National Weather Service United States Department of Commerce
A -  Symbol used on long-term climate outlooks issued by the Climate Prediction Center to indicate areas that are likely to be above normal for the specified parameter (temperature, precipitation, etc.).
A AMS - Arctic Air Mass
A IndexA daily index of geomagnetic activity derived as the average of the eight 3-hourly a indices.
AAASAmerican Association for the Advancement of Science
AAWUAlaskan Aviation Weather Unit
AblationDepletion of snow and ice by melting and evaporation.
ABNDTAbundant
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.
AbsorptionThe process in which incident radiant energy is retained by a substance by conversion to some other form of energy.
ABTAbout
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.
ABV Above
AC1. Abbreviation for Altocumulus - a cloud of a class characterized by globular masses or rolls in layers or patches, the individual elements being larger and darker than those of cirrocumulus and smaller than those of stratocumulus. These clouds are of medium altitude, about 8000-20,000 ft (2400-6100 m).  2. Convective outlook issued by the Storm Prediction Center. Abbreviation for Anticipated Convection; the term originates from the header coding [ACUS1] of the transmitted product.
ACCAS(usually pronounced ACK-kis) - AltoCumulus CAStellanus; mid-level clouds (bases generally 8 to 15 thousand feet), of which at least a fraction of their upper parts show cumulus-type development. These clouds often are taller than they are wide, giving them a turret-shaped appearance. ACCAS clouds are a sign of instability aloft, and may precede the rapid development of thunderstorms.
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.
ACCUMSaccumulation
AccuracyDegree of conformity of a measure to a standard or true value; in other words, how close a predicted or measured value is to the true value.
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.
Acid RainRain containing relatively high concentrations of acid-forming chemicals that have been released into the atmosphere and combined with water vapor; harmful to the environment.
ACLDAbove Cloud Level
ACPYAccompany
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.
ACRSAcross
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.
ACTVActive. In solar-terrestrial terms, solar activity levels with at least one geophysical event or several larger radio events (10cm) per day (Class M Flares)
ACYCAnticyclone - A large-scale circulation of winds around a central region of high atmospheric pressure, clockwise in the Northern Hemisphere, counterclockwise in the Southern Hemisphere.
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.
ADASAutomated Data Acquisition System
Additive DataA group of coded remarks that includes pressure tendency, amount of precipitation, and maximum/minimum temperature during specified periods of time
ADDSAviation Digital Data Service
AdiabatA line on a thermodynamic chart relating the pressure and temperature of a substance (such as air) that is undergoing a transformation in which no heat is exchanged with its environment.
AdiabaticChanges in temperature caused by the expansion (cooling) or compression (warming) of a body of air as it rises or descends in the atmosphere, with no exchange of heat with the surrounding air.
Adiabatic 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.
ADJAdjacent
ADPCAcoustic Doppler Current Profiler
ADVCTNAdvection- Transport of an atmospheric property by the wind.
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.
ADVISIn hydrologic terms, a program which combines the Antecedent Precipitation Index (API) method of estimating runoff with unit hydrograph theory to estimate streamflow for a headwater basin.
Advisory(Abbrev. ADVY)- Highlights special weather conditions that are less serious than a warning. They are for events that may cause significant inconvenience, and if caution is not exercised, it could lead to situations that may threaten life and/or property.
ADVNAdvance
ADVYAdvisory - Highlights special weather conditions that are less serious than a warning. They are for events that may cause significant inconvenience, and if caution is not exercised, it could lead to situations that may threaten life and/or property.
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.
AFBAir Force Base
AFCTAffect
AFDArea Forecast Discussion - This 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.
AFOSAutomation of Field Operations and Services. Computer system linking NWS offices for the transmission of weather data. This system was installed in the early to mid 1980s and it is being replaced by Advanced Weather Interactive Processing System (AWIPS).
AFREDAbbreviation for the A Index for Fredericksburg.
AFSSAutomated Flight Service Station
AFTAfter
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.
AFTNAfternoon
AFTRafter
AFWAAir Force Weather Agency
AGCAutomatic Gain Control
AGDISPA particular atmospheric disperison model used for treating the transport and diffusion of aerially sprayed pest control agents in agricultural applications.
AGFSAviation Gridded Forecast System
AgglomerateAn ice cover of floe formed by the freezing together of various forms of ice.
AGLAbove Ground Level
AGNAgain
AHDAhead
AHOSAutomatic Hydrologic Observing System
AHOS-SAutomatic Hydrologic Observing System - Satellite
AHOS-TAutomatic Hydrologic Observing System - Telephone
AirThe mixture of gases comprising the earth's atmosphere.
Air MassA body of air covering a relatively wide area and exhibiting horizontally uniform properties.
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 PollutantHarmful substance or product introduced into the atmosphere.
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 StagnationA meteorological situation in which there is a major buildup of air pollution in the atmosphere. This usually occurs when the same air mass is parked over the same area for several days. During this time, the light winds cannot "cleanse" the buildup of smoke, dust, gases, and other industrial air pollution.
Air Stagnation AdvisoryThis National Weather Service product is issued when major buildups of air pollution, smoke, dust, or industrial gases are expected near the ground for a period of time. This usually results from a stagnant high pressure system with weak winds being unable to bring in fresh air.
Air ToxinToxic air pollutant.
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)
AIVAviation Impact Variables
Alaska CurrentA North Pacific Ocean current flowing counterclockwise in the Gulf of Alaska. It is the northward flowing (warm) division of the Aleutian Current
AlbedoReflectivity; 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.
ALFAloft
ALGAlong
AlgorithmA computer program (or set of programs) which is designed to systematically solve a certain kind of problem. WSR-88D radars (NEXRAD) employ algorithms to analyze radar data and automatically determine storm motion, probability of hail, VIL, accumulated rainfall, and several other parameters.
ALIASINGThe process by which frequencies too high to be analyzed with the given sampling interval appear at a frequency less than the Nyquist frequency.
AlluviumSediments deposited by erosional processes, usually by streams.
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.
ALQDSAll Quadrants
ALTHOalthough
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.
AltocumulusA cloud of a class characterized by globular masses or rolls in layers or patches, the individual elements being larger and darker than those of cirrocumulus and smaller than those of stratocumulus. These clouds are of medium altitude, about 8000-20,000 ft (2400-6100 m).
AltostratusA cloud of a class characterized by a generally uniform gray sheet or layer, lighter in color than nimbostratus and darker than cirrostratus. These clouds are of medium altitude, about 8000 to 20,000 ft (2400-6100 m).
AmbientOf the surrounding area or environment.
AMDAmend
AMOSAutomatic Meteorological Observing System
AmplifierA device used to increase the strength of an analog signal
AmplitudeThe maximum magnitude of a quantity. Often used to refer to the maximum height of a wave.
AMS1. Air Mass - a body of air covering a relatively wide area and exhibiting horizontally uniform properties.  2. American Meteorological Society
AMTAmount
AMVERAutomated 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.
AnabranchA diverging branch of a river which re-enters the main stream.
Analog1. Class of measuring devices in which the output varies continuously as a function of the input (non-digital).  2. A historical instance of a given meteorological scenario or feature that is used for comparison with another scenario or feature. For example, a long-range forecaster predicting conditions for the upcoming winter may make comparisons to analog seasons in which meteorological factors were similar to those of the upcoming season.
Analog SignalA signal, such as voice, that varies in a continuous manner.
ANBURSAlphanumeric Backup Replacement System
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).
AngstromA unit of length equal to 10-8 cm.
Annual FloodIn hydrologic terms, the maximum discharge peak during a given water year (October 1 - September 30).
ANOMALOUS PROPAGATION (AP)Non-standard atmospheric temperature or moisture gradients will cause all or part of the radar beam to propagate along a non-normal path. When non-standard index-of-refraction distributions prevail, "abnormal" or "anomalous" propagation occurs. When abnormal downward bending occurs, it is called "superrefraction." If the beam is refracted downward sufficiently, it will illuminate the ground and return signals to the radar from distances further than is normally associated with ground targets. The term "subrefraction" is applied when there is abnormal upward bending of the radar beam.
AnomalyThe deviation of a measurable unit (e.g., temperature or precipitation) over a period in a given region from the long-term average, often the thirty-year mean, for that region.
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.
Anti-windThe upper or return branch of an along-valley wind system, as confined within a valley, and blowing in a direction opposite to the winds in the lower altitudes of the valley.
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 Hemisphere
Anticyclonic RotationRotation in the opposite sense as the Earth's rotation, i.e., clockwise in the Northern Hemisphere as would be seen from above. The opposite of cyclonic rotation.
Antilles CurrentA current which originates in the vicinity of the Leeward Islands as part of the Atlantic North Equatorial Current.
AnvilThe flat, spreading top of a cumulonimbus cloud, often shaped like an anvil. Thunderstorm anvils may spread hundreds of miles downwind from the thunderstorm itself, and sometimes may spread upwind.
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.
Anvil ZitsSlang for frequent (often continuous or nearly continuous), localized lightning discharges occurring from within a thunderstorm anvil.
AOArctic Oscillation - the Arctic Oscillation is a pattern in which atmospheric pressure at polar and middle latitudes fluctuates between negative and positive phases. The negative phase brings higher-than-normal pressure over the polar region and lower-than-normal pressure at about 45 degrees north latitude. The negative phase allows cold air to plunge into the Midwestern United States and western Europe, and storms bring rain to the Mediterranean. The positive phase brings the opposite conditions, steering ocean storms farther north and bringing wetter weather to Alaska, Scotland and Scandinavia and drier conditions to areas such as California, Spain and the Middle East. In recent years research has shown, the Arctic Oscillation has been mostly in its positive phase. Some researchers argue that the North Atlantic Oscillation is in fact part of the AO.
AOAAt or above
AOBAt or below
AOPAAircraft Owners and Pilots Association
APAnomalous Propagation. Radar term for false (non-precipitation) echoes resulting from nonstandard propagation of the radar beam under certain atmospheric conditions.
AP IndexIn solar-terrestrial terms, an averaged planetary A Index based on data from a set of specific stations.
APDOn a buoy report, the average wave period (seconds) of all waves during the 20-minute period.
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.
APRCHApproach
APRCHGapproaching
APRNTapparent
APSTAviation Products and Services Team
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.
ARAMAviation, Range, and Aerospace Meteorology
Arch DamA concrete arch dam is used in sites where the ratio of width between abutments to height is not great and where the foundation at the abutments is solid rock capable of resisting great forces. The arch provides resistance to movement. When combined with the weight of concrete (arch-gravity dam), both the weight and shape of the structure provide great resistance to the pressure of 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.
ArcticThe region within the Arctic Circle, or, loosely, northern regions in general, characterized by very low temperatures.
Arctic frontThe boundary or front separating deep, cold arctic air from shallower, relatively less cold polar air.
Arctic Oscillation(abbrev. AO)- The Arctic Oscillation is a pattern in which atmospheric pressure at polar and middle latitudes fluctuates between negative and positive phases. The negative phase brings higher-than-normal pressure over the polar region and lower-than-normal pressure at about 45 degrees north latitude. The negative phase allows cold air to plunge into the Midwestern United States and western Europe, and storms bring rain to the Mediterranean. The positive phase brings the opposite conditions, steering ocean storms farther north and bringing wetter weather to Alaska, Scotland and Scandinavia and drier conditions to areas such as California, Spain and the Middle East. In recent years research has shown, the Arctic Oscillation has been mostly in its positive phase. Some researchers argue that the North Atlantic Oscillation is in fact part of the AO.
Arctic Sea SmokeSteam fog, but often specifically applied to steam fog rising from small open water within sea ice.
ArcusA low, horizontal cloud formation associated with the leading edge of thunderstorm outflow (i.e., the gust front). Roll clouds and shelf clouds both are types of arcus clouds.
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 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.
AridAn adjunctive applied to regions where precipitation is so deficient in quantity, or occurs at such times, that agriculture is impracticable without irrigation.
ARINCAeronautical Radio, Incorporated
ARNDAround
ARRArrive/Arrival
ArroyoIn hydrologic terms, a water-carved channel or gully in arid country, usually rather small with steep banks, dry most of the time, due to infrequent rainfall and the shallowness of the cut which does not penetrate below the level of permanent ground water.
ARSIAtmospheric Research System, Inc.
ARTThe Automatic Radiotheodolite. A ground-based radio direction finder that automatically tracks a ballon-borne radiosonde.
ARTCCAir Route Traffic Control Center
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.
Artificial ControlIn hydrologic terms, a weir or other man-made structure which serves as the control for a stream-gaging station.
AS(NOTE: if this appears in an Area Forecast Discussion or other text product in context as the word "as," disregard the technical definition below). 

Abbreviation for Altostratus, a cloud of a class characterized by a generally uniform gray sheet or layer, lighter in color than nimbostratus and darker than cirrostratus. These clouds are of medium altitude, about 8000 to 20,000 ft (2400-6100 m).
ASAP1. AHOS SHEF Automatic Processing System .  2. As soon as possible (may be used in Area Forecast Discussions)
ASAPTRANThe software component of ASAP.
ASBAviation Support Branch
Ashfall AdvisoryAn advisory issued for conditions associated with airborne ash plume resulting in ongoing deposition at the surface. Ashfall may originate directly from a volcanic eruption, or indirectly by wind suspending the ash.
ASLAbove Sea Level
ASOSAutomated Surface Observing System. A list of stations currently active and available through the NWS website can be found here.
ASOS IDsEach Automated Surface Observing System an a four character identifier assigned to it. A list of stations currently active and available through the NWS website can be found here.
ASR-9Airport Surveillance Radar (FAA)
ASSOCIATED PRINCIPAL USERA Principal User with dedicated communications to a WSR-88D unit.
Astronomical DawnThe time at which the sun is 18 degrees below the horizon in the morning. Astronomical dawn is that point in time at which the sun starts lightening the sky. Prior to this time during the morning, the sky is completely dark.
Astronomical DuskThis is the time at which the sun is 18 degrees below the horizon in the evening. At this time the sun no longer illuminates the sky.
Astronomical Unit(abbrev. AU)- The mean earth-sun distance, equal to 1.496x1013 cm, or 214.94 solar radii.
ATCAir Traffic Control
ATDTDCSAutomated Tone Dial Telephone Data Collection System - Data collection system where cooperative observers collect precipitation, stage, and temperature data then transmit the data to the NWS ATDTDCS computer through the telephone lines. The ATDTDCS computer transmits the data to AFOS.
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 Pressure - The 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.
ATMPOn a buoy report, the air temperature (Celsius).
Attenuation - 1. It 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.  2. Any process in which the flux density (power) of a beam of energy is dissipated.
ATTMAt this time
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.
AuroraA faint visual phenomenon associated with geomagnetic activity, which occurs mainly in the high-latitude night sky; typical auroras are 100 to 250 km above the ground
Aurora AustralisSame as Aurora Borealis, but in the Southern Hemisphere. Also known as the southern 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.
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.
Auroral OvalIn solar-terrestrial terms, an oval band around each geomagnetic pole which is the locus of structured aurorae
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.
AUTOMATIC GAIN CONTROLAny method of automatically controlling the gain of a receiver, particularly one that holds the output level constant regardless of the input level.
AutumnThe season of the year that is the transition period from summer to winter, occurring as the sun approaches the winter solstice. Meteorological autumn (different from standard/astronomical autumn) begins September 1 and ends November 30.
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.
AVBLAvailable
AVGAverage
AVHRRAdvanced Very High Resolution Radiometer. Main sensor on U.S. polar orbiting satellites.
AVNThe Aviation model (120-hour numerical model of the atmosphere). The output from this model is now part of what is known as the GFS model.
AVPOn a buoy report, Average Wave Period is the average period (seconds) of the highest one-third of the wave observed during a 20 minute sampling period.
AWCAviation Weather Center
AWHPSArea Wide Hydrologic Prediction System
AWIPSAdvanced Weather Interactive Processing System. This system replaced the Automation of Field Operations and Services (AFOS). This system allows the operator to overlay meteorological data from a variety of sources.
AWOSAutomated Weather Observation System
AzimuthA direction in terms of a 360° compass. North is at 0°, east is at 90°, south is at 180°, and west is at 270°.
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.
 
 
 
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