Partial-Beam Filling - a limitation of the rainfall estimation techniques used by NEXRAD. At far ranges from the radar, a storm may occupy only a portion of the radar beam (which may be several miles across). However, the radiation received by the radar antenna consists of the average reflectivity across the entire beam, so the reflectivity and associated rainfall rates are underestimated.

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.