NEPHANALYSIS.—The analysis of a synop-ticchart in terms of the types and amounts of clouds and precipitation.
NEPHCURVE.—In nephanalysis, a linebounding a significant portion of a cloud system—for example, a clear-skyline, a precipita-tion line, a cloud-type line, or a ceiling-height line.
NEUTRAL EQUILIBRIUM.—A property ofthe steady state of a system which exhibits neither instability nor stability according to the particular criterion under consideration. A disturbance in-troduced into such an equilibrium will thus be neither amplified nor damped.
NEUTRAL STABILITY.—The state of anunsaturated or saturated column. of air in the atmosphere when its environmental lapse rate of temperature is equal to the dry-adiabatic lapse rate or the saturation-adiabatic lapse rate, respectively. Under such conditions a parcel of air displaced vertically will experience no buoyant acceleration.
NEUTRAL WAVE.—Any wave whoseamplitude does not change with time. In most contexts these waves are referred to as stable waves, the term neutral wave being used when it is important to emphasize that the wave is neither damped nor amplified.
NORTHEAST TRADES.—The trade windsof the Northern Hemisphere.
OCCLUDED FRONT.—(commonly calledocclusion; also called frontal occlusion) A com-posite of two fronts, formed as a cold front overtakes a warm front or quasi-stationary front. This is a common process in the late stages of wave-cyclone development, but it is not limited to occurrence within a wave cyclone.
OCCLUSION.—Same as OCCLUDEDFRONT.
OCEAN WEATHER STATION.—As de-finedby the World Meteorological Organization, a specific maritime location occupied by a ship equipped and staffed to observe weather and sea conditions and report the observations by inter-national exchange.
OROGRAPHIC LIFTING.—The lifting ofan air current caused by its passage up and over mountains.
OVERRUNNING.—A condition existingwhen an air mass is in motion aloft above another air mass of greater density at the surface. This term is usually applied in the case of warm air ascending the surface of a warm or quasi-stationary front.
PARAMETER.—(1) In general, any quantityof a problem that is not an independent variable. More specifically, the term is often used to distinguish, from dependent variables, quantities that may be more or less arbitrarily assigned values for purposes of the problem at hand. (2) Commonly and carelessly used by many meteorologists for almost any meteorological quantity or element.
PARTIAL PRESSURE.—The pressure of asingle component of a gaseous mixture, according to Dalton’s Law.
PERTURBATION.—Any departure in-troducedinto an assumed steady state of a system. In synoptic meteorology, the term most often refers to any departure from zonal flow within the major zonal currents of the atmosphere. It is especially applied to the wave-like disturbances within the tropical easterlies.
PHOTOSPHERE.—The intensely bright por-tionof the Sun visible to the unaided eye. It is a shell a few hundred miles in thickness marking the boundary between the dense interior gases of the Sun and the more diffuse cooler gases in the outer portions of the Sun.
PLANETARY BOUNDARY LAYER.—(akocalled friction layer or atmospheric boundary layer) That layer of the atmosphere from Earth’s surface to the geostrophic wind level, including therefore, the surface boundary layer and the Eckman layer.
PLANETARY CIRCULATION.—The sys-temof large-scale disturbances in the troposphere when viewed on a hemispheric or worldwide scale. Same as GENERAL CIRCULATION.
POLAR AIR.—A type of air whosecharacteristics are developed over high latitudes, especially within the subpolar highs. Continen-tal polar air (cP) has low surface temperature, low moisture content, and, especially in its source regions, great stability in the lower layers. It is shallow in comparison with arctic air.
POLAR EASTERLIES.—The rather shallowand 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-fronttheory, the semipermanent, semicontinuous 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 origi-natedby 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 acold 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 meteoro-logy,a wave trough in the westerlies having suf-ficient 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 eleva-tions it is characterized by westerly winds. It moves generally from west to east and is accom-panied by considerable cloudiness at all levels. Cumulus congestus 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 abody 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 squallline or instability line located in the warm sector of a wave cyclone, about 50 to 300 miles in ad-vance 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 ofdecrease (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.—Theforce 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-metriccharacteristics of atmospheric pressure distribution as revealed by isobars on a constant-height chart, usually the surface chart.
PRESSURE SYSTEM.—An individual cy-clonic-scale feature of atmospheric circulation; commonly used to denote either a high or low, less frequently a ridge or trough.
PRIMARY CIRCULATION.—The prevail-ingfundamental atmospheric circulation on a planetary scale that must exist in response to (a) radiation differences with latitude, (b) the rota-tion 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 pro-tuberancefrom the chromosphere of the Sun.
QUASI-STATIONARY FRONT.—(Commonlycalled stationary front) A front that is stationary or nearly so. Conventionally, a front that is mov-ing 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).