected geographic area of the event, typically by state, county (or parish), or unique NWS zone (land and marine). The only exception to the above is to define the weather synopsis part of certain marine products. Universal Time (UT)By international agreement, the local time at the prime meridian, which passes through Greenwich, England. Prior to 1972, this time was called Greenwich Mean Time (GMT) but is now referred to as Coordinated Universal Time or Universal Time Coordinated (UTC). It is a coordinated time scale, maintained by the Bureau International des Poids et Mesures (BIPM). It is also known a "Z time" or "Zulu Time".
More about UTC, and a table to convert UTC to your local time is
posted at:
http://www.srh.noaa.gov/srh/jetstream/doppler/radarfaq.htm#utcUnsettledIn meteorological use: A colloquial term used to describe a condition in the atmosphere conducive to precipitation. This term typically is associated with the passage of surface or upper level low pressure systems, fronts or other phenomenon when precipitation expected.
In solar-terrestrial use: With regard to geomagnetic levels, a descriptive word specifically meaning that 8 is less than or equal to the Ap Index which is less than or equal to 15.Unstable AirAir that is able to rise easily, and has the potential to produce clouds, rain, and thunderstorms.Up-valley WindA diurnal thermally driven flow directed up a valley's axis, usually occurring during daytime; part of the along-valley wind system.Upper LevelIn weather observing, the term applies to the portion of the atmosphere that is above the lower troposphere, generally 850 hPa and above.Upper Level DisturbanceA disturbance in the upper atmospheric flow pattern which is usually associated with clouds and precipitation. This disturbance is characterized by distinct cyclonic flow, a pocket of cold air, and sometimes a jet streak. These features make the air aloft more unstable and conducive to clouds and precipitation.Upper Level SystemA general term for any large-scale or mesoscale disturbance capable of producing upward motion (lift) in the middle or upper parts of the atmosphere. This term sometimes is used interchangeably with impulse or shortwave.Upper-air Weather ChartWeather maps that are produced for the portion of the atmosphere above the lower troposphere, generally at and above 850 mb. Isolines on these maps usually represent the heights of a constant pressure surface, such as the 500 mb surface.Upslope FlowSame as Orographic Lifting; air that flows toward higher terrain, and hence is forced to rise. The added lift often results in widespread low cloudiness and stratiform precipitation if the air is stable, or an increased chance of thunderstorm development if the air is unstable.Upslope FogA fog that forms when moist, stable air is carried up a mountain slope.UpstreamTowards the source of flow, or located in the area from which the flow is coming.Upstream SlopeThe part of the dam which is in contact with the reservoir water. On earthen dams, this slope must be protected from the erosive
action of waves by rock riprap or concrete.UpwellingIn ocean dynamics, the upward motion of sub-surface water toward the surface of the ocean. This is often a source of cold, nutrient-rich water. Strong upwelling occurs along the equator where easterly winds are present. Upwelling also can occur along coastlines, and is important to fisheries and birds in California and Peru.Urban and Small Stream Flood AdvisoryThis advisory alerts the public to flooding which is generally only an inconvenience (not life-threatening) to those living in the affected area. Issued when heavy rain will cause flooding of streets and low-lying places in urban areas. Also used if small rural or urban streams are expected to reach or exceed bankfull. Some damage to homes or roads could occur.Urban and Small Stream FloodingFlooding of small streams, streets, and low-lying areas, such as railroad underpasses and urban storm drains. This type of flooding is mainly an inconvenience and is generally not life threatening nor is it significantly damaging to property.Urban Flash Flood GuidanceA specific type of flash flood guidance which estimates the average amount of rain needed over an urban area during a specified period of time to initiate flooding on small, ungaged streams in the urban area.Urban Heat IslandThe increased air temperatures in urban areas in contrast to cooler surrounding rural areas.UV IndexUltraviolet Index- This index provides important information to help you plan your outdoor activities in ways that prevent overexposure to the sun's rays. It was designed by the National Weather Service (NWS) and the Environmental Protection Agency (EPA). Unlike some countries' indices, the United States UV Index is not based upon surface observations. Rather, it is computed using forecasted ozone levels, a computer model that relates ozone levels to UV incidence on the ground, forecasted cloud amounts, and the elevation of the forecast cities. The calculation starts with measurements of current total ozone amounts for the entire globe, obtained via two satellites operated by the National Oceanic and Atmospheric Administration (NOAA). These data are then used to produce a forecast of ozone levels for the next day at various points around the country.
Category | UV Index | Time to Burn | Actions to Take |
Minimal | 0 - 2 | 60 min. + | Apply SPF sunscreen. |
Low | 3 - 4 | 45 min. | Apply SPF sunscreen, wear a hat. |
Moderate | 5 - 6 | 30 min. | Apply SPF 15, wear a hat. |
High | 7 - 9 | 15 - 24 min. | Apply SPF 15 to 30, wear a hat and sunglasses. Limit midday exposure. |
Very High | 10+ | 10 min. | Apply SPF 30; wear a hat, sunglasses, and protective clothing; limit midday exposure. |
VAD Wind ProfileA radar plot of horizontal winds, derived from VAD data, as a function of height above a Doppler Radar. The display is plotted with height as the vertical axis and time as the horizontal axis (a so-called time-height display), which then depicts the change in wind with time at various heights. This display is useful for observing local changes in vertical wind shear, such as backing of low-level winds, increases in speed shear, and development or evolution of nearby jet streams (including low-level jets). This product often is referred to erroneously as a VAD.Vadose ZoneThe locus of points just above the water table where soil pores may either contain
air or water. This is also called the zone of aerationValid TimeThe period of time during which a forecast or warning, until it is updated or superseded by a new forecast issuance, is in effect. Valid Time Event Code(VTEC) - The Valid Time Event Code (VTEC) always is used in conjunction with, and provides supplementary information to, the Universal Geographic Code (UGC), to further aid in the automated delivery of National Weather Service text products to users. The VTEC is included in many event driven or non-routine products and in some routine Marine forecasts. The VTEC provides information on the event, while the UGC describes the affected geographic area. Valley Exit JetA strong elevated down-valley air current issuing from a valley above its intersection with the adjacent plain.Valley Volume EffectThe reduction in volume of a valley (or basin) as compared to an equal depth volume with a horizontal floor. Because the valley volume is smaller, equivalent heat fluxes will cause larger changes in temperature in the valley volume than in the flat-floor volume.ValveIn hydrologic terms, a device fitted to a pipeline or orifice in which the closure member is either rotated or moved in some way as to control or stop
flow.Vapor PressureThe partial pressure of water vapor in an air-water system.Variable Infiltration Capacity (VIC)A large-scale, semi-distributed hydrologic model that solves full water and energy balances. As such, it shares several basic features with other land surface models that are commonly coupled to global circulation models.Variable WindSame as Variable Wind Direction; a condition when (1) the wind direction fluctuates by 60° or more during the 2-minute evaluation period and the wind speed is greater than 6 knots; or (2) the direction is variable and the wind speed is less than 6 knots.Variable Wind DirectionA condition when (1) the wind direction fluctuates by 60° or more during the 2-minute evaluation period and the wind speed is greater than 6 knots; or (2) the direction is variable and the wind speed is less than 6 knots.VarianceA measure of variability.VeeringA clockwise shift in wind direction (for example, south winds shifting to the west).Veering WindsWinds which shift in a clockwise direction with time at a given location (e.g., from southerly to westerly), or which change direction in a clockwise sense with height (e.g., southeasterly at the surface turning to southwesterly aloft). The latter example is a form of directional shear which is important for tornado formation. Compare with backing winds.Velocity Azimuth DisplayA WSR 88-D product which shows the radar derived wind speeds at various heights. This radar product shows the wind speeds from 2,000 to 55,000 feet above the ground. VAD and EVAD (Extended VAD) are methods of guessing the large scale two-dimensional winds from one-dimensional radial velocity data. They are essentially multivariate regressions which fit a simple, large scale wind model to the observed winds. EVAD also estimates the large scale horizontal divergence and particle fall speed. See VWP.Velocity Cross SectionThis WSR-88D radar product displays a vertical cross section of velocity on a grid with heights up to 70,000 feet on the vertical axis and distance up to 124 nm on the horizontal axis. The two end points to create cross section are radar operator selected along a radial or from one AZRAN to another AZRAN within 124 nm of the radar that are less than 124 nm apart. It is used to: 1) Examine storm structure features such as location of updrafts/downdrafts, strength of storm top divergence, and the depth of mesocyclones; 2) Locate areas of convergence/divergence (when generated along a radial; and 3) Analyze areas of rotation (when generated from one AZRAN to another). Velocity ZonesIn hydrologic terms, areas within the floodplain subject to potential high damage from waves. These sometimes appear on flood insurance rate mapsVentilation IndexProduct of the mixing depth and transport wind speed, a measure of the potential of the atmosphere to disperse airborne pollutants from a stationary source. Sometimes referred to as a Clearing Index.Venturi EffectThe speedup of air through a constriction due to the pressure rise on the upwind side of the constriction and the pressure drop on the downwind side as the air diverges to leave the constriction.Ver High Frequency (VHF)That portion of the radio frequency spectrum
from 30 to 300 MHzVertical VelocityThe component of velocity (motion) in the vertical. The evaluation of areas of upward vertical velocity is key to forecasting areas of active
weather.Vertical Wind Shearthe change in the wind's direction and speed with height. This is a critical factor in determining whether severe thunderstorms will develop.Vertically Stacked SystemA low-pressure system, usually a closed low or cutoff low, which is not tilted with height, i.e., located similarly at all levels of the atmosphere. Such systems typically are weakening and are slow-moving, and are less likely to produce severe weather than tilted systems. However, cold pools aloft associated with vertically-stacked systems may enhance instability enough to produce severe weather.
Very Low Frequency (VLF)That portion of the radio frequency spectrum from
3 to 30 kHzVery Windy30 to 40 mph winds.Virtual Potential TemperatureThe virtual potential temperature is the temperature a parcel at a specific pressure level and virtual temperature would have if it were lowered or raised to 1000 mb. This is defined by Poisson's equation.Virtual TemperatureThe virtual temperature is the temperature a parcel which contains no moisture would have to equal the density of a parcel at a specific temperature and humidity.Visibility Protection ProgramThe program specified by the Clean Air Act to achieve a national goal of remedying existing impairments to visibility and preventing future visibility impairment throughout the United States.Visible Infra-Red Imaging Radiometer (VIIRS)A medium-resolution sensor flown aboard the NOAA-20 and Suomi-NP satellites.Visible Satellite ImageryThis type of satellite imagery uses reflected sunlight (this is actually reflected solar radiation) to see things in the atmosphere and on the Earth's surface. Clouds and fresh snow are excellent reflectors, so they appear white on the imagery. Clouds can be distinguished from snow, because clouds move and snow does not move. Meanwhile, the ground reflects less sunlight, so it appears black on the imagery. The satellite uses its 0.55 to 0.75 micrometer (um) channel to detect this reflected sunlight. Since this imagery relies on reflected imagery, it cannot be used during night.Visual SpectrumThe portion of the electromagnetic spectrum to which the eye is sensitive, i.e., light with wavelengths between 0.4 and 0.7 micrometers. Compare shortwave radiation and longwave radiation.Volume ScanA radar scanning strategy in which sweeps are made at successive antenna elevations (i.e., a tilt sequence), and then combined to obtain the three-dimensional structure of the echoes.Volume Velocity ProcessingA way to guess the large-scale 2-dimensional winds, divergence and fall speeds from one-dimensional radial velocity data. Essentially a multivariate regression which fits a simple wind model to the observed radial velocities. Very similar to VAD and EVAD, except it uses different functions for the fit.Voluntary Observing Ship Program(VOS) - An international voluntary marine observation program under the auspices of the World Meteorological Organization (WMO). Observations are coded in a special format known as the ships synoptic code, or "BBXX" format. They are then distributed for use by meteorologists in weather forecasting, by oceanographers, ship routing services, fishermen, and many others. VortexA whirling mass of air in the form of a column or spiral. It need not be oriented vertically but, for example, could be rotating around a horizontal axis.
VTEC(Valid Time Event Code) - The Valid Time Event Code (VTEC) always is used in conjunction with, and provides supplementary information to, the Universal Geographic Code (UGC), to further aid in the automated delivery of National Weather Service text products to users. The VTEC is included in many event driven or non-routine products and in some routine Marine forecasts. The VTEC provides information on the event, while the UGC describes the affected geographic area. WakeThe region of turbulence immediately to the rear of a solid body caused by the flow of air over or around the body.Warm AdvectionTransport of warm air into an area by horizontal winds. Low-level warm advection sometimes is referred to (erroneously) as overrunning. Although the two terms are not properly interchangeable, both imply the presence of lifting in low levels.Warm Core LowA low pressure area which is warmer at its center than at its periphery. Tropical cyclones exhibit this temperature pattern. Unlike cold core lows, these lows produce much of their cloud cover and precipitation during the nighttime.Warm SectorA region of warm surface air between a cold front and a warm front.Watch CancellationThis product will be issued to let the public know when either a Tornado Watch or Severe Thunderstorm Watch has been canceled early. It is issued by the Storm Prediction Center (SPC) in Norman, Oklahoma. In the text of the statement it will specify the severe weather watch number and the area which the watch covered.Watch Redefining StatementThis product tells the public which counties/parishes are included in the watch. This is done not only by writing them all out, but by using the county FIPS codes in the Header of the product. It is issued by the local National Weather Service Forecast Office (WFO).Watch Status ReportsThis product lets the NWFO know of the status of the current severe weather watch (Tornado or Severe Thunderstorm). During the severe weather watch, the Storm Prediction Center (SPC) will issue these reports periodically. These reports will describe, in plain language, the current evaluation of the severe weather situation and whether the watch will expire or be reissued. A status report is not issued if a cancellation or replacement has been issued at least 1 hour prior to the expiration time of the original watch.Water EquivalentThe liquid content of solid precipitation that has accumulated on the ground (snow depth). The accumulation may consist of snow, ice formed by freezing precipitation, freezing liquid precipitation, or ice formed by the refreezing of melted snow.Water PollutionThe alteration of the constituents of a body of water by man to such a degree that the water loses its value as a natural resource.Water Supply OutlookA seasonal volume forecast, generally for a period centered around the time of spring snowmelt (e.g., April-July). The outlooks are in units of acre-feet and represent the expected volume of water to pass by a given point during a snowmelt season. The outlook categories include Most Probable, Reasonable Maximum, and Reasonable Minimum.Water TableThe level below the earth's surface at which the ground becomes saturated with water. The water table is set where hydrostatic pressure equals atmospheric pressure.Water Vapor PlumeThis appears in the water vapor satellite imagery. It is a plume-like object that extends from the Intertropical Convergence Zone (ITCZ) northward or southward into the higher latitudes. It is usually located over a 850 to 700 mb theta-e ridge axis. As a result, it is a favored location for the development of a Mesoscale Convective Complex (MCC). Researchers have found it to be a favored region for very heavy rain. It is thought that the ice crystals located in this plume help thunderstorms to become highly efficient rainfall producers. In North America, this is sometimes called the "Mexican Connection".Water YearThe time period from October 1 through September 30.WatercourseAny surface flow such as a river, stream, tributary.WatershedLand area from which water drains toward a common watercourse in a natural basin.WaterspoutIn general, a tornado occurring over water. Specifically, it normally refers to a small, relatively weak rotating column of air over water beneath a Cb or towering cumulus cloud. Waterspouts are most common over tropical or subtropical waters.
The exact definition of waterspout is debatable. In most cases the term is reserved for small vortices over water that are not associated with storm-scale rotation (i.e., they are the water-based equivalent of landspouts). But there is sufficient justification for calling virtually any rotating column of air a waterspout if it is in contact with a water surface.Wave CrestThe highest part of a waveWave HeightDistance from wave trough to wave crest.Wave PeriodTime, in seconds, between the passage of consecutive wave crests past a fixed point.
Wave SpectrumThe distribution of wave energy with respect to wave frequency or period. Wave spectra assist in differentiating between wind waves and swell. Wave SteepnessThe ratio of wave height to wavelength and is an indicator of wave stability. When wave steepness exceeds a 1/7 ratio; the wave typically becomes unstable and begins to break.Wave TroughThe lowest part of the wave.WavelengthDistance between crests or troughs of a wave.WAVEWATCH IIIOne of the operational forecast models run at NCEP. The WWIII is run four times daily, with forecast output out to 126 hours.Weak Echo Region(Abbrev. WER) - A WSR-88D radar product which displays reflectivity for up to 8 elevation angles for a radar operator selected location as a set presentation of a storm. The plains in this product are presented in an ascending order, lowest plain is lowest elevation angle selected. It is used to depict storm tilt and to identify Weak Echo Regions (WER) and Bounded Weak Echo Regions (BWER) in thunderstorms.WeatherThe state of the atmosphere with respect to wind, temperature, cloudiness, moisture, pressure, etc. Weather refers to these conditions at a given point in time (e.g., today's high temperature), whereas Climate refers to the "average" weather conditions for an area over a long period of time (e.g., the average high temperature for today's date).Weather Forecast Office(Abbrev. WFO) - this type of National Weather Service office is responsible for issuing advisories, warnings, statements, and short term forecasts for its county warning areaWeatherfaxSee RADIOFACSIMILEWedge TornadoSlang for a large tornado with a condensation funnel that is at least as wide (horizontally) at the ground as it is tall (vertically) from the ground to cloud base. The term "wedge" often is used somewhat loosely to describe any large tornado. However, not every large tornado is a wedge. A true wedge tornado, with a funnel at least as wide at the ground as it is tall, is very rare.
Wedges often appear with violent tornadoes (F4 or F5 on the Fujita Scale), but many documented wedges have been rated lower. And some violent tornadoes may not appear as wedges (e.g., Xenia, OH on 3 April 1974, which was rated F5 but appeared only as a series of suction vortices without a central condensation funnel). Whether or not a tornado achieves "wedge" status depends on several factors other than intensity - in particular, the height of the environmental cloud base and the availability of moisture below cloud base. Therefore, spotters should not estimate wind speeds or F-scale ratings based on visual appearance alone. However, it generally is safe to assume that most (if not all) wedges have the potential to produce strong (F2/F3) or violent (F4/F5) damage.WEFAXSystem for transmitting weather charts and imagery via satellite. Occasionally used as an abbreviation for radiofacsimile via HF radio. Weighing-Type Precipitation GageA rain gage that weighs the rain or snow which falls into a bucket set on a platform of a spring or lever balance. The increasing weight of its contents plus the bucket are recorded on a chart. The record thus shows the accumulation of precipitation.WeirIn hydrologic terms, (a) A low dam built across a stream to raise the upstream water level (fixed-crest weir when uncontrolled); (b) A structure built
across a stream or channel for the purpose of measuring flow (measuring or gaging weir).WERWeak Echo Region - Radar term for a region of relatively weak reflectivity at low levels on
the inflow side of a thunderstorm echo, topped by stronger reflectivity in the form of an echo
overhang directly above it. The WER is a sign of a strong updraft on the inflow side of a
storm, within which precipitation is held aloft. When the area of low reflectivity extends upward into,
and is surrounded by, the higher reflectivity aloft, it becomes a BWER. West African Disturbance LineA line of convection about 300 miles long, similar to a squall line. It forms over west Africa north of the equator and south of 15 degrees North latitude. It moves faster than an Easterly Wave between 20 and 40 mph. They move off the African coast every 4 to 5 days mainly in the summer. Some reach the American tropics and a few develop into tropical cyclones.West WallThe coast side boundary of the Gulf Stream, typically south of Cape Hatteras. See also North Wall WesterliesThe prevailing winds that blow from the west in the mid-latitudes.Wet Bulb Zero(Abbrev. WBZ) - the height where the wet-bulb temperature goes below 0°C. It is important because WBZ heights between 7000 ft and 10,500 ft (above ground level) correlate well with large hail at the surface when storms develop in an airmass primed for strong convection. Higher values infer mid and upper level stability and also indicate a large melting area for falling hail. Lower WBZ heights indicate that the low level atmosphere is often too cool and stable to support large hail.Wet FloodproofingIn hydrologic terms, an approach to floodproofing which usually is a last resort. Flood waters are intentionally allowed into the building to minimize
water pressure on the structure. Wet Floodproofing can include moving a few valueable items to a higher place or completely
rebuilding the floodable area. Wet floodproofing has an advantage over other approaches: no matter how little is done, flood
damage will be reduced. Thousands of dollars in damage can be avoided just by moving furniture and appliances out of the
flood-prone area.Wet MicroburstA microburst accompanied by heavy precipitation at the surface. A rain foot may be a visible sign of a wet microburst.Wet-Bulb TemperatureThe lowest temperature that can be obtained by evaporting water into the air.WetlandIn hydrologic terms, an area that is regularly wet or flooded and has a water table that stands at or above the land surface for at least part of the year.White Light (WL)Sunlight integrated over the visible portion of the spectrum (4000 - 7000 angstroms) so that all colors are blended to appear white to the eye.White Light FlareIn solar-terrestrial terms, a major flare in which small parts become visible in
white light. Such flares are usually strong X-ray, radio, and
particle emitters.WhitecapThe breaking crest of a wave, usually white and frothy.WidespreadAreal coverage of non-measurable, non-convective weather and/or restrictions to visibility affecting more than 50 percent of a forecast zone(s).WildfireAny free burning uncontainable wildland fire not prescribed for the area which consumes the natural fuels and spreads in response to its environment.Wind CoupletAn area on the radar display where two maximum wind speeds are blowing in opposite directions.Wind DirectionThe true direction from which the wind is blowing at a given location (i.e., wind blowing from the north to the south is a north wind). It is normally measured in tens of degrees from 10 degrees clockwise through 360 degrees. North is 360 degrees. A wind direction of 0 degrees is only used when wind is calm. Wind FieldThe three-dimensional spatial pattern of winds.Wind RoseA diagram, for a given locality or area, showing the frequency and strength of the wind from various directions.Wind ShearThe rate at which wind velocity changes from point to point in a given direction (as, vertically). The shear can be speed shear (where speed changes between the two points, but not direction), direction shear (where direction changes between the two points, but not speed) or a combination of the two.Wind Shear ProfileThe change in wind speed and/or direction usually in the vertical. The characteristics of the wind shear profile are of critical importance in
determining the potential for and type of severe weather.Wind Shift LineA long, but narrow axis across which the winds change direction (usually veer).Wind SpeedThe rate at which air is moving horizontally past a given point. It may be a 2-minute average speed (reported as wind speed) or an instantaneous speed (reported as a peak wind speed, wind gust, or squall).Wind WavesLocal, short period waves generated from the action of wind on the water surface (as opposed to swell). Commonly referred to as waves. In a National Weather Service Coastal Marine Forecast or Offshore Forecast, wind waves are used when swells are described in the forecast. or Waves generated by the local wind blowing at the time of observation.WinterTypically the coldest season of the year during which the sun is farthest from overhead. In the Northern Hemisphere, winter customarily includes the
months of December, January and February.Winter PoolThe pool, or height of the water surface, of a reservoir during the winter. This pool is usually a specific height and is maintained for the control of late
winter and spring flooding.Winter SolsticeThe time at which the sun is farthest south in the Southern Hemisphere, on or around December 21.Winter Storm WarningThis product is issued by the National Weather Service when a winter storm is producing or is forecast to produce heavy snow or significant ice accumulations. The criteria for this warning can vary from place to place.Winter Storm WatchThis product is issued by the National Weather Service when there is a potential for heavy snow or significant ice accumulations, usually at least 24 to 36 hours in advance. The criteria for this watch can vary from place to place.Winter Weather AdvisoryThis product is issued by the National Weather Service when a low pressure system produces a combination of winter weather (snow, freezing rain, sleet, etc.) that present a hazard, but does not meet warning criteria.Wire Weight GageIn hydrologic terms, a river gage comprised of a weight which is lowered to the water level. The weight is attached to a cable; and as the weight is
lowered, a counter indicates the length of cable released. The stage is determined from the length of cable required to reach the
water level.Wolf NumberA historic term for Sunspot Number. In 1849, R. Wolf of
Zurich originated the general procedure for computing the sunspot
numberWSR-88D SystemThe summation of all hardware, software, facilities, communications, logistics, staffing, training, operations, and procedures specifically associated with the collection, processing, analysis, dissemination and application of data from the WSR-88D unit.X-Ray Flare ClassIn solar-terrestrial terms, rank of a flare based on its X-ray energy output. Flares are classified by the order of magnitude of the peak burst intensity (I) measured at the earth in the 1 to 8 angstrom band as follows:
Class | Intensity (in Watts/m2) |
B | I < 10-6 |
C | 10-6 <= I < 10-5 |
M | 10-5 <= I < 10-4 |
X | I >= 10-4 |
XCITEDExcitedXSECCross SectionYearThe period during which the Earth completes one revolution around the sun.Yellow SnowSnow given a golden or yellow appearance by the presence in it of pine, cypress pollen, or anthropogenic material or animal-produced material.Zero DatumIn hydrologic terms, a reference "zero" elevation for a stream or river gage. This "zero" can be referenced (usually within ten feet of the bottom of the
channel) to mean sea level, or to any other recognized datum.Zone of AerationIn hydrologic terms, the locus of points just above the water table where soil pores may either contain air or water. This is also called the vadose zoneZone of SaturationIn hydrologic terms, the locus of points below the water table where soil pores are filled with water. This is also called the phreatic zoneZoned Embankment DamIn hydrologic terms, an embankment dam which is comprised of zones of selected materials having different degrees of porosity, permeability and
density.Zulu (Z) TimeFor practical purposes, the same as Coordinated Universal Time (UTC). The
notation formerly used to identify time Greenwich MeanTime. The word "Zulu" is notation in
the phonetic alphabet corresponding to the letter "Z" assigned to the time zone on the Greenwich
Prime Meridian. Z\/R RelationshipAn empirical relationship between radar reflectivity factor z (in mm^6 / m^3 ) and rain rate ( in mm / hr ), usually expressed as Z = A R^b; A and b are empirical constants.
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