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Absolute humidity
In a system of moist air, the ratio of the mass of water vapor to the total volume of the system. Usually expressed as grams per cubic meter (g/m³).

Absolute instrument
An instrument whose calibration can be determined by means of simple physical measurements on the instrument. Compare to secondary instrument.

Absolute temperature
Temperature based on an absolute scale.

Absolute temperature scale
A temperature scale based on absolute zero. Same as Kelvin temperature scale.

Absolute zero
A hypothetical temperature characterized by a complete absence of heat and defined as 0 K, -273.15 C, or -459.67 F.

Absorption
The process in which incident radiation is retained by a substance. A further process always results from absorption.

Absorption hygrometer
A type of hygrometer which measures the water vapor content of the atmosphere by means of the absorption of vapor by a hygroscopic chemical.

Accretion
Growth of a cloud or precipitation particle by the collision and union of a frozen particle with a super-cooled water drop.

Accuracy
The degree of conformity of an indicated value to an accepted standard value, or ideal value. See accuracy rating, measured accuracy.

Accuracy rating
A number of quantity defining a limit that errors will not exceed when a device is used under specified operating conditions. Accuracy rating can be expressed in a number of forms, i.e. in terms of the measured variable (+/- 1 C), percent of span (+/-0.5% of span), percent of upper range value (+/-0.5% of upper range value F.S.), percent of scale length (+/-0.5% of scale length), or percent of actual output reading (+/- 1% of actual output reading).

Acre-foot
The volume of water required to cover one acre to a depth of one foot: 43,560 cubic feet.

Actinometer
An instrument which measures the intensity of radiation by determining the amount of chemical change( or fluorescence produced by that radiation.

Actual pressure
The atmospheric pressure at the level of the barometer. May or may not be the same as station pressure.

Adfreezing
The process by which one object becomes adhered to another by the binding action of ice.

Adiabatic process
A thermodynamic change of state in a system in which there is no transfer of heat or mass across the boundaries of the system. In an adiabatic process, compression always results in warming, expansion in cooling. Compare to diabatic process.

Aeolian
Pertaining to the action or effect of the wind. Derived from the name of the Greek god of the winds, Aeolus.

Aeolian anemometer
An anemometer utilizing the principle that the pitch of the aeolian tones generated by air moving past an obstacle is a function of the speed of the air. Largely a curiosity and has been put to no practical application in modem meteorology.

Aerial
Of or pertaining to the air, atmosphere, or aviation. Also, same as antenna.

Aerograph
In general, any self-recording instrument carried aloft by any means to obtain meteorological data.

Aerometeorograph
A self-recording instrument used on aircraft for the simultaneous recording of atmospheric pressure, temperature, and humidity.

Air current
Very generally, any moving- stream of air. It has no particular technical connotation.

Air density
The mass density of a parcel of air expressed in units of mass per volume.

Airlight formula
See Koschmieder's law.

Air meter
A small anemometer with flat vanes which indicates the number of linear feet or meters of air which have passed the instrument during its exposure.

Albedo
The ratio of the amount of electromagnetic radiation reflected by a body to the amount incipient upon it, commonly expressed as a percentage. The albedo is to be distinguished from the reflectivity, which refers to one specific wavelength.

Albedometer
An instrument used for the measurement of the reflecting, power (the albedo) of a surface. A pyranometer adapted for the measurement of radiation reflected from the earth's surface is sometimes employed as an albedometer.

ALERT
Automated Local Evaluation in Real Time. National Weather Service flood warning program.

Allard's law
A basic equation in night visual range theory, relating the illuminance of a point source of light to distance and the transmissivity of the atmosphere.

Alter shield
A type of rain gauge shield consisting of freely hanging, evenly spaced slats arranged circularly around the gauge. The advantage of this shield is that the slats do not easily accumulate snow, permitting its use on unattended gauges. See rain gauge shield.

Altimeter
An instrument which determines the altitude of an object with respect to a fixed level. There are two general types of altimeters: (a) the pressure altimeter, which gives an approximate measure of altitude from a pressure measurement and an assumed standard temperature distribution; and (b) the radio altimeter, which deduces altitude by electronic techniques.

Altimeter setting
The value of atmospheric pressure to which the scale of a pressure altimeter is set so as to indicate airport elevation. The altimeter setting is included as part of an aviation weather observation.

Anabatic wind
An up slope wind due to local surface heating. Opposite of katabatic wind.

Analog
Pertaining to measurements or devices in which the output varies continuously, i.e. voltage or rotation signals. Compare to digital.

Anemo-biagraph
A recording pressure-tube anemometer in which the wind scale of the float manometer has been made linear by the use of springs, i.e. Dines anemometer.

Anemoclinograph
A recording anemoclinometer.

Anemoclinometer
General name for a type of instrument which measures the inclination of the wind to the horizontal plane. See bivane.

Anemograph
A recording anemometer.

Anemometer
A general term for instruments designed to measure the speed or force of the wind.

Aneroid
Literally "not wet", containing no liquid.

Aneroid barograph
An aneroid barometer arranged so that the deflection of the aneroid capsule actuates a pen which graphs a record on a rotating drum. Sometimes called aneroidograph.

Aneroid barometer
A barometer which measures atmospheric pressure using one or a series of aneroid capsules. Also called holosteric barometer.

Aneroid capsule
A thin metal disc partially evacuated of air used to measure atmospheric pressure by measuring its expansion and contraction.

Aneroidogram
The record of an aneroid barograph.

Angstrom compensation pyrheliometer
An absolute instrument developed by K. Angstrom for the measurement of direct solar radiation. The radiation receiver station consists of two identical manganin strips whose temperatures are measured by attached thermocouples. One of the strips is shaded, while the other is exposed to sunlight. An electrical heating current is passed through the shaded strip so as to raise its temperature to that of the exposed strip. The electric power required to accomplish this is a measure of the solar radiation.

Angstrom pyrgeometer
An instrument developed by K. Angstrom for measuring the effective terrestrial radiation. It consists of four manganin strips, of which two are blackened and two are polished. The blackened strips are allowed to radiate to the atmosphere while the polished strips are shielded. The electrical power required to equalize the temperature of the four strips is taken as a measure of the solar radiation.

Antenna
A conductor or system of conductors for radiating and/or receiving radio energy. Also called aerial.

Antenna feed
See feed.

Antenna gain
See gain.

Antenna pattern
Same as radiation pattern. Anticyclone-An area of high atmospheric pressure which has a closed circulation that is anticyclonic (clockwise in northern hemisphere and counterclockwise in southern hemisphere).

Antitriptic wind
In Jeffreys' classification, a wind for which the pressure force exactly balances the viscous force, in which the vertical transfers of momentum predominate.

Apparent freezing point
Same as freezing point.

Approximate absolute temperature scale
A temperature scale with the ice point at 273 degrees and boiling point of water at 373 degrees. It is intended to approximate the Kelvin temperature scale with sufficient accuracy for many sciences, notably meteorology.

ARDC model atmosphere
See standard atmosphere.

ASCII
American Standard Code for Information Interchange. A standard code used to represent data using 8 bits (7 data bits and I parity bit) per character.

ASOS
Automated Surface Observing System. A network of instrumented weather stations deployed primarily by the National Weather Service to make weather observations without operator involvement.

Aspiration meteorograph
An instrument, for the recording of two or more meteorological parameters, in which the ventilation is provided by a suction fan.

Aspiration psychrometer
A psychrometer in which the ventilation is provided by a suction fan.

Aspiration thermograph
A thermograph in which ventilation is provided by a suction fan.

Aspirator
A device attached to a meteorological instrument to provide ventilation; usually a suction fan.

Assmann psychrometer
A special form of the aspiration psychrometer. developed by Assmann, in which the thermometric elements are well shielded from radiation. Psychrometric measurements may be taken with the instrument in the presence of direct solar radiation.

Asynchronous
Lacking a relationship to a time base or clock. In asynchronous communications, individual data characters are sent at an arbitrary rate.

Atmidometer
Same as atmometer.

Atmometer
General name for an instrument which measures the evaporation rate of water into the atmosphere. See clay atmometer, evaporation pan, evapotranspirometer, Livingston sphere, Piche evaporimeter, radio atmometer.

Atmoradiograph
A device for measuring the frequency of occurrence of atmospherics whose intensity is greater than a predetermined level.

Atmosphere
The envelope of air surrounding the earth and bound to it more or less permanently by virtue of the earth's gravitational attraction. The system whose chemical Properties. dynamic motions, and physical processes constitute the subject matter of meteorology. Also, a unit of pressure. See standard atmosphere.

Atmospheric pressure (barometric pressure)
The pressure exerted by the atmosphere as a consequence of gravitational attraction exerted upon the "column" of air lying directly above the point in question.

Atmospheric radiation
infrared radiation emitted by or being propagated through the atmosphere.

Attenuation
In physics, any process in which the flux density (or power, amplitude, intensity, illuminance, etc.) of a "parallel beam" of energy decreases with increasing distance from the source. Attenuation is always due to the action of the transmitting medium itself, mainly by absorption and scattering. In meteorological optics, the attenuation of light is termed extinction.

Audio-modulated radiosonde
A radiosonde whose carrier wave is modulated by audio-frequency signals whose frequency is controlled by the sensing elements of the instrument.

Aviation weather forecast
A forecast of weather elements of particular interest to aviation; including ceiling, visibility, upper winds, icing, turbulence, precipitation types, and storms.

Aviation weather observation
An evaluation, according to set procedures, of those weather elements which are most important for aircraft operations. Always includes cloud height or vertical visibility, sky cover, visibility, obstructions to vision, certain atmospheric phenomena, and wind speed and direction. Complete observations include sea level pressure, temperature, dew point temperature, and altimeter setting. Compare to synoptic weather observation.

AWOS
Automated Weather Observing Station. A self-contained weather station designed to make aviation weather observations without operator involvement.