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Weather Glossary: B's

Back-Building Thunderstorm
A 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 Door Cold Front
A cold front moving south or southwest along the Atlantic seaboard and Great Lakes.
The backing up of water through a conduit or channel in the direction opposite to normal flow.
It is the counterclockwise turning of the wind direction as we move up through the atmosphere. For example, the wind direction would change from the north at the ground to the northwest. This is indicative of the airmass cooling.
Backing Winds

Winds which shift in a counterclockwise direction with time at a given location (e.g. from south to southeast), or change direction in a counterclockwise sense with height (example: west wind at the surface but becoming more southerly aloft). The opposite of veering winds.

In storm spotting, a backing wind usually refers to the turning of a south or southwest surface wind with time to a more east or southeasterly direction. Backing of the surface wind can increase the potential for tornado development by increasing the directional shear at low levels.

Back Scatter
It refers to the portion of the radar beam energy that returns back towards the radar after striking a target.
Back-Sheared Anvil
Slang for 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.
A rod reading taken on a point of known elevation, a benchmark or a turning point. Backsights are added to the known elevation to arrive at the height of the instrument. With a known height of the instrument, the telescope can be used to determine the elevation of other points in the vicinity.
Backwater Curve
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 Effect
The effect which a dam or other obstruction has in raising the surface of the water upstream from it.
Backwater Flooding
Upstream flooding caused by downstream conditions such as channel restriction and/or high flow in a downstream confluence stream.
Ball Lightning
A relatively rare form of lightning consisting of a luminous ball, often reddish in color, which moves rapidly along solid objects or remains floating in mid-air. Also known as globe lightning.
See frequency band.
Bandpass Filter
A filter whose frequencies are between given upper and lower cutoff values, while substantially attenuating all frequencies outside these values (this band).
Band Width
The number of cycles per second between the limits of a frequency band.
The margins of a channel. Banks are called right or left as viewed facing in the direction of the flow.
Bank Storage
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 Stage/Elevation
An established river stage/water surface elevation at a given location along a river which is intended to represent the maximum water level that will not overflow the river banks or cause any significant damages from flooding.
Bankfull Stage
An established river stage at a certain point along a river which is intended to represent the maximum safe water level which will not overflow the river banks or cause any significant damage within the reach of the river.
An obstacle formed at the shallow entrance to the mouth of a river or bay which empties into the ocean.
Barber Pole
Slang for 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 Zone
A 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.
A graphic record of air pressure produced by a barograph.
A recording barometer.
An instrument used for measuring air pressure. The two most common types are the mercury barometer and the aneroid barometer.
Barometric Pressure
The actual pressure value indicated by a pressure sensor.
Barometric Tendency
The amount and direction of change in barometer readings over a three-hour period.
Barotropic System

A 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.

Any artificial obstruction placed in water to increase water level or divert it. Usually the idea is to control peak flow for later release.
Base Flood
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.
Streamflow which results from precipitation that infiltrates into the soil and it eventually moves through the soil to the stream channel.
Base Reflectivity (R)

This WSR-88D radar product depicts a full 360° sweep of echo intensity data. It is available for every elevation angle that is sampled in a volume scan.

It is used to observe precipitation intensity and movement; determine storm structure; estimate hail potential; locate boundaries (cold front, outflow, lake breeze, etc.); identify cloud layers; and detect light snow, drizzle, birds, insects, and smoke plumes.

Base Velocity (V)

This WSR-88D radar product depicts a full 360° sweep of radial velocity data. It is available for every elevation angle that is sampled in a volume scan.

It is used to estimate wind speed and direction; determine regions of significant shear (convergence, etc.); locate boundaries (cold front, outflow, lake breeze, etc.); identify areas of circulation; and determine storm structure.

Base Width
The time duration of a unit hydrograph.
Basic Fire Weather Services
Routine daily forecasts; spot forecasts; prescribed burn forecasts; smoke management forecasts and information, advisories, observations, summaries, and briefings produced in and by a National Weather Service office during normal working hours, plus warnings of critical weather conditions. Generally, these basic services are tailored to meet the specific needs of user agencies.
A surface area having drainage system consisting of a surface stream and its tributaries and impounded bodies of water. It is also called a "Drainage Basin".
Basin Boundary
The topographic dividing line around the perimeter of a basin, beyond which overland flow (i.e.; runoff) drains away into another basin.
Basin Lag
The time it takes from the centroid of rainfall for the hydrograph to peak.
Basin Recharge
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.
Bathymetric Chart
A map delineating the form of the bottom of a body of water, usually by means of depth contours (see Isobaths).
Bathythermograph (BT)
A device for obtaining a record of temperature against depth (strictly speaking pressure) in the upper 300 meters of the ocean from a ship underway. Some of these devices are expendable and designated as XBT.
Beach Erosion
The carrying away of beach materials by wave action, currents, tides, or wind.
Beam Width
The angular width of the radar beam. Usually that width where the power density is one-half that on the axis of the beam ("half-power" or "3 dB" point).
Bear's Cage
Slang for 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 High Precipitation (HP) storm. The term reflects the danger involved in observing such an area visually, which must be done at close range in low visibility.
Beaver('s) Tail

Slang for 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.

Spotters should note the distinction between a beaver tail and a tail cloud. A "true" tail cloud typically is attached to the wall cloud and has a cloud base at about the same level as the wall cloud itself.

A beaver tail, on the other hand, is not attached to the wall cloud and has a cloud base at about the same height as the updraft base (which by definition is higher than the wall cloud). Unlike the beaver tail, the tail cloud forms from air that is flowing from the storm's main precipitation cascade region (or outflow region). Thus, it can be oriented at a large angle to the pseudo-warm front.

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. Then it may be referred to as the Azores High.

Best Track
A subjectively smoothed path, versus a precise and very erratic fix-to-fix path, used to represent tropical cyclone movement. It is based on an assessment of all available data.
Bimetallic Thermometer
A temperature measuring devise usually consisting of two dissimilar metals that expand and contract differentially as the temperature changes.
Bistatic Radar
A radar which uses separate antennas for transmission and reception; usually the transmitter and receiver are at different locations. Bistatic radars depend upon forward scattering of the signal from transmitter to receiver.
Black Blizzard
A local term for a violent dust storm on the south-central Great Plains that darkens the sky and casts a pall over the land. Also called a black roller.
Black Ice
Thin, new ice on fresh or saltwater, appearing dark in color because of its transparency; also, popularly applied to thin hazardous ice coverings on roads.
A winter storm which produces the following conditions for at least 3 hours:
  • Sustained winds or frequent gusts to 35 miles per hour (56 km/h) or greater, and
  • Considerable falling and/or blowing snow reducing visibility to less than 1/4 mile (0.4 kilometer).
Blizzard Warning
This product is issued by the National Weather Service when blizzard conditions are life threatening. The criteria for this warning vary from state to state.
Blocking High
The development of a warm ridge or cutoff high aloft at high latitudes which becomes associated with a cold high at the surface, causing a split in the westerly winds. Such a high will move very slowly, tending to move west during intensification and east during dissipation. It prevents the movement of migratory cyclones across its latitudes. Two examples are a cut-off high and an Omega block.
A descriptor used to amplify observed weather phenomena (dust, sand, snow, and spray) whenever the phenomena are raised to a height of 6 feet (2 meters) or more above the ground and reduces horizontal visibility to less than 7 statute miles (11 kilometers).
Blowing Dust (BLDU)
Wind-driven dust that significantly reduces surface visibility to less than 7 miles (11 kilometers).
Blowing Snow (BLSA)
Wind-driven sand that significantly reduces surface visibility to less than 7 miles (11 kilometers).
Blowing Snow (BLSN)
Wind-driven snow that significantly reduces surface visibility to less than 7 miles (11 kilometers).
Blue Watch (or Blue Box)
Slang for a severe thunderstorm watch.
15 to 25 mph (24 to 40 km/h) winds, generally used in cold conditions. Same wind speed criteria as brisk and breezy.
Border Ice
An ice sheet in the form of a long border attached to the bank or shore.; shore ice.
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.

Bounded Weak Echo Region (BWER)

A radar feature that identifies where the strongest updraft is located in a supercell thunderstorm. This updraft is so strong that large particles do not have time to form in the lower- and mid-levels of the thunderstorm and they are prevented from falling back into the updraft core from updraft.

The weak echo region is bounded when, in a horizontal section, the weak echo region is completely surrounded or bounded by higher reflectivity values.

Bow Echo
A rapidly moving crescent shaped echo on a radar which is convex in the direction of motion. It is associated with strong, straight-line winds. 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. See also Line Echo Wave Pattern (LEWP).
Box (or Watch Box)
Slang for a severe thunderstorm or tornado watch. See blue box and red box.
Braided Stream
Characterized by successive division and rejoining of streamflow with accompanying islands. A braided stream is composed of anabranches.
Brash Ice
Accumulation of floating ice made up of fragments not more than 2 meters across; the wreckage of other forms of.
The failed opening in a dam.
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 Jam
Ice jam that occurs as a result of the accumulation of broken ice pieces.
Breakup Period
The period of disintegration of an ice cover.
15 to 25 mph (24 to 40 km/h) winds, generally used in warm conditions. Same wind speed criteria as blustery and brisk.
Bright Band
The enhanced layer of radar echo caused by the difference in radar reflectivity of ice and water particles. This echo is interpreted as the delineation on a radar display between frozen and liquid precipitation. It shows where the snow is melting and becoming rain.
15 to 25 mph (24 to 40 km/h) winds, generally used in cool conditions. Same wind speed criteria as blustery and breezy.
An acronym for Bulk Richardson Number.
Broken (BKN)
An official sky cover classification for aviation weather observations, descriptive of a sky cover of 5/8 to 7/8. This is applied only when obscuring phenomenon aloft are present - that is, not when obscuring phenomenon are surface-based, such as fog.
Fear of thunder and thunderstorms. See Astraphobia, Astrapophobia, Ceraunophobia, Keraunophobia, and Tonitrophobia.
Brown Snow
Snow intermixed with dust particles. A not uncommon phenomenon in many parts of the world. Snows of other colors, such as yellow snow, are similarly explainable.
Bubble High
A 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 Gage
A water stage recording device that is capable of attaching to a LARC for data automation purposes.
Bulk Richardson Number (BRN)

It is the ratio of the buoyancy (CAPE) of a lifted parcel to the vertical wind shear of the environment in which the parcel is lifted. It correlates well with observed storm type (single, multicell, supercell), especially for CAPEs between 1,500 and 3,000 J/kg.

BRN's less than 45 tend to support supercell structures, but multicellular convection is favored over 45. While the BRN has shown some value as a predictor of storm type, it is a poor predictor of storm rotation because BRN Shear is a "bulk" measure.

For example, it does not take in account the specific effects of directional and speed shear components. High values indicate unstable and/or weakly-sheared environments; low values indicate weak instability and/or strong vertical shear.

Burn Index
A fire control management system that integrates the effects of selected fire danger factors into one or more qualitative or numerical indices from which ease of ignition and probable fire behavior may be estimated. This is also called a Fire Danger Rating.
Slang for an inaccurate forecast or an unsuccessful storm chase; usually a situation in which thunderstorms or severe weather are expected, but do not occur.
Buttress Dam

Buttress 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% 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.

An acronym for Bounded Weak Echo Region.