P
- Parachute radiosonde
- Same as dropsonde.
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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.
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Paraselene
- A weakly colored lunar halo identical in form and optical origin to the
solar parhelion.
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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.
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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.
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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.
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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.
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Parity
- The addition of one or more redundant bits to information to verify its
accuracy.
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Pascal
- Name given to the unit of pressure in the International System of Units
(SI). Equal to 1 newton/meter² or 0.01 millibar.
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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.
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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.
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P-E index
- Abbreviation for precipitation effectiveness index.
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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.
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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.
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P-E quotient
- Abbreviation for precipitation-evaporation quotient.
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P-E ratio
- Abbreviation for precipitation-evaporation ratio.
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Percolation
- The gravity flow of water within soil.
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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.
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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.
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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.
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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.
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Phot
- A photometric unit of illuminance or illumination equal to one lumen per
square centimeter.
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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.
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Photoelectric photometer
- See photometer.
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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.
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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.
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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.
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Photopolarimeter
- A polarimeter utilizing a Wollaston prism as a polarizer and a Nicol prism
as an analyzer.
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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.
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Pibal
- Contraction for pilot balloon observation.
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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.
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Pilot balloon
- A small balloon whose ascent is followed by a theodolite in order to
obtain data for the computation of winds aloft.
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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.
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Pitot-static tube
- Same as pitot tube.
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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.
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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.
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Pluvial
- Pertaining to rain, or more broadly, to precipitation.
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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.
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Pluviograph
- Same as recording rain gauge.
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Pluviometer
- Same as rain gauge.
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Pluvioscope
- Apparatus from which the nature and time of precipitation may be
determined.
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Polarimeter
- An instrument for determining the degree of polarization of light. See
photopolarimeter.
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Polariscope
- An instrument for studying, or examining substances in, polarized light.
See Savant polariscope.
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Potential evaporation
- Same as evaporative power.
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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.
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Potential temperature
- Temperature assumed by an unsaturated air parcel when brought
adiabatically to a standard pressure (1,000 mb).
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Potentiometer
- An instrument for measuring differences in electric potential.
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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.
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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.
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Precipitation
- Any and all forms of water particles, liquid or solid, that fall from the
atmosphere and reach the ground.
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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.
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Precipitation-effectiveness ratio
- Same as precipitation-evaporation ratio.
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Precipitation-evaporation index
- Same as precipitation-effectiveness index.
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Precipitation-evaporation quotient
- A measure of long-term precipitation effectiveness. The ratio of the
normal annual rainfall to the normal annual evaporation.
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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.
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Precipitation gauge
- General term for any device that measures precipitation, principally a
rain gauge or snow gauge.
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Pressure altimeter
- An aneroid barometer with a scale graduated in altitude instead of
pressure units.
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Pressure gradient
- The rate of decrease of pressure per unit distance at a fixed time.
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Pressure jump
- A sudden, significant increase in station pressure.
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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.
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Pressure tendency (barometric tendency)
- The change in barometric pressure within a specified period of time
(typically 3 hours for meteorological observations).
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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.
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PROM
- Programmable Read-Only Memory. Read-only memory which can be programmed by
the user using a special hardware programmer.
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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.
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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°.
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Propeller anemometer
- A rotation anemometer which has a horizontal axis upon which helicoidal
shaped vanes are mounted. See windmill anemometer.
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Protected thermometer
- A reversing thermometer which is encased in a strong glass outer shell
that protects it against hydrostatic pressure. Compare to unprotected
thermometer.
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Protocol
- A set of rules or conventions used to standardize data transfer between
devices.
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- World class sales and service depot of
meteorological instruments and navigation and communication electronics.
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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.
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Psychrograph
- A self-recording psychrometer.
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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.
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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.
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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.
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p.t.u.
- Abbreviation for the pressure, temperature, and humidity data obtained by
a radiosonde observation.
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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.
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Pyranograph
- An instrument for recording global solar radiation.
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Pyranometer
- An instrument which measures combined direct solar radiation and diffuse
sky radiation. See pyrheliometer, Robitzsch actinograph, solarimeter. See also
albedometer.
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Pyrgeometer
- An instrument which measures the effective terrestrial radiation. See
Angstrom pyrgeometer.
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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.
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Pyrradiometer
- An instrument for the measurement of both solar and terrestrial radiation.
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