parameterA value or measurement that varies with circumstances, and is used as a reference to quantify a situation or a process.
Parapet WallA solid wall built along the top of a dam (upstream and/or downstream edge) used for ornamentation, for safety of vehicles and pedestrians, or to prevent…
ParcelA volume of air small enough to contain uniform distribution of its meteorological properties and large enough to remain relatively self-contained and…
ParhelionThe scientific name for sun dogs. Either of two colored luminous spots that appear at roughly 22 degrees on both sides of the sun at the same elevation.
Partial Beam FillingA 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…
Partial-Duration Flood SeriesA list of all flood peaks that exceed a chosen base stage or discharge, regardless of the number of peaks occurring in a year.
particle size distributionThe number of particles present in any given volume of air within a specified size range.
Particle Trajectory ModelA computer sub-model that tracks the trajectories of multiple particles that are released into an atmospheric flow model.
ParticulateA small, discrete mass of solid or liquid matter that remains individually dispersed in gas or liquid emissions.
Particulate Matter (PM)Very small pieces of solid or liquid matter such as particles of soot, dust, fumes, mists or aerosols.
ParticulatesThe fine liquid or solid particles contained in combustion gases. The quantity and size of particulates emitted by cars, power and industrial plants, wood…
Partly CloudyBetween 3/8 and 5/8 of the sky is covered by clouds.
Partly SunnyBetween 3/8 and 5/8 of the sky is covered by clouds. The term "Partly Sunny" is used only during daylight hours.
Parts per billion (ppb)A measurement of concentration on a weight or volume basis. Equivalent to micrograms per liter.
Parts per million (ppm)A measurement of concentration on a weight or volume basis. Equivalent to milligrams per liter (mg/l).
PascalThe unit of pressure produced when one newton acts on one square meter (1 N/m2). It is abbreviated Pa.
Past weatherPast weather Predominant characteristic of the weather which had existed at an observing station during a given period of time (during the preceding hour…
PATIn blackjack, an unbusted hand worth at least 17 points. In draw poker, a hand that does not need any more hands.
PC-GRIDDSPC-Gridded Interactive Display and Diagnostic System - Allows the forecaster to view fields of gridded model output in contour or vector format.
PCTPatent Cooperation Treaty (more) provides a mechanism by which an applicant can file a single application that, when certain requirements have been…
PDOPacific Decadal Oscillation - a recently described pattern of climate variation similar to ENSO though on a timescale of decades and not seasons.
PDSParticularly Dangerous Situation (PDS) wording is used in rare situations when long-lived, strong and violent tornadoes are possible.
PDS WatchThe Particularly Dangerous Situation (PDS) wording is used in rare situations when long-lived, strong and violent tornadoes are possible.
PDSIPalmer Drought Severity Index - an index used to gage the severity of drought conditions by using a water balance equation to track water supply and…
Peak DischargeHighest rate of discharge of a volume of water passing a given location during a given period of time (during the year, or a flood event, etc..).
Peak GustThe highest instantaneous wind speed observed or recorded.
Peak PulseThe amount of power transmitted by a radar during a given pulse. Note that because these pulses are widely spaced, the average power will be much smaller.
Peak Wind SpeedThe maximum instantaneous wind speed (or velocity) that occurs within a specific period of time or interval.
Pendant EchoRadar signature generally similar to a hook echo, except that the hook shape is not as well defined.
Penetrating TopSame as Overshooting Top; a dome-like protrusion above a thunderstorm anvil, representing a very strong updraft and hence a higher potential for severe…
Pentanes plusA mixture of hydrocarbons, mostly pentanes and heavier fractions, extracted from natural gas. See hydrocarbons.
PenumbraIn solar-terrestrial terms, the sunspot area that may surround the darker umbra or umbrae.
Perched GroundwaterLocal saturated zones above the water table which exist above an impervious layer of limited extent.
PercolationThe filtering of a liquid passed through a medium with many fine spaces.
Percolation PathThe course followed by water moving or percolating through any other permeable material, or under a dam which rests upon a permeable foundation.
Perennial StreamA stream that flows continually throughout the year.
Perfluorocarbons (PFCs)A group of human-made chemicals composed of carbon and fluorine only. These chemicals (predominantly CF4 and C2F6) were introduced as alternatives, along…
PerigeeThe closest distance between moon and earth or the earth and sun.
PerihelionThe point on the annual orbit of a body (about the sun) that is closest to the sun; at present, the earth reaches this point on about 5 January.
PermeabilityThe rate at which liquids pass through soil or other materials in a specified direction.
Permeability CoefficientThe rate of flow of a fluid through a cross section of a porous mass under a unit hydraulic gradient, at a temperature of 60 degrees Fahrenheit.
PermeameterDevice for containing the soil sample and subjecting it to fluid flow in order to measure permeability or hydraulic conductivity.
persistenceAbility of an organism to remain in a particular setting for a period of time after it is introduced.
Persistence ForecastA forecast that the current weather condition will persist and that future weather will be the same as the present (e.g.
Perturbation ModelA computer model used to calculate air pollution concentrations. A perturbation model produces a wind field from solutions to a simplified set of…
Pervious ZoneA part of the cross section of an embankment dam comprising material of high permeability.
Petrochemical feedstockFeedstock derived from petroleum, used principally for the manufacture of chemicals, synthetic rubber, and a variety of plastics.
PetrochemicalsChemicals obtained by refining (i.e., distilling) crude oil. They are used as raw materials in the manufacture of most industrial chemicals, fertilizers…
PetroleumAny petroleum-based substance composed of a complex blend of hydrocarbons derived from crude oil, including motor fuel, jet oil, lubricants, petroleum…
Petroleum cokeA residue that is the final product of the condensation process in cracking.
Phenomenological ModelA computer model used to calculate air pollution concentrations. A phenomenological model focuses on an individual phenomenon, such as plume impingement…
PhiezometerIn hydrologic terms, an instrument used to measure pressure head in a conduit, tank, soil, etc. They are used in dams to measure the level of saturation.
Photochemical SmogAir pollution containing ozone and other reactive chemical compounds formed by the reaction of nitrogen oxides and hydrocarbons in the presence of…
PhotosphereThe Sun’s surface, from which the light we actually see (with the human eye) is emitted. Temperatures average 5,800 K (9,900°F).
PhotosynthesisA chemical process by which green plants synthesize organic compounds from carbon dioxide and water in the presence of sunlight.
Photovoltaic and solar thermal energyEnergy radiated by the sun as electromagnetic waves (electromagnetic radiation) that is converted into electricity by means of solar (i.e.
Phreatic waterWater within the earth that supplies wells and springs; water in the zone of saturation where all openings in rocks and soil are filled, the upper surface…
PIBALPilot balloon. A small helium-filled meteorological balloon that is tracked as it rises through the atmosphere to determine how wind speed and direction…
Pilot Balloon(Abbrev. PIBAL)- A small helium-filled meteorological balloon that is tracked as it rises through the atmosphere to determine how wind speed and direction…
Pilot Report(Abbrev. PIREP)- A report of inflight weather by an aircraft pilot or crew member.
PingoA large frost mound of more than one-year duration.
PIREPPilot Report. A report of inflight weather by an aircraft pilot or crew member.
Pitot TubeA device for measuring the velocity of flowing water using the velocity head of the stream as an index of velocity.
PlageIn solar-terrestrial terms, an extended emission feature of an active region that exists from the emergence of the first magnetic flux until the widely…
Plage CorridorIn solar-terrestrial terms, a space in chromospheric plage lacking plage intensity, coinciding with polarity inversion line.
PlagenilIn solar-terrestrial terms, spotless disc free of calcium plage.
Plan Position IndicatorAn acronym for Plan Position Indicator. A PPI displays radar data horizontally using a map projection.
Planetary Boundary LayerThe layer within the atmosphere between 1 km and the earth's surface where friction affects wind speed and wind direction.
PlanktonCollectively, all organisms (plants and animals) suspended in the water which are not independent of water movements.
planktonic foraminiferaMarine zooplankton that passively float or weakly swim, and have calcium carbonate skeletons that are present in large numbers on the surface of the ocean.
Plasma(PLAZ-muh) The clear, yellowish, fluid part of the blood that carries the blood cells. The proteins that form blood clots are in plasma.
platformA generic radar term, often used to encompass the pedestal and antenna assembly; sometimes including the radar control, display and analysis hardware and…
Pleistocene epochAn interval of the Quaternary period, from 1.8 million to about 11,500 years before present.
Plow WindA term used in the midwestern United States to describe strong, straight-line winds associated with the downdrafts spreading out in advance of squall…
Plume BlightVisibility impairment caused by air pollution plumes aggregated from individual sources.
Plume ImpingementThe collision of a plume with topography that rises above the plume altitude; often a temporary condition that occurs as the plume sweeps by the face of a…
Plume-dominated FireA fire whose behavior is governed primarily by the local wind circulation produced in response to the strong convection above the fire rather than by the…
PluvialIn hydrology, anything that is brought about directly by precipitation.
PNSPublic Information Statement - a narrative statement issued by a National Weather Service Forecast Office that can be used for: 1) A current or expected…
POHProbability of Hail - a product from the NEXRAD hail detection algorithm that estimates the likelihood that hail is present in a storm.
Point PrecipitationPrecipitation at a particular site, in contrast to the mean precipitation over an area.
Point sourceAny discernible, confined, or discrete conveyance from which pollutants are or may be discharged, including, but not limited to, any pipe, ditch, channel…
Polar Cap Absorption (PCA)In solar-terrestrial terms, an anomalous condition of the polar ionosphere whereby HF and VHF (3 - 300 MHz) radiowaves are absorbed, and LF and VLF (3 -…
Polar dayPolar day In polar regions, the portion of the year when the Sun is continuously in the sky.