B

Abbreviation used on long-term climate outlooks issued by CPC to indicate areas that are likely to be below normal for a given parameter (temperature, precipitation, etc.).

Back Door Cold Front

A cold front moving south or southwest along the Atlantic seaboard and Great Lakes; these are especially common during the spring months.

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-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.

Backfire

A fire started to stop an advancing fire by creating a burned area in its path.

Backflow

In hydrologic terms, the backing up of water through a conduit or channel in the direction opposite to normal flow.

Backing

(abbrev. BCKG)- A counterclockwise shift in wind direction (for example, south winds shifting to the east).

Backing Winds

Winds which shift in a counterclockwise direction with time at a given location (e.g. from southerly to southeasterly), or change direction in a counterclockwise sense with height (e.g. westerly 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.

Backscatter

The portion of power scattered back in the incident direction.

Backsight

In hydrologic terms, 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

In 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 Effect

In hydrologic terms, the effect which a dam or other obstruction has in raising the surface of the water upstream from it.

Backwater Flooding

Hydrologic terms, upstream flooding caused by downstream conditions such as channel restriction and/or high flow in a downstream confluence stream.

BAM

Binary Angular Measure

BANDPASS FILTER

A filter whose frequencies are between given upper and lower cutoff values, while substantially attenuating all frequencies outside these values (this band).

Bandwidth

The frequency range between the lowest and highest frequencies that are passed through a component, circuit, or system with acceptable attenuation.

Bank

In hydrologic terms, the margins of a channel. Banks are called right or left as viewed facing in the direction of the flow.

Bank Storage

In 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

The water level, or stage, at which a stream, river or lake is at the top of its banks and any further rise would result in water moving into the flood plain.

Bankfull Stage

An 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 Cloud

A cloud plume often observed to extend downwind behind isolated mountain peaks, even on otherwise cloud-free days.

BAPSU

Bay Area Public Service Unit. Public Service section of the San Francisco Bay Area Weather Service Forecast Office.

Bar

An obstacle formed at the shallow entrance to the mouth of a river or bay.

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 shield

A cloud pattern on satellite images - frequently noted in advance of formation of a low pressure center.

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.

Baroclinity

A measure of the state of stratification in a fluid in which surfaces of constant pressure (isobaric) intersect surfaces of constant density (isosteric).

Barogram

An analog record of pressure produced by a barograph

Barograph

A barometer that records its observations continuously.

Barometer

An instrument that measures atmospheric pressure.

Barometric Pressure

The pressure of the atmosphere as indicated by a barometer.

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.

Barotropy

The state of a fluid in which surfaces of constant density (or temperature) are coincident with surfaces of constant pressure; it is the state of zero baroclinity.

Barrage

In 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 Jet

A 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 Number

The 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 DATA

Those digital fields of reflectivity, mean radial velocity, and spectrum width data in spherical coordinates provided at the finest resolution available from the radar.

Base Flood

In 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 PRODUCTS

Those 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 Reflectivity

Base 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 scan

Base Station

In 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 Width

In hydrologic terms, the time duration of a unit hydrograph.

Baseflow

In 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

An area having a common outlet for its surface runoff. 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

In hydrologic terms, the time it takes from the centroid of rainfall for the hydrograph to peak.

Basin Recharge

In 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.

Bathymetry

The science of measuring depths of the oceans, lakes, seas, etc.

BCKG

Backing- A counterclockwise shift in wind direction (for example, south winds shifting to the east).

BCM

Become

BCMNG

Becoming

BD

Blowing Dust

Beach Erosion

The movement of beach materials by some combination of high waves, currents and tides, or wind.

BEAM FILLING

The 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 WIDTH

Angular 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 Scale

The 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 Load

In 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 Freezup

In hydrologic terms, date on which ice forming a stable winter ice cover is first observed on the water surface

Beginning of the Breakup

In 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 Process

The process by which ice crystals in a cloud grow at the expense of supercooled liquid water droplets.

Bergy Bit

A 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 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.

Best Track

A 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.

BFR

Before

BGN

Begin

BHND

Behind

BIAS

A systematic difference between an estimate of and the true value of a parameter.

Billow Cloud

A cloud consisting of broad parallel bands oriented perpendicular to the wind.

BIN

Radar sample volume.

BINOVC

Breaks in Overcast

BKN

Broken

BL

Abbreviation for Boundary Layer; 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.

Black Ice

1. 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.

Blackbody

A hypothetical "body" that absorbs all of the electromagnetic radiation striking it - it does not reflect or transmit any of the incident radiation. A blackbody not only absorbs all wavelengths, but emits at all wavelengths with the maximum possible intensity for any given temperature.

Blackbody Radiation

The electromagnetic radiation emitted by an ideal blackbody adhering to the radiation laws; it is the theoretical maximum amount of electromagnetic radiation of all wavelengths that can be emitted by a body at a given temperature.

BLD

Build

BLDUP

Buildup

Blizzard

(abbrev. BLZD)- A blizzard means that the following conditions are expected to prevail for a period of 3 hours or longer: 

• Sustained wind or frequent gusts to 35 miles an hour or greater; and
• Considerable falling and/or blowing snow (i.e., reducing visibility frequently to less than ¼ mile)

Blizzard Warning

Issued for winter storms with sustained or frequent winds of 35 mph or higher with considerable falling and/or blowing snow that frequently reduces visibility to 1/4 of a mile or less. These conditions are expected to prevail for a minimum of 3 hours.

BLO

Below

Blocked Flow

Flow approaching a mountain barrier that is too weak or too stable to be carried over the barrier.

Blowing

A descriptor used to amplify observed weather phenomena whenever the phenomena are raised to a height of 6 feet or more above the ground

Blowing Dust or Sand

Strong winds over dry ground, that has little or no vegetation, can lift particles of dust or sand into the air. These airborne particles can reduce visibility, cause respiratory problems, and have an abrasive affect on machinery. A concentration reducing the visibility to ¼ mile or less often poses hazards for travelers.

Blowing Snow

Blowing snow is wind-driven snow that reduces surface visibility. Blowing snow can be falling snow or snow that has already accumulated but is picked up and blown by strong winds. Blowing snow is usually accompanied by drifting snow.

Blowing Snow Advisory

Issued when wind driven snow reduces surface visibility, possibly, hampering traveling. Blowing snow may be falling snow, or snow that has already accumulated but is picked up and blown by strong winds.

Blue Watch or Blue Box

[Slang], a severe thunderstorm watch.

Blustery

Same as Breezy; 15 to 25 mph winds.

BLV

before

BLZD

Blizzard- A blizzard means that the following conditions are expected to prevail for a period of 3 hours or longer: 

• Sustained wind or frequent gusts to 35 miles an hour or greater; and
• Considerable falling and/or blowing snow (i.e., reducing visibility frequently to less than ¼ mile)

BN

Blowing Sand

BNDRY

Boundary

Bomb

Popular expression of a rapid intensification of a cyclone (low pressure) with surface pressure expected to fall by at least 24 millibars in 24 hour.

Bora

A regional downslope wind whose source is so cold that it is experienced as a cold wind, despite compression warming as it descends the lee slope of a mountain range.

Border Ice

In hydrologic terms, 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 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.

BOVC

Base of Overcast

Bow Echo

A 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 Ratio

For 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 Model

A 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.

BR

Mist

Brackish Ice

In hydrologic terms, ice formed from brackish water.

Braided Stream

In hydrologic terms, characterized by successive division and rejoining of streamflow with accompanying islands. A braided stream is composed of anabranches.

Brash Ice

In hydrologic terms, accumulation of floating ice made up of fragments not more than 2 meters across; the wreckage of other forms of ice.

BRD

Border

Breach

In hydrologic terms, the failed opening in a dam.

Breakers

Waves that break, displaying white water. Depends on wave steepness and bottom bathymetry.

Breakup

In 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 Date

In hydrologic terms, date on which a body of water is first observed to be entirely clear of ice and remains clear thereafter.

Breakup Jam

In hydrologic terms, an ice jam that occurs as a result of the accumulation of broken ice pieces.

Breakup Period

In hydrologic terms, the period of disintegration of an ice cover.

Breezy

15 to 25 mph winds

BRF

Brief

Bright Band

A distinct feature observed by a radar that denotes the freezing level of the atmosphere. The term originates from a horizontal band of enhanced reflectivity that can result when a radar antenna scans vertically through precipitation. The freezing level in a cloud contains ice particles that are coated with liquid water. These particles reflect significantly more radiation (appearing to the radar as large raindrops) than the portions of the cloud above and below the freezing layer. The bright band can affect the ability of the NEXRAD algorithms to produce accurate rainfall estimates at far ranges because the algorithm may interpret reflectivity from the bright band as an overestimate of precipitation reaching the surface.

Bright Band

The enhanced radar echo of snow as it melts to rain.

Bright 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.

Brightness

A basic visual sensation describing the amount of light that appears to emanate from an object, or more precisely, the luminance of an object

Brisk

15 to 25 mph winds

Brisk Wind Advisory

A Small Craft Advisory issued by the National Weather Service for ice-covered waters.

BRK

Break

BRN

(Bulk Richardson Number) A non-dimensional number relating vertical stability and 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.

Broadband

A method of signaling in which multiple signals share the bandwidth of the transmission by the subdivision of the bandwidth into channels based on frequency.

Brocken Specter

An 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 Level

A layer of the atmosphere with 5/8 to 7/8 sky cover (cloud cover).

BS

Blowing Snow

BTR

Better

BTWN

Between

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

In hydrologic terms, a water stage recording device that is capable of attaching to a LARC for data automation purposes.

BUFKIT

A software tool used by forecasters to examine the vertical profile and other aspects of the atmosphere.

Bulk Richardson Number

A 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.

Buoyancy

The tendency of a body to float or to rise when submerged in a fluid; the power of a fluid to exert an upward force on a body placed in it.

Burst

In solar-terrestrial terms, a transient enhancement of the solar radio emission, usually associated with an active region or flare.

Bust

Slang for an inaccurate forecast, especially one where significant weather (e.g., heavy snowfall) is predicted but does 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 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.

BWER

Abbreviation 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.

BYD

            Beyond

 

 

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