Glossary
Parcel
an imaginary "bubble" of air that contains a great number of molecules, but is small enough so that its properties are uniform within it. It is usually assumed that there is no transfer of heat into or out of this hypothetical volume, which means that only adiabatic temperature changes are allowed. It is usually considered to be approximately beach ball-size. Same as Air Parcel.
Parhelion (Plural: Parhelia)
the scientific name for sundogs, which are two colored luminous spots that appear at roughly 22 degrees on both sides of the sun at the same elevation. They are caused by the refraction of sunlight passing through ice crystals (mainly in cirrus-type clouds). They are also known as mock suns.
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.
Particularly Dangerous Situation (PDS)
the Storm Prediction Center uses this wording in rare situations when long-lived, strong and violent tornadoes are possible. This wording may also accompany severe thunderstorm watches for intense convective wind storms.
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 continue (the future weather will be the same as the present)
Photosynthesis
the process in which a plant, algae, or bacteria uses carbon dioxide, water, and sunlight to produce oxygen and carbohydrates (food).
Pilot Balloon (PIBAL)
a small, helium-filled weather balloon that is tracked as it rises through the atmosphere to determine how wind speed and direction change with altitude.
Plan Position Indicates No Echoes (PPINE)
referring to the fact that a radar is not detecting any precipitation within its range
Plan Position Indicator (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.
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.
Polar Coordinate System
a two-dimensional coordinate system in which each point is determined by an angle and a distance from a central location. Let’s use a spider web as our polar coordinate system. An insect is trapped at this location: on the intersection of the top part of a straight thread perpendicular to the ground and the third ring from the center of the web. If the top part of the straight thread is called 90° (the right part of the left-right thread is 0°), then the insect is at 90° on the web. Each ring around the web is exactly half an inch wide, so the distance from the center of the web to the insect is 1.5 inches. The polar coordinates of the insect would then be (1.5, 90°).
Polar Orbiting Satellite
a satellite with an orbit nearly parallel to the earth’s meridian lines that crosses the polar region on each orbit
Polar Region
between 60 degrees and 90 degrees N or S, this area receives the least amount of solar energy (compared with the mid-latitudes and the tropics)
Polar-Orbiting Operational Environmental Satellite (POES)
see Polar Orbiting Satellite. This group includes the TIROS, NOAA, and MetOp series satellites.
Popcorn Convection
slang for showers and thunderstorms that form on a scattered basis with little or no apparent organization, usually during the afternoon in response to diurnal heating. They are also very common when a Maritime Tropical air mass is over the area. Individual thunderstorms typically are of the type sometimes referred to as air-mass thunderstorms; they are small, short-lived, very rarely severe, and they almost always dissipate near or just after sunset.
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. See Convective Available Potential Energy.
Positive Vorticity Advection (PVA)
the advection of higher values of vorticity into an area; often PVA is associated with upward motion (lifting) of the air. Also referred to as Cyclonic Vorticity Advection (CVA).
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 Energy (PE)
energy stored within a system that has the potential to be converted into other forms of energy, such as kinetic energy, and to do work in the process. Mathematically, potential energy is equal to the product of a body’s mass times gravity times the height of the body.
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
Precipitable Water (PW or PWAT)
the depth of liquid water that would be at the surface if all of the water vapor in a column were condensed and precipitated over a location. The layer that is typically used is from the surface to 300 mb.
Precipitation (PCPN, PRCP)
any form of water particles, either liquid or solid, which falls from the atmosphere and reaches the ground
Precipitation Core
the area of a supercell where the heaviest precipitation, including rain and hail, is usually found
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 Gradient Force (PGF)
a three-dimensional force vector that accelerates air parcels away from high pressure and toward low pressure in response to an air pressure gradient. It is usually split into vertical pressure gradient force and horizontal pressure gradient force.
Principal User Processor (PUP)
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.
Probability Forecast
a forecast of the probability that one set of mutually exclusive 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 1 inch in diameter 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 air mass).
Psychrometer
an instrument used for measuring the water vapor content of the atmosphere; a hygrometer consisting of two similar thermometers. The bulb of one is kept wet and the cooling that results from evaporation makes it register a lower temperature than the dry one. The difference between the readings provides a measure of the dryness of the atmosphere
Pulse
a short burst of electromagnetic radiation that a radar sends out in a straight line to detect a precipitation target. The straight line that this pulse travels along is called a radar beam.
Pulse Repetition Frequency (PRF)
the amount of time between successive pulses, or bursts, of electromagnetic radiation 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.
Pulse Storm
a thunderstorm in 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.
Pyranometer
a type of radiation sensor that measures the combined intensity of incoming direct solar radiation and the diffuse sky radiation.
