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AMS Glossary
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Section BB index501-509 of 517 terms
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  		•  buoyant instability—That static instability in a system in which buoyancy or reduced gravity is the only restoring force on displacements.
    In general, a fluid is buoyantly unstable when the environmental lapse rate of density is greater than the process lapse rate of density. For an incompressible fluid this requires an increase of density with height; for the atmosphere, when lifting is assumed to be adiabatic, it requires the lapse rate (of temperature) to be greater than the adiabatic lapse rate.
  		• buoyant subrange—A range of turbulent eddies in a stably stratified atmosphere, too small to be influenced by shear but large enough to be affected by buoyancy.
    Dimensional considerations give a power law dependence of energy versus wavenumber, or spectral slope, of −11/5. See turbulence spectrum.
  		• buran—A strong northeast wind in Russia and central Asia.
    It is most frequent in winter when it resembles a blizzard, that is, very cold and lifting snow from the ground; as such it is called white buran or, on the tundra, purga. A similar wind in Alaska is called burga. The buran also occurs, but less frequently, in summer, when it raises dust clouds; it is then called karaburan.
  		• burga—(Also spelled boorga.) A northeasterly storm in Alaska, bringing sleet or snow; it is similar to the winter buran or purga of Russia and Siberia.
    See blizzard.
  		• Burger number—A dimensionless number, Bu, for atmospheric or oceanographic flow expressing the ratio between density stratification in the vertical and the earth's rotation in the horizontal:

    where N is the Brunt–Väisälä frequency, Ω is the angular rotation rate of the earth, H is the scale height of the atmosphere, L is a horizontal length scale of typical motions, Ro is the Rossby number, Fr is the Froude number, and RD is the Rossby deformation radius.
    Bu is often of order one for many atmospheric phenomena, meaning that both stratification and rotation play nearly equal roles in governing vertical and other motions in the fluid.
              Cushman–Roisin, 1994: Introduction to Geophysical Fluid Dynamics, 320 pp.
  		• Burger's vortex—Exact solution of the Navier–Stokes equations for a steady vortex in which the diffusion of vorticity is balanced by vortex stretching in an external strain field.
    The vortex has a Gaussian shape. For example, for a vertical axisymmetric vortex in an external velocity field given by

    the vorticity is

    in which νにゅー is the kinematic viscosity.
  		• buria—Bulgarian term for bora.
  		• burn off—With reference to fog or low stratus cloud layers, to dissipate by heating from the sun, primarily during the early morning hours.
  		• burning index—A relative number related to the contribution that fire behavior makes to the amount of effort needed to contain a fire of a specified fuel type.
    The calculated burning index falls on a scale of 1–100: 1–11 is no fire danger; 12–35 is medium danger; 40–100 is high danger. See fire-danger meter.
  		• burraxka silch—Hailstorm occurring in the Mediterranean, near Malta.
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