P

Partly cloudy – sky condition when between 3/10ths and 7/10ths of the sky is covered. Used more frequently at night. In the United Kingdom, it is defined as less than half cloud cover.

Partly sunny – similar to partly cloudy. Used to emphasize daytime sunshine.

Permafrost – a soil layer below the surface of tundra regions that remains frozen permanently.

Polar air – a mass of very cold, very dry air that forms in polar regions.

Polar front – the semi-permanent, semi-continuous front that encircles the northern hemisphere separating air masses of tropical and polar origin.

Polar Stratospheric Clouds (PSCs) – high altitude clouds that form in the stratosphere above Antarctica during the Southern Hemisphere winter. Their presence seems to initiate the ozone loss experienced during the ensuing southern hemisphere spring.

Polar vortex – a circumpolar wind circulation which isolates the Antarctic continent during the cold Southern Hemisphere winter, heightening ozone depletion.

Poor visibility – one to five kilometers’ visibility.

POP – Probability of Precipitation. Probability forecasts are subjective estimates of the chances of encountering measurable precipitation at some time during the forecast period.

Popcorn convection – clouds, showers and thundershowers that form on a scattered basis with little or no apparent organization, usually during the afternoon in response to diurnal heating.

Precipitation – liquid or solid water that falls from the atmosphere and reaches the ground.

Pressure – the force exerted by the interaction of the atmosphere and gravity. Also known as atmospheric pressure.

Pressure change – the net difference between pressure readings at the beginning and ending of a specified interval of time.

Pressure falling rapidly – a decrease in station pressure at a rate of 0.06 inch of mercury or more per hour which totals 0.02 inches or more.

Pressure rising rapidly – an increase in station pressure at a rate of 0.06 inch of mercury or more per hour which totals 0.02 inches or more.

Pressure tendency – the character and amount of atmospheric pressure change during a specified period of time, usually the three-hour period preceding an observation.

Prevailing westerlies – winds in the middle latitudes (approximately 30 degrees to 60 degrees) that generally blow from west to east.

Prevailing wind – the direction from which the wind blows most frequently in any location.

Pulse storm – a thunderstorm within which a brief period (pulse) of strong updraft occurs, during and immediately after which the storm produces a short episode of severe weather. These storms generally are not tornado producers, but often produce large hail and/or damaging winds. See overshooting top, cyclic storm.