Weather Glossary - P


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Perfect Gas - see ideal gas

Perigee - the point in the moon's orbit when it is closest to the earth; opposite of apogee

Perihelion - the point in the earth's orbit nearest the sun (147 million kilometers from the sun), on about 3 January; opposite of aphelion

Period - (1) the time interval for a planet or satellite to complete one revolution on its orbit or (2) the time interval between passages, at a fixed point, of the crest of a wave (e.g.,an electromagnetic wave). Mathematically, it is the reciprocal of frequency.

Permafrost - a permanently frozen layer of soil or bedrock at a variable depth below the surface of the earth in frigid regions

Persistence Forecast - a forecast that the current weather condition will persist and that future weather will be the same as the present

Phase - the physical state of a substance; solid, liquid or gas

Phase - the position along a wave as measured from a reference point on the wave

Planck's Law - the amount of radiation emitted by a blackbody is uniquely determined by its absolute temperature

Planetary Boundary Layer (PBL) - the layer within the atmosphere between 1 km and the earth's surface where friction affects wind speed and wind direction.

Planetary Scale - see global scale

Polar Orbiting Satellite - a satellite with an orbit nearly parallel to the earth's meridian lines which crosses the polar regions on each orbit

Positive Area - the area on a sounding representing the layer in which a lifted parcel would be warmer than the environment; thus, the area between the environmental temperature profile and the path of the lifted parcel

Positive-Tilt Trough - an upper level system which is tilted to the east with increasing latitude (i.e., from southwest to northeast); often it is a sign of a weakening weather system

Potential Temperature - the temperature a parcel of dry air would have if brought adiabatically (i.e., without transfer of heat or mass) to a standard pressure level of 1000 mb

PPI - an acronym for Plan Position Indicator. A PPI displays radar data horizontally using a map projection. In PPI mode, the radar makes a 360-degree sweep with the antenna at a specific elevation angle. A PPI display is the familiar radar display shown on the television weather programs.

PPINE (Plan Position Indicates No Echoes) - referring to the fact that a radar detects no precipitation within its range

Precipitation - any form of water particles, either liquid or solid, which falls from the atmosphere and reaches the ground

Precipitation Mode - the standard, or default, operational mode of the WSR-88D. The radar automatically switches into precipitation mode from clear-air mode if the measured reflectivity exceeds a specific threshold value. The precipitation mode of NEXRAD is more sensitive than previous weather radars. The minimum detectable reflectivity in NEXRAD's precipitation mode is 5 dBZ, compared to 28 dBZ with the old WSR-57.

Pressure - a force per unit area or a stress characterized by uniformity in all directions

Pressure Gradient - the change in pressure over a given distance at a given time

Pressure Tendency - the change in pressure over a given time at a given location

Prevailing Westerlies - the westerly winds that dominant in middle latitudes

Prevailing Wind - the wind direction most commonly observed during a given period

Probability - the chance that a given event will occur

Probability Forecast - a forecast of the probability that one or more of a mutually exclusive set of weather conditions will occur

Probability of Hail - a product from the NEXRAD hail detection algorithm that estimates the likelihood that hail is present in a storm

Probability of Severe Hail - a product from the NEXRAD hail detection algorithm that estimates the likelihood of hail exceeding 3/4 inch in diameter is present in a storm

Profiler - an instrument designed to measure horizontal winds directly above its location, and thus measure the vertical wind profile. Profilers operate on the same principles as Doppler radar.

Propagation - the movement of an atmospheric phenomenon. This term is typically applied to the motion of thunderstorms into regions favorable for their continued development (into a maritime tropical airmass).

Psychrometer - an instrument used for measuring the water vapor content of the atmosphere; a hygrometer consisting essentially of two similar thermometers with the bulb of one being kept wet so that the cooling that results from evaporation makes it register a lower temperature than the dry one and with the difference between the readings constituting a measure of the dryness of the atmosphere

Pulse Repetition Frequency (PRF) - the amount of time between successive pulses, or bursts, of electromagnetic energy that is transmitted by a radar. The PRF determines the maximum range at which echoes can be detected and also the maximum radial velocity that can be detected by a Doppler radar. See also Doppler Dilemma.

PUP - an acronym for Principal User Processor. A PUP is the hardware component of the NEXRAD system that consists of a user workstation with two graphics monitors and a graphics tablet. The PUP receives NEXRAD products from an RPG, and the user can interact with the products using the PUP.

PVA (Positive Vorticity Advection) - the advection of higher values of vorticity into an area; often PVA is associated with upward motion (lifting) of the air

Pyranometer - a type of radiation sensor that measures the combined intensity of incoming direct solar radiation and the diffuse sky radiation.


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