Back-sheared anvil-
Colloquial expression for a cumulonimbus anvil that spreads
upwind into relatively strong winds aloft.
Balance equation- In
general, an equation expressing a balance of quantities in the sense that the
local or individual rates of change are zero.
Ball lightning- A rare
and randomly occurring bright ball of light observed floating or moving through
the atmosphere close to the ground.
Banner cloud- A
cloud plume often observed to extend downwind from isolated, sharp, often
pyramid-shaped mountain peaks, even on otherwise cloud-free
days.
Baroclinic instability-
A hydrodynamic instability arising from the existence of a meridional
temperature gradient (and hence vertical shear of the mean flow and a thermal
wind) in an atmosphere in quasigeostrophic equilibrium and possessing static
stability.
Baroclinic leaf
- A synoptic-scale cloud pattern frequently observed in
satellite imagery just prior to the onset of
cyclogenesis.
Barometric pressure-
The pressure exerted by the atmosphere as a consequence of
gravitational attraction exerted upon the column of air lying directly above the
point in question.
Barometric tendency-
The character and amount of atmospheric pressure change during a
specified period of time, often a three-hour period preceding an observation.
Barometric wave-
Any wave in the atmospheric pressure field.
Barotropic distrubance-
A wave disturbance in a two-dimensional flow, the driving
mechanism for which lies in the variation of vorticity of the basic current
and/or in the variation of the vorticity of the earth about the local vertical.
Barrier jet-
A jet on the windward side of a mountain barrier, blowing
parallel to the barrier.
Barrier layer-
The depth range, where it exists, between the bottom of the
oceanic surface mixed layer and the thermocline, usually at a depth between 30
and 80 m.
Beaded lightning-
A particular aspect of a normal lightning flash occasionally
seen when the observer happens to view end-on a number of segments of the
irregular channel (zigzag lightning) and hence receives an impression of higher
luminosity at a series of locations along the channel.
Bergeron-Findeisen
process- A theoretical explanation of the
process by which precipitation particles may form within a mixed cloud (composed
of both ice crystals and liquid water drops).
Bernoulli's theorem-
As originally formulated, a statement of the conservation of
energy (per unit mass) for an inviscid fluid in steady motion.
Billow cloud-
A cloud variety composed of merged or separate elements that are
elongated and parallel, either suggestive of ocean waves or arranged in ranks
and files.
Black squall-
A squall accompanied by dark clouds and generally by heavy rain.
In an adiabatic process any change in internal energy (for a system of fixed
mass) is solely a consequence of working. For an ideal gas and for most
atmospheric systems, compression results in warming, expansion results in
cooling. See dry-adiabatic process, moist-adiabatic process.
Blue jets-
Weakly luminous upward propagating discharges, blue in color,
emanating from the tops of thunderstorms.
Boltzmann's constant-
The ratio of the universal gas constant to Avogadro's number;
equal to 1.3804 × 1023 W K-1.
Book-end vorticites-
Mesoscale vortices observed at the ends of a line segment of
convective cells, usually cyclonic on the northern end of the system and
anticyclonic on the southern end, for an environment of westerly vertical wind
shear.
Bouguer's Law (Beer-Lambert
Law)- Attenuation of a beam of light by
an optically homogeneous (transparent) medium.
Boussinesq approximation-
An approximation to the dynamical equations of motion whereby
density is assumed to be constant except in the buoyancy term of the vertical
velocity equation.
Bow echo-
A bow-shaped line of convective cells that is often associated
with swaths of damaging straight-line winds and small tornadoes.
Bowen ratio-
The ratio of sensible to latent heat fluxes from the earth's
surface up into the air.
Boyden index-
A measure of the mean thermodynamic stability in a layer beneath
700 mb.
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.
Brightness temperature-
A descriptive measure of radiation in terms of the temperature
of a hypothetical blackbody emitting an identical amount of radiation at the
same wavelength.
Brown cloud-
Phenomenon particularly associated with smog episodes in cities
such as Denver, Colorado.
Brownian motion-
The rapid and chaotic motion of particles suspended in a fluid
at rest as a consequence of fluctuations in the rate at which fluid molecules
collide with the particles.
Brunt-Vaisala frequency-
The frequency at which a displaced air parcel will oscillate
when displaced vertically within a statically stable environment.
Bulk heat flux-
Heat flux averaged over a layer of air, such as the boundary
layer; heat-flux divergence or difference between the top and bottom of a layer
Bulk Richardson number-
An approximation to the gradient Richardson number formed by
approximating local gradients by finite difference across layers.
Buoyancy-
That property of an object that enables it to float on the
surface of a liquid, or ascend through and remain freely suspended in a
compressible fluid such as the atmosphere.
Buoyancy flux-
The vertical kinematic flux of virtual potential temperature,
which when multiplied by the buoyancy parameter yields a flux that is
proportional to buoyancy.
Buoyancy velocity-
A vertical velocity scale for convective boundary layers, that
is related to the buoyant driving force for convection and the depth of the
mixed layer.
Buys Ballot's law-
A law describing the relationship of the horizontal wind
direction in the atmosphere to the pressure distribution.
