A B C D E F G H I J K L M N O P Q R S T U V W X Y Z #

Back Door Cold Front

cold front moving west or southwest (from the east or northeast). Typically, a cold front moves from the north, northwest, or west. A back door cold front can occur when the clockwise rotation around a high-pressure system wraps cold air around, bringing the cold air toward the south and west.

Back-Building Thunderstorm

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

thunderstorm anvil which spreads upwind, against the flow aloft, often implying that a very strong updraft and, hence, a high severe weather potential exists

Backing Winds

 winds that 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). 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, which might increase the shear. The opposite of veering winds.

Ball Lightning

a relatively rare form of lightning consisting of a luminous ball that moves rapidly along solid objects or remains floating in mid-air. Also known as globe lightning.


the range of frequencies (in Hertz) between the limits of a frequency band. Bandwidth is a measure of how well radio energy input is passed through the receiver without distortion or loss of data. It is one of the variables determining the minimum detectable signal of a radar unit. The shorter the pulse duration, the larger the bandwidth required to preserve the same quality of receiver output pulses.

Baroclinic Zone

a region in which a temperature gradient exists on a surface of constant pressure; not barotropic. Baroclinic zones are favored areas for strengthening and weakening weather systems. Wind shear is characteristic of a baroclinic zone.

Baroclinity (or Baroclinicity)

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


an instrument for determining the pressure of the atmosphere

Barometric Pressure

see Atmospheric Pressure


this term usually is used in a relative sense to describe systems in which the isotherms and height contours are nearly parallel everywhere on a surface of constant pressuredirectional wind shear is weak; as a rule, a true equivalent barotropic system can never be achieved in the real atmosphere


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.

Base Reflectivity

one of the three fundamental quantities (along with base [radial] velocity and spectrum width) that a Doppler radar measures. Reflectivity is related to the power, or intensity, of the reflected radiation that is sensed by the radar antenna. Base reflectivity is expressed on a logarithmic scale in units called dBZ. The term "base" refers to the product being "basic", with little advanced processing performed on the data. Base reflectivity is related to rainfall intensity (e.g., drop size and rainfall rate) and hail size (for large values of reflectivity).

Base Velocity

one of the three fundamental quantities (along with base reflectivity and spectrum width) that a Doppler radar measures. Base [radial] velocity is the average velocity (towards or away from the radar looking in a specific direction) of the hydrometeors detected in the radar pulse volume. Base velocity is expressed as being positive or negative, with positive values (warm colors) being interpreted as flow away from the radar and negative values (cool colors) being interpreted as flow towards the radar. The term "base" refers to the product being "basic" with little advanced processing performed on the data.










blowing dust

Beam Width

the angle between the center of the radar beam and the point in the beam where the power of the transmitted energy is one-half of the power at the center's maximum. A WSR-88D radar’s beam width is approximately 1 degree.

Bear's Cage

a region of rotation in a thunderstorm that is wrapped in heavy precipitation. This can often be seen on radar as the area of the hook echo. In order for a spotter to observe this area for signs of a possible tornado, he or she must try to peer through low visibility at close range, which is very dangerous.

Beaufort Scale

a system used to estimate and report wind speeds when no method of measurement is available.


Beaufort Number
Wind Speed (mph)
Effect on Sea Surface
Less than 1
Sea surface smooth and mirror-like
Smoke rises vertically
1 to 3.5
Light Air
Scaly ripples, no foam crests
Smoke drift indicates wind direction, still wind vanes
4.5 to 7
Light Breeze
Small wavelets, crests glassy, no breaking waves
Wind felt on face, leaves rustle, vanes begin to move
8 to 11.5
Gentle Breeze
Large wavelets, crests begin to break, scattered whitecaps
Leaves and small twigs constantly moving, light flags extended
12.5 to 18.5
Moderate Breeze
Small waves 1 to 4 feet, becoming longer, numerous whitecaps
Dust, leaves, and loose paper lifted, small tree branches move
19.5 to 24
Fresh Breeze
Moderate waves 4 to 8 feet taking longer form, many whitecaps, some spray
Small trees in leaf begin to sway
25 to 31
Strong Breeze
Larger waves 8 to 13 feet, whitecaps common, more spray
Larger tree branches moving, whistling in wires
32 to 38
Near Gale
Sea heaps up, waves 13 to 20 feet, white foam streaks off breakers
Whole trees moving, resistance felt walking against wind
39 to 46
Moderate high (13 to 20 feet) 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
47 to 54
Strong Gale
High waves (20 feet), sea begins to roll, dense streaks of foam, spray may reduce visibility
Slight structural damage occurs, slate blows off roofs
55 to 63
Very high waves (20 to 30 feet) with overhanging crests, sea white densely blown foam, heavy rolling, lowered visibility
Seldom experienced on land, trees broken or uprooted, "considerable structural damage"
64 to 72.5
Violent Storm
Exceptionally high (30 to 45 feet) waves, foam patches cover sea, visibility more reduced
Greater than 73.5
Air filled with foam, waves over 45 feet, sea completely white with driving spray, visibility greatly reduced

Beaver Tail

a type of inflow band with a broad, flat appearance that looks like a beaver’s tail.

Bergeron Process

also known as the Wegener-Bergeron-Findeisen process or Bergeron-Findeisen process - the process by which ice crystals in a cloud grow at the expense of supercooled liquid water droplets.

Bermuda High

the semi-permanent atmospheric subtropical anticyclone (high pressure system) over the North Atlantic Ocean, so named especially when it is located in the western part of the ocean, near Bermuda (near 30°N).

Bernoulli's Principle

air flowing over an airfoil results in an increase in flow speed over the upper curved surface. Since a velocity increase in fluid flow results in a corresponding pressure decrease, the increased airflow over the upper surface of the airfoil produces a lift on the airfoil because of lower pressure exerted on the upper surface. Named for Daniel Bernoulli (1700 -1782), a Swiss physicist who discovered the effect.








see Broken


see Boundary Layer

Black Frost

freeze in which there is no visible frost, but plants freeze. After thawing, the plants often turn limp and black. There is no visible frost because the below-freezing air temperature is not close enough to the frost point for deposition of ice to occur.

Black Ice

patchy ice on roadways that cannot easily be seen.


a hypothetical "body" that absorbs all of the electromagnetic radiation striking it - it does not reflect or transmit any of the incident radiation. The radiation emitted is consistent with Planck’s law. In accordance with Kirchhoff’s law, a blackbody not only absorbs all wavelengths, but emits at all wavelengths with the maximum possible intensity for any given temperature. Contrast with whitebody and graybody.

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.





Blizzard (BLZD)

severe weather condition characterized by low temperatureswinds of 32 mph or higher, and sufficient snow for visibility to be reduced to less than 500 ft




see Blizzard


blowing sand

Boiling Point

the temperature at which a liquid boils

Boltzmann's Constant

the ratio of the universal gas constant to Avogadro’s number; equal to 1.38062 X 10-23 joules per Kelvin. Named for Ludwig Boltzmann (1844 -1906), an Austrian physicist.

Bomb Cyclogenesis

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

Bookend Vortices (singular: vortex)

also known as line end vortices. Areas of circulation that may form at the ends of a bow echoTornado formation may occur in these vortices.

Boundary Layer (BL)

in general, a layer of air near 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 solar heating 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)

also known as a vault; 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, in the field.

Bow Echo

radar echo that is linear but bent outward in the shape of a bow (i.e., used by an archer). Damaging straight-line winds often occur near the "crest" or center of a bow echo. The left (usually northern) end of the bow is a preferred location for the formation of tornadoes (see Bookend Vortices).

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.

Boyle's Law

the empirical generalization that for many so-called perfect gases, the product of pressure and volume is constant in an isothermal process. Named for Robert Boyle (1627-1691), a British chemist who formulated this relationship.


see Mist





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.


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

Brightness Temperature

the apparent temperature of a celestial object, based on the assumption that it radiates as a blackbody



Broken (BKN)

a classification for sky cover used when 0.6 (six tenths) to 0.9 (nine tenths) of the sky is covered by clouds


blowing snow





Bubble High

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.

Bulk Richardson Number (or BRN)

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.


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


Slang for an inaccurate forecast, especially one where significant weather (e.g., heavy snowfall, tornado outbreak, etc.) is predicted but does not occur.