CONDENSATION—The physical process by which a vapor becomes a liquid or solid. CONDITIONAL   INSTABILITY—The   state   of   a column    of    air    in    the    atmosphere    when    its temperature lapse rate is less than the dry adiabatic lapse rate but greater than the saturation adiabatic lapse rate. CONVECTION—Atmospheric motions that are predominantly   vertical,   resulting   in   the   vertical transport and mixing of atmospheric properties. CORONA—(1)  A  set  of  one  or  more  prismatically colored rings of small radii, concentrically surrounding  the  disk  of  the  Sun,  Moon,  or  other luminary  when  veiled  by  a  thin  cloud.   A  corona maybe  distinguished  from  the  relatively  common 22° halo by its color sequence, which is from blue inside to red outside, the reverse of that of the 22° halo.  Diffraction and reflection of light from water droplets  produce  coronas.  (2)  The  pearly  outer envelope of the Sun. COUNTERRADIATION—(also called back radiation)   The   downward   flow   of   atmospheric radiation  passing  through  a  given  level  surface, usually taken as Earth’s surface. It is the principal factor in the GREENHOUSE EFFECT. CUT-OFF HIGH—A warm high displaced and lying poleward of the basic westerly current. CUT-OFF  LOW—A  cold  low  displaced  and  lying equatorward of the basic westerly current. CYCLOGENESIS—Any development or strengthening of cyclonic circulation in the atmosphere. The  initial  appearance  of  a  low  or trough, as well as the intensification of an existing cyclonic flow. CYCLOLYSIS—Any weakening of cyclonic circulation in the atmosphere. CYCLONIC—A   counterclockwise   rotation   in   the Northern  Hemisphere  and  a  clockwise  rotation  in the Southern Hemisphere. DISPERSION—The   process   in   which   radiation   is separated   into   its   component   wavelengths. It results when an optical process, such as diffraction, refraction,    or    scattering,    varies    according    to wavelength.   All   of   the   coloration   displayed   by atmospheric  optical  phenomena  are  the  result  of dispersion. DOLDRUMS—A   nautical   term   for   the   equatorial trough,   with   special   reference   to   the   light   and variable nature of the winds. DOWNWIND—The direction toward which the wind is blowing; with the wind. DRY   AIR—In    atmospheric    thermodynamics    and chemistry, air that contains no water vapor. ELECTROMAGNETIC  WAVES—Disturbances  in electric and magnetic fields in space or in material media, resulting in the propagation of electromagnetic energy (radiation). EQUINOX—(1) Either of the two points of intersection of the Sun’s apparent annual path and the plane of Earth’s equator. (2) Popularly, the time at which the Sun passes directly above the equator; the    “time    of    the    equinox.”    In    the    Northern Hemisphere, the vernal equinox falls on or about 21 March,  and  the  autumnal  equinox  on  or  about  22 September.    These    dates    are    reversed    in    the Southern Hemisphere. EVAPORATION—The  physical  process  by  which  a liquid or solid is transformed to the gaseous state. FRONT—The  interface  or  transition  zone  between two air masses of different density. Since temperature   distribution   is   the   most   important regulator  of  atmospheric  density,  a  front  almost invariably separates air masses of different temperature. FRONTAL  INVERSION—A  temperature  inversion in   the   atmosphere,   encountered   upon   vertical ascent through a sloping front. FRONTAL  SURFACE—Refers   specifically   to   the warmer side of the frontal zone. FRONTAL SYSTEM—Simply, a system of fronts as they appear on a synoptic chart. This is used for (a) a continuous front and its characteristics along its entire  extent,  including  its  warm,  cold,  stationary, and occluded sectors, its variations of intensity, and any    frontal    cyclones    along    it;    and    (b)    the orientation   and   nature   of   the   fronts   within   the circulation of a frontal cyclone. FRONTAL ZONE—The transition zone between two adjacent air masses of different densities bounded by a frontal surface. FRONTOGENESIS—The initial formation of a front or frontal zone. AI-2