Glossary
Back Door Cold Front
a 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
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
a 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.
Bandwidth
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).
Barotropic
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 pressure; directional wind shear is weak; as a rule, a true equivalent barotropic system can never be achieved in the real atmosphere
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.
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.
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.
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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.
Black Frost
a 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.
Blackbody
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 temperatures, winds of 32 mph or higher, and sufficient snow for visibility to be reduced to less than 500 ft
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 echo. Tornado 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
a 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.
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.
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
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
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.
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.
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
