POLAR   EASTERLIES—The   rather   shallow   and diffuse body of easterly winds located poleward of the subpolar low-pressure belt. In the mean in the Northern Hemisphere, these easterlies exist to an appreciable extent only north of the Aleutian low and Icelandic low. POLAR    FRONT—According    to    the    polar-front theory, the semi permanent, semi continuous front separating air masses of tropical and polar origin. This is the major front in terms of air mass contrast and susceptibility to cyclonic disturbance. POLAR-FRONT THEORY—A theory originated by the Scandinavian school of meteorologists whereby  a  polar  front,  separating  air  masses  of polar  and  tropical  origin,  gives  rise  to  cyclonic disturbances, which intensify and travel along the front, passing through various phases of a characteristic life history. POLAR OUTBREAK—The movement of a cold air mass   from   its   source   region;   almost   invariably applied  to  a  vigorous  equatorward  thrust  of  cold polar  air,  a  rapid  equatorward  movement  of  the polar front. POLAR TROUGH—In tropical meteorology, a wave trough  in  the  wastrels  having  sufficient  amplitude to reach the tropics in the upper air. At the surface it is reflected as a trough in the tropical easterlies, but at   moderate   elevations   it   is   characterized   by westerly  winds.  It  moves  generally  from  west  to east and is accompanied by considerable cloudiness  at  all  levels. Cumulus  congests  and cumulonimbus  clouds  are  usually  found  in  and around the trough lines. The early and late season hurricanes   of   the   western   Caribbean   frequently form in polar troughs. POTENTIAL  ENERGY—The  energy  that  a  body possesses  as  a  consequence  of  its  position  in  the field  of  gravity;  numerically  equal  to  the  work required   to   bring   the   body   from   an   arbitrary standard level, usually taken as mean sea level, to its given position. PRE-FRONTAL SQUALL LINE—A  squall  line  or instability line located in the warm sector of a wave cyclone,  about  50  to  300  miles  in  advance  of  the cold front, usually oriented roughly parallel to the cold front and moving in about the same manner as the cold front. PRESSURE CENTER—On a synoptic chart, a point of local minimum or maximum pressure; the center of  a  low  or  high.  It  is  also  a  center  of  cyclonic  or anticyclonic circulation. PRESSURE   GRADIENT—The   rate   of   decrease (gradient) of pressure in space at a fixed time. The term  is  sometimes  loosely  used  to  denote  simply the magnitude of the gradient of the pressure field. PRESSURE  GRADIENT  FORCE—The  force  due to  differences  of  pressure  within  a  fluid  mass.  In meteorological  literature  the  term  usually  refers only to horizontal pressure force. PRESSURE   PATTERN—The   general   geo-metric characteristics of atmospheric pressure distribution as  revealed  by  isobars  on  a  constant-height  chart, usually the surface chart. PRESSURE SYSTEM—An individual cyclonic scale feature of atmospheric circulation; commonly used to  denote  either  a  high  or  low,  less  frequently  a ridge or trough. PRIMARY CIRCULATION—The prevailing fundamental atmospheric circulation on a planetary  scale  that  must  exist  in  response  to  (a) radiation differences with latitude, (b) the rotation of Earth, and (c) the particular distribution of land and   oceans;   and   which   is   required   from   the viewpoint of conservation of energy. PROMINENCE—A filament-like Protuberance from the chromosphere of the Sun. QUASI-STATIONARY FRONT—(Commonly called stationary front) A front that is stationary or nearly so. Conventionally, a front that is moving at a   speed   less   than   about   5   knots   is   generally considered to be quasi-stationary. In synoptic chart analysis, a quasi-stationary front is one that has not moved  appreciably  from  its  position  on  the  last (previous) synoptic chart (3 or 6 hours before). RADIATION—(1) The process by which electromagnetic  radiation  is  propagated  through free space by virtue of joint undulatory variations in  the  electric  and  magnetic  fields  in  space.  This concept is to be distinguished from convection and conduction.  (2)  The  process  by  which  energy  is propagated  through  any  medium  by  virtue  of  the wave motion of that medium, as in the propagation of sound waves through the atmosphere, or ocean waves along the water surface. AI-6