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O

Obscuring phenomenon
An atmospheric phenomenon, other than clouds, which obscures a portion of the sky from the point of observation. Also called obscuration.

Observational error
The difference between the true value of some quantity and its observed value. Every observation is subject to certain errors. Systematic errors affect the whole of a series of observations in nearly the same way. For example, the scale of an instrument may be out of adjustment. These instrument errors can be detected and corrected by comparison with a standard. The personal equation of an observer may lead him or her to make small systematic errors in his or her readings, for example, if the scale is not at eye level. Random errors, which appear in any series of observations, are generally small and as likely to be positive as negative. Their magnitudes are usually distributed according to the error distribution. Mistakes are widely discrepant readings.

Ombrometer
A rain gauge capable of measuring very small amounts of precipitation. Also called micropluviometer, trace recorder.

Ombroscope
An instrument which indicates the presence of precipitation. The ombroscope consists of a heated, water-sensitive surface which indicates by mechanical or electrical techniques the occurrence of precipitation.

Operational weather limits
The limiting values of ceiling, visibility, and wind, or runway visual range, established as safety minimum for aircraft landings and take-offs.

Operating conditions
Conditions to which a device is subjected, not including the variable measured by the device. See normal operating conditions, reference operating conditions.

Operating influence
The change in a performance characteristic caused by a change in a specified operating condition from reference operating condition, all other conditions being held within the limits of reference operating conditions.

Orographic precipitation
Precipitation caused by the ascent of moist air over an orographic barrier such as a Mountain range.

OSHA
Occupational Safety and Health Administration. A regulatory office of the U.S. Department of Labor.

Owens dust recorder
An instrument for rapidly obtaining samples of airborne dust; a type of dust counter. Particles pass through a cylindrical chamber, are drawn at high velocity through a narrow slit, and then impinge upon a microscope cover glass located a short distance from the slit. Analysis for quantity and size of the particles is made using a microscope. The vacuum required to operate the instrument is developed by an attached hand Pump.

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.

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.

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 timeinterval 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, waterflow pyrheliometer.

Pyrradiometer
An instrument for the measurement of both solar and terrestrial radiation.