Glossary of Meteorological Terms

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P

Parachute radiosonde: Same as dropsonde.
Paranthelion: A refraction phenomenon similar to a parahelion, but occurring generally at a distance of 120° (occasionally 90° and 140°) from the sun, on the parhelic circle.
Paraselene: A weakly colored lunar halo identical in form and optical origin to the solar parhelion.
Paraselenic circle: A halo phenomenon consisting of a horizontal circle passing through the moon, corresponding to the parhelic circle through the sun. Produced by reflection of moonlight from ice crystals.
Parahelic circle: A halo consisting of a faint white circle passing through the sun and running parallel to the horizon for as much as 360° of azimuth. Produced by reflection of sunlight from ice crystals.
Parhelion: Either of two colored luminous spots that appear at points 22° (or somewhat more) on both sides of the sun and at the same elevation as the sun. Also called mock sun, sun dog.
Parallel data transmission (parallel output): A form of data transmission in which the bits of each character are all sent simultaneously, resulting in extremely fast communication but requiring a communication path for each bit. Compare to serial data transmission.
Parity: The addition of one or more redundant bits to information to verify its accuracy.
Pascal: Name given to the unit of pressure in the International System of Units (SI). Equal to 1 newton/meter² or 0.01 millibar.
Pascal's law: A hydrostatic principle that pressure supplied to an enclosed fluid is transmitted undiminished to every portion of the fluid and to the walls of the containing vessel.
Peak gust: In United States weather observing practice, the highest "instantaneous" wind speed recorded at a station during a specified period, usually the 24-hour observation day. Therefore, a peak gust need not be a true gust of wind.
P-E index: Abbreviation for precipitation effectiveness index.
Pendulum anemometer: A pressure-plate anemometer consisting of a plate which is free to swing about a horizontal axis in its own plane above its center of gravity. The angular deflection of the plate is a function of the wind speed. This instrument is not used for station measurements because of the false reading which results when the frequency of the wind gusts and the natural frequency of the swinging plate coincide. This was the earliest form of anemometer.
Penetrometer: A pointed device which indicates the amount of resistance encountered when it is forced into a material such as snow or soil. See ram penetrometer.
P-E quotient: Abbreviation for precipitation-evaporation quotient.
P-E ratio: Abbreviation for precipitation-evaporation ratio.
Percolation: The gravity flow of water within soil.
Permeability: Capacity of a soil or other surface to be penetrated by water sinking into the ground under the force of gravity. It thus expresses the rate of percolation.
Personal equation: A systematic observational error due to the characteristics of the observer. The uncertainty in a reading made by an observer may be acertained by a statistical analysis of his or her readings.
Pers sunshine recorder: A sunshine recorder of the type in which the time scale is supplied by the motion of the sun. The instrument, which is pointed at the celestial pole, consists of a hemispherical mirror mounted externally on the optical axis of a camera. The lens of the camera forms an image of the sun which is reflected by the hemispherical mirror so that as the sun moves across the sky, the image traces an arc of a circle on the photographic paper.
Phenolic: A plastic molding component formed by the reaction of phenol and formaldehyde. It can be heavily reinforced or "filled" with glass fibers or other materials. Phenolics are known for their high impact strength, excellent wear characteristics, and dimensional stability over a wide temperature range.
Phot: A photometric unit of illuminance or illumination equal to one lumen per square centimeter.
Photoelectric cell: A transducer which converts electromagnetic radiation in the infrared, visible, and ultraviolet regions into electrical quantities such as voltage, current, or resistance. Also called photo cell.
Photoelectric photometer: See photometer.
Photoelectric transmittance-meter: An instrument for measuring the transmissivity of the atmosphere; a type of transmissometer. It consists of a constant-intensity collimated light source located at a suitable distance from a photoelectric cell. Variation in the turbidity of the atmosphere causes changes in the intensity of the light received by the photo cell, thereby varying its electrical output. Also called photoelectric transmissometer.
Photographic barograph: A mercury barometer arranged so that the position of the upper or lower meniscus may be measured photographically. In one design the image of the meniscus is formed on a rotating drum covered with sensitized paper so that a continuous record of pressure as a function of time is obtained.
Photometer: An instrument for measuring the intensity of light or the relative intensity of a pair of lights. Also called an illuminometer. If the instrument is designed to measure the intensity of light as a function of wavelength, it is called a spectrophotometer. Photometers may be divided into two classes: photoelectric photometers in which a photoelectric cell is used to compare electrically the intensity of an unknown light with that of a standard light; and visual photometers in which the human eye performs the function of a photo cell. A photometer used to measure the intensity of a distant light is referred to as a telephotometer or transmissometer.
Photopolarimeter: A polarimeter utilizing a Wollaston prism as a polarizer and a Nicol prism as an analyzer.
Phytometer: A device, similar to a potometer, for measuring transpiration, consisting of a vessel containing soil in which one or more plants are rooted and sealed so that water can escape only by transpiration from the plant.
Pibal: Contraction for pilot balloon observation.
Piche evaporimeter: An atmometer which uses a filter paper disc as the evaporating element. The amount of water evaporated through the paper is read at the graduated tube reservoir.
Pilot balloon: A small balloon whose ascent is followed by a theodolite in order to obtain data for the computation of winds aloft.
Pilot balloon observation: A method of winds aloft observation in which the elevation and azimuth angles of a theodolite are read while visually tracking a pilot balloon. Balloon height data is estimated from assumed balloon ascension rates.
Pitot-static tube: Same as pitot tube.
Pitot tube: An instrument for measuring the relative speed of a fluid. It consists of a concentric pipe arrangement in which the inner pipe is open at one end and the outer pipe is perforated and closed at both ends. Each pipe is connected to a manometer. The unit is operated with the open end pointing upstream, so that the inner pipe measures the total pressure and the outer pipe measures the static pressure. The difference between these pressures, the dynamic pressure, is proportional to the square of the fluid speed.
Pitot tube anemometer: A pressure tube anemometer, consisting of a pitot tube mounted on the windward end of a wind vane and a suitable manometer to measure the developed pressure and calibrated in units of wind.
Pluvial: Pertaining to rain, or more broadly, to precipitation.
Pluvial index: The amount of precipitation falling in one day, or other specified period, that is likely to be equalled or exceeded in any given place only once in a century. That is, a precipitation amount that has a return period of 100 years.
Pluviograph: Same as recording rain gauge.
Pluviometer: Same as rain gauge.
Pluvioscope: Apparatus from which the nature and time of precipitation may be determined.
Polarimeter: An instrument for determining the degree of polarization of light. See photopolarimeter.
Polariscope: An instrument for studying, or examining substances in, polarized light. See Savant polariscope.
Pole-star recorder: An instrument used to determine approximately the amount of cloudiness during the dark hours. It consists of a fixed long-focus camera positioned so that Polaris is permanently within its field of view. The apparent motion of the star appears as a circular arc on the photograph and is interrupted as clouds come between the star and the camera.
Potential evaporation: Same as evaporative power.
Potential evapotranspiration: The amount of moisture which, if available, would be removed from a given land area by evapotranspiration. Expressed in units of water depth.
Potential temperature: Temperature assumed by an unsaturated air parcel when brought adiabatically to a standard pressure (1,000 mb).
Potentiometer: An instrument for measuring differences in electric potential.
Potometer: A device, similar to a phytometer, for measuring transpiration. It consists of a small vessel containing water and sealed so that the only escape of moisture is by transpiration from a leaf, twig, or small plant with its cut end inserted in the water.
Precipitable water: Amount of water, expressed as a depth or as a mass, which would be obtained if all the water vapor in a specified column of the atmosphere were condensed and precipitated.
Precipitation: Any and all forms of water particles, liquid or solid, that fall from the atmosphere and reach the ground.
Precipitation-effectiveness index: For a given location, a measure of the long-range effectiveness of precipitation in promoting plant growth. Also called precipitation-evaporation index.
Precipitation-effectiveness ratio: Same as precipitation-evaporation ratio.
Precipitation-evaporation index: Same as precipitation-effectiveness index.
Precipitation-evaporation quotient: A measure of long-term precipitation effectiveness. The ratio of the normal annual rainfall to the normal annual evaporation.
Precipitation-evaporation ratio: For a given locality and month, an empirical expression devised for the purpose of classifying climates numerically on the basis of precipitation and evaporation.
Precipitation gauge: General term for any device that measures precipitation, principally a rain gauge or snow gauge.
Precipitation scavenging: Removal of pollutants from the air by either rain or snow.
Pressure altimeter: An aneroid barometer with a scale graduated in altitude instead of pressure units.
Pressure gradient: The rate of decrease of pressure per unit distance at a fixed time.
Pressure jump: A sudden, significant increase in station pressure.
Pressure-plate anemometer: An anemometer which measures wind speed in terms of the drag which the wind exerts on a solid body. See bridled-cup anemometer, normal-plate anemometer, pendulum anemometer.
Pressure tendency (barometric tendency): The change in barometric pressure within a specified period of time (typically 3 hours for meteorological observations).
Pressure-tube anemometer: An anemometer which derives wind speed from measurements of dynamic wind pressures. Wind blowing into a tube develops a pressure greater than the static pressure, while wind blowing across a tube develops a pressure less than the static. This pressure differential, which is proportional to the square of the wind speed, is measured by a suitable manometer. See anemo-biagraph, Dines anemometer, Pitot tube.
PROM: Programmable Read-Only Memory. Read-only memory which can be programmed by the user using a special hardware programmer.
Price meter: A current meter consisting of six conical cups, mounted around a vertical axis, which rotate and generate a signal with each rotation. Tail vanes and a heavy weight stabilize the instrument.
Primary rainbow: The most common of the principal rainbow phenomena, which appears as an arc of about 42° about the observers antisolar point. On occasion, inside the primary rainbow one or more supernumerary rainbows may be seen. The secondary rainbow lies outside the primary rainbow at an angular radius of about 50°.
Propeller anemometer: A rotation anemometer which has a horizontal axis upon which helicoidal shaped vanes are mounted. See windmill anemometer.
Protected thermometer: A reversing thermometer which is encased in a strong glass outer shell that protects it against hydrostatic pressure. Compare to unprotected thermometer.
Protocol: A set of rules or conventions used to standardize data transfer between devices.
PSK: Phase Shift Keying. A form of phase modulation of a data signal performed by a modem for transmission over dedicated wire or phone lines.
Psychrograph: A self-recording psychrometer.
Psychrometer: An instrument used to measure the water vapor content of the air. A type of hygrometer. It consists of a wet-bulb and a dry-bulb thermometer. See aspiration psychrometer, Assmann psychrometer, hygrodeik, sling psychrometer.
Psychrometric calculator: A device for computing certain psychrometric data, usually the dew point and the relative humidity, from known values of the dry-bulb and wet-bulb temperatures and the atmospheric pressure. One type is the circular slide-rule form and, like the psychrometric tables, it is derived from the psychrometric formula.
Psychrometric tables: Tables prepared from the psychrometric formula and used to obtain vapor pressure, relative humidity, and dew point from values of wet-bulb and dry-bulb temperatures.
p.t.u.: Abbreviation for the pressure, temperature, and humidity data obtained by a radiosonde observation.
Pulse-time-modulated radiosonde: A radiosonde which transmits the indications of the meteorological sensing elements in the form of pulses spaced in time. The meteorological data are evaluated from the intervals between the pulses. Also called time-interval radiosonde.
Pyranograph: An instrument for recording global solar radiation.
Pyranometer: An instrument which measures combined direct solar radiation and diffuse sky radiation. See pyrheliometer, Robitzsch actinograph, solarimeter. See also albedometer.
Pyrgeometer: An instrument which measures the effective terrestrial radiation. See Angstrom pyrgeometer.
Pyrheliometer: An instrument for measuring the intensity of direct solar radiation at normal incidence. See Angstrom compensation pyrheliometer, Eppley pyrheliometer, Michaelson actinograph, silver-disc pyrheliometer, spectropyrheliometer, water-flow pyrheliometer.
Pyrradiometer: An instrument for the measurement of both solar and terrestrial radiation.

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Thanks to everyone who has contributed to our glossary, especially All Weather Inc and the AMS Glossary of Meteorology. We always welcome feedback, corrections, and suggestions. Please let us know how we can improve!

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