Aeronautical Terms beginning with P

Pressure altitude

The altitude in the standard atmosphere at which the pressure is the same as at the point in question. Since an altimeter operates solely on pressure, this is the uncorrected altitude indicated by an altimeter set at standard sea level pressure of 29.92 inches or 1013 millibars.


Precipitation attenuation

Reduction of power density because of absorption or reflection of energy by precipitation.


Parcel

A small volume of air, small enough to contain uniform distribution of its meteorological properties, and large enough to remain relatively self-contained and respond to all meteorological processes. No specific dimensions have been defined, however, the order of magnitude of 1 cubic foot has been suggested.


Partial obscuration

A designation of sky cover when part of the sky is hidden by surface based obscuring phenomena.


Pilot balloon

A small free-lift balloon used to determine the speed and direction of winds in the upper air.


Pilot balloon observation (commonly called PIBAL)

A method of winds-aloft observation by visually tracking a pilot balloon.


Plan position indicator (PPI) scope

A radar indicator scope displaying range and azimuth of targets in polar coordinates.


Plow wind

The spreading downdraft of a thunderstorm; a strong, straight-line wind in advance of the storm. Also called first gust.


Polar air

An air mass with characteristics developed over high latitudes, especially within the subpolar highs. Continental polar air (cP) has cold surface temperatures, low moisture content, and, especially in its source regions, has great stability in the lower layers. It is shallow in comparison with Arctic air. Maritime polar (mP) initially possesses similar properties to those of continental polar air, but in passing over warmer water it becomes unstable with a higher moisture content. Compare tropical air.


Polar front

The semipermanent, semicontinuous front separating air masses of tropical and polar origins.


Power density

In radar meteorology the amount of radiated energy per unit cross sectional area in the radar beam.


Precipitation

Any or all forms of water particles, whether liquid or solid, that fall from the atmosphere and reach the surface. It is a major class of hydrometeor, distinguished from cloud and virga in that it must reach the surface.


Pressure altimeter

An aneroid barometer with a scale graduated in altitude instead of pressure using standard atmospheric pressure-height relationships; shows indicated altitude (not necessarily true altitude); may be set to measure altitude (indicated) from any arbitrarily chosen level.


Pressure gradient

The rate of decrease of pressure per unit distance at a fixed time.


Pressure jump

A sudden, significant increase in station pressure.


Pressure tendency

Barometric tendency. The change of barometric pressure within a specified period of time. In aviation weather observation, routinely determined periodically, usually for a 3-hour period.


Prevailing easterlies

The broad current or pattern of persistent easterly winds in the Tropics and in polar regions.


Prevailing visibility

In the U.S., the greatest horizontal visibility which is equaled or exceeded throughout half of the horizon circle; it need not be a continuous half.


Prevailing westerlies

The dominant west-to-east motion of the atmosphere, centered over middle latitudes of both hemispheres.


Prevailing wind

Direction from which the wind blows most frequently.


Prognostic chart (contracted PROG)

A chart of expected or forecast conditions.


Pseudo-adiabatic lapse rate

Saturated-adiabatic lapse rate. The rate of decrease of temperature with height as saturated air is lifted with no gain or loss of heat from outside sources; varies with temperature, being greatest at low temperatures.


Psychrometer

An instrument consisting of a wet-bulb and a dry-bulb thermometer for measuring wet-bulb and dry-bulb temperature; used to determine water vapor content of the air.


Pulse

Pertaining to radar, a brief burst of electromagnetic radiation emitted by the radar; of very short time duration.


Pulse length

Pertaining to radar, the dimension of a radar pulse; may be expressed as the time duration or the length in linear units. Linear dimension is equal to time duration multiplied by the speed of propagation (approximately the speed of light).


Positive vorticity

Vorticity caused by cyclonic turning; it is associated with upward motion of the air.


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