POLAR EASTERLIESThe 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 FRONTAccording 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 THEORYA theory originated by
whereby a polar front, separating air masses of
polar and tropical origin, gives rise to cyclonic
disturbances, which intensify and travel along the
characteristic life history.
POLAR OUTBREAKThe 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 TROUGHIn 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
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 ENERGYThe 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 LINEA 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 CENTEROn 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
PRESSURE GRADIENTThe 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 FORCEThe force due
to differences of pressure within a fluid mass. In
meteorological literature the term usually refers
only to horizontal pressure force.
PRESSURE PATTERNThe general geo-metric
characteristics of atmospheric pressure distribution
as revealed by isobars on a constant-height chart,
usually the surface chart.
PRESSURE SYSTEMAn individual cyclonic scale
feature of atmospheric circulation; commonly used
to denote either a high or low, less frequently a
ridge or trough.
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.
PROMINENCEA filament-like Protuberance from
the chromosphere of the Sun.
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).
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.