POLAR EASTERLIES—The rather shallow anddiffuse body of easterly winds located poleward ofthe subpolar low-pressure belt. In the mean in theNorthern Hemisphere, these easterlies exist to anappreciable extent only north of the Aleutian lowand Icelandic low.POLAR FRONT—According to the polar-fronttheory, the semi permanent, semi continuous frontseparating air masses of tropical and polar origin.This is the major front in terms of air mass contrastand susceptibility to cyclonic disturbance.POLAR-FRONT THEORY—A theory originated bytheScandinavianschoolofmeteorologistswhereby a polar front, separating air masses ofpolar and tropical origin, gives rise to cyclonicdisturbances, which intensify and travel along thefront,passingthroughvariousphasesofacharacteristic life history.POLAR OUTBREAK—The movement of a cold airmass from its source region; almost invariablyapplied to a vigorous equatorward thrust of coldpolar air, a rapid equatorward movement of thepolar front.POLAR TROUGH—In tropical meteorology, a wavetrough in the wastrels having sufficient amplitudeto reach the tropics in the upper air. At the surface itis reflected as a trough in the tropical easterlies, butat moderate elevations it is characterized bywesterly winds. It moves generally from west toeastandisaccompaniedbyconsiderablecloudiness at all levels.Cumulus congests andcumulonimbus clouds are usually found in andaround the trough lines. The early and late seasonhurricanes of the western Caribbean frequentlyform in polar troughs.POTENTIAL ENERGY—The energy that a bodypossesses as a consequence of its position in thefield of gravity; numerically equal to the workrequired to bring the body from an arbitrarystandard level, usually taken as mean sea level, toits given position.PRE-FRONTAL SQUALL LINE—A squall line orinstability line located in the warm sector of a wavecyclone, about 50 to 300 miles in advance of thecold front, usually oriented roughly parallel to thecold front and moving in about the same manner asthe cold front.PRESSURE CENTER—On a synoptic chart, a pointof local minimum or maximum pressure; the centerof a low or high. It is also a center of cyclonic oranticyclonic circulation.PRESSURE GRADIENT—The rate of decrease(gradient) of pressure in space at a fixed time. Theterm is sometimes loosely used to denote simplythe magnitude of the gradient of the pressure field.PRESSURE GRADIENT FORCE—The force dueto differences of pressure within a fluid mass. Inmeteorological literature the term usually refersonly to horizontal pressure force.PRESSURE PATTERN—The general geo-metriccharacteristics of atmospheric pressure distributionas revealed by isobars on a constant-height chart,usually the surface chart.PRESSURE SYSTEM—An individual cyclonic scalefeature of atmospheric circulation; commonly usedto denote either a high or low, less frequently aridge or trough.PRIMARY CIRCULATION—Theprevailingfundamentalatmosphericcirculationonaplanetary scale that must exist in response to (a)radiation differences with latitude, (b) the rotationof Earth, and (c) the particular distribution of landand oceans; and which is required from theviewpoint of conservation of energy.PROMINENCE—A filament-like Protuberance fromthe chromosphere of the Sun.QUASI-STATIONARY FRONT—(Commonlycalled stationary front) A front that is stationary ornearly so. Conventionally, a front that is moving ata speed less than about 5 knots is generallyconsidered to be quasi-stationary. In synoptic chartanalysis, a quasi-stationary front is one that has notmoved appreciably from its position on the last(previous) synoptic chart (3 or 6 hours before).RADIATION—(1)Theprocessbywhichelectromagnetic radiation is propagated throughfree space by virtue of joint undulatory variationsin the electric and magnetic fields in space. Thisconcept is to be distinguished from convection andconduction. (2) The process by which energy ispropagated through any medium by virtue of thewave motion of that medium, as in the propagationof sound waves through the atmosphere, or oceanwaves along the water surface.AI-6
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