Glossary of Meteorological Terms (C)

C

Calibration

The process whereby a position on the scale of an instrument is identified with the magnitude of the signal (or input force) actuating the instrument.

Calibration error

The inaccuracy that the manufacturer permits when the unit is calibrated in the factory.

Calm

Wind with a speed below 1 knot (1 mph); Beaufort scale number 0.

Calorie

A unit of heat originally defined as the amount of heat required to raise the temperature of water through one degree centigrade (the gram-calorie or small calorie), but this proved to be insufficiently precise. The 15° gram-calorie (cal15) is the amount of heat required to raise the temperature of one gram of water from 14.5° to 15.5°C, and is equal to 4.1855 joules. The kilogram calorie or large calorie (Kcal, kg-cal, or Cal) is 1,000 times as large as a calorie.

Calorimeter

An instrument designed to measure quantities of heat. Sometimes used in meteorology to measure solar radiation.

Campbell-Stokes recorder

A sunshine recorder of the type in which the time scale is supplied by the motion of the sun. It consists essentially of a spherical lens which burns an image of the sun upon a specially prepared card.

CAMEO

Computer-Aided Management of Emergency Operations. A system of software applications used to plan for and respond to chemical emergencies. Developed by the U.S. Environmental Protection Agency (EPA) and the National Oceanic and Atmospheric Administration (NOAA).

Canadian hardness-gauge

A type of disk hardness-gauge, especially useful in relatively soft snow. See disk hardness gauge.

Candela

Unit of luminous intensity. One candela is one lumen per steradian. Formerly called the candle.

Candle

A unit of luminous intensity of a light source. See candela.

Candlepower

Luminous intensity expressed in candelas.

Capacity correction

The correction applied to a mercury barometer with a nonadjustable cistern in order to compensate for the change in level of the cistern as the atmospheric pressure changes. Thus, as the pressure falls, the height of the cistern increases, due to the exchange of mercury between the barometer tube and its cistern. This correction is not required if the scale is calibrated as in the Kew barometer. See also barometric corrections.

Capillary collector

An instrument for collecting liquid water from the atmosphere.

Captive balloon

A buoyant balloon kept from rising freely by means of a line secured to a point on the ground, as opposed to a free balloon. See kytoon.

Carbon-film hygrometer element

An electrical hygrometer element constructed of a plastic strip coated with a film of carbon black dispersed in a hygroscopic binder. Variations in atmospheric moisture content vary the volume of the binder and thus change the resistance of the carbon coating. This element is characterized by high sensitivity and rapid response.

Cardinal winds

Winds from the four cardinal points of the compass; that is, north, east, south, and west winds.

Carrier frequency

The frequency of a carrier wave.

Carrier wave

Transmitted energy which is modulated in order to carry information. Usually, it is in the form of a radio-frequency sine wave, modulated either in amplitude or in frequency.

Carry-over

The portion of the streamflow during any month or year derived from precipitation in previous months or years.

Catch

The amount of precipitation captured by a rain gauge.

Ceiling

The height ascribed to the lowest layer of clouds or obscuring phenomena when it is reported as broken, overcast, or obscuration and not classified as "thin" or "partial." The ceiling is termed unlimited when these conditions are not satisfied.

Ceiling balloon

A small balloon used to determine the height of the cloud base. The height can be computed from the ascent velocity of the balloon and the time required for its disappearance into the cloud.

Ceiling classification

A description or explanation of the manner in which the height of the ceiling is determined, i.e. aircraft ceiling, balloon ceiling, estimated ceiling, indefinite ceiling, measured ceiling, precipitation ceiling.

Ceiling light

A type of cloud height indicator which uses a searchlight to project vertically a narrow beam of light onto the cloud base. The height of the cloud is determined using a clinometer, located at a known distance from the ceiling light, to measure the angle included by the illuminated spot on the cloud, the observer, and the ceiling light.

Ceiling projector

Same as ceiling light.

Ceilometer

An automatic, recording cloud height indicator.

Celsius temperature scale

International thermometric scale on which the freezing point of water equals 0° and the boiling point equals 100° at standard atmospheric pressure (760 mm Hg). Named for Swedish astronomer Anders Celsius (1701-1744), who devised the system in 1742.

Centibar

The pressure unit of the meter-ton-second system of physical units, equal to 10 millibars or 104 dynes per cm².

Centigrade temperature scale

The older name for the Celsius temperature scale. Officially abandoned by international agreement in 1948, but still in common use.

Centimeter-gram-second system

A system of physical units based on the use of the centimeter, gram, and the second as elementary quantities of length, mass, and time.

Ceptometer

An instrument that measures photosynthetically-active radiation (PAR) as a measure of light interception in plant canopies.

Channel storage

The water volume within a specified portion of a stream channel.

Character

Part of a computer word that has meaning in itself; often, a byte.

Chronograph

A clock-driven device for recording the time of occurrence of an event or the time interval between the occurrence of events.

Chronometric radiosonde

A radiosonde whose carrier wave is switched on and off in such a manner that the interval of time between the transmission of signals if a function of the magnitude of the meteorological elements being measured.

Chronothermometer

A thermometer consisting of a clock mechanism the speed of which is a function of temperature.

Cistern barometer

A mercury barometer in which the lower mercury surface is larger in area than the upper surface. The basic construction of a cistern barometer is as follows: A glass tube one meter in length, sealed at one end, is filled with mercury, and then inverted. The tube is mounted so that its mount penetrates the upper surface of a reservoir of mercury called the cistern of the barometer. See Fortin barometer, Kew barometer.

Class A pan

See evaporation pan.

Clay atmometer

An atmometer consisting of a porous porcelain or ceramic container connected to a calibrated reservoir filled with distilled water. Evaporation is determined by the depletion of water in the reservoir.

Clear-air turbulence

Turbulence encountered by aircraft when flying through air space devoid of clouds. Thermals and wind shear are the main causes.

Clinometer

An instrument for measuring angles of inclination. Used in conjunction with a ceiling light to measure cloud height at night.

Cloud

A hydrometeor consisting of a visible aggregate of minute water and/or ice particles in the atmosphere above the earth’s surface. Cloud differs from fog only in that the latter is, by definition, in contact with the earth’s surface.

Cloud base

For a given cloud or cloud layer, the lowest level in the atmosphere at which the air contains a perceptible quantity of cloud particles.

Cloud height

The height of the cloud base above the local terrain.

Cloudburst

Any sudden and heavy rain, almost always of the shower type.

CMOS

Complementary Metal-Oxide Semiconductor. A method of making silicon chips that results in low power consumption by the circuits.

Coalescence

Formation of a single water drop by the union of two or more colliding drops.

Cockeyed bob

A colloquial term in western Australia for a squall, associated with thunder, on the northwest coast in summer.

Code-sending radiosonde

A radiosonde which transmits the indication of the meteorological sensing elements in the form of a code consisting of combinations of dots and dashes.

Collector

A class of instruments employed to determine the electric potential at a point in the atmosphere, and ultimately the atmospheric electric field.

Color temperature

An estimate of the temperature of an incandescent body, determined by observing the wavelength at which it is emitting with peak intensity (its color) and using that wavelength in Wien’s law.

Combined error

The total of all deviations of a transducer’s output from a specified straight line in a constant environment.

Comb nephoscope

A direct-vision nephoscope which is constructed in the following manner: a comb consisting of a cross-piece containing equispaced vertical rods is attached to one end of a column eight to ten feet long and is supported on a mounting that is free to rotate about its vertical axis. In use, the comb is turned so that the cloud appears to move parallel to the tips of the vertical rods.

Compensation of instruments

The use of electromechanical devices to reduce (compensate for) the sensitivities of meteorological sensors to other parameters (e.g., the effect of temperature on a pressure sensor).

Commutator

See radiosonde commutator.

Condenser-discharge anemometer

A contact anemometer connected to an electrical circuit which is so arranged that the average wind speed is indicated.

Conductivity

A unit measure of electrical conduction. The facility with which a substance conducts electricity, as represented by the current density per unit electrical-potential gradient in the direction of flow. Electrical conductivity is the reciprocal of electrical resistivity and is expressed in units such as mhos (reciprocal ohms) per cm. It is an intrinsic property of a given type of material under given physical conditions (dependent mostly on temperature). Conductance, on the other hand, varies with the dimensions of the conducting system and is the reciprocal of the electrical resistance.

Compass points

The cardinal points of the compass, i.e. north, south, east, west.

Compensated pyrheliometer

Pyrheliometer based on the comparison of the heating of two identical metal strips, one exposed to radiation, the other to a joule effect.

Condensation

The process by which a vapor becomes a liquid. In meteorology, it occurs when water vapor changes to dew, fog, or becomes a cloud.

Condensation nucleus

Small particle on which water vapor condenses.

Conformal coating

A protective coating applied to circuits.

Constant-level balloon

A balloon designed to float at a constant pressure level. This may be accomplished by a pressure valve which controls the release of ballast so as to maintain flight above a selected pressure level until the supply of ballast is exhausted. See Moby Dick balloon, skyhook balloon, transosonde.

Constant-pressure balloon

Same as constant-level balloon.

Contact anemometer

Anemometer which generates an electrical contact output with a frequency proportional to wind speed.

Contact-cup anemometer

Same as contact anemometer.

Cooling-power anemometer

The general term for anemometers operating on the principle that the heat transfer to air from an object at an elevated temperature is a function of the air speed. Examples are the hot-wire anemometer and the katathermometer.

Coordinated Universal Time (UTC)

The international standard of time, kept by atomic clocks around the world. Formerly known as Greenwich Mean Time (GMT), local time at zero degrees longitude at the Greenwich Observatory, England. UTC uses a 24-hour clock.

Coriolis force

In meteorology, a deflecting force acting on a body in motion and resulting from the earth’s rotation. It deflects air currents to the right in the northern hemisphere and to the left in the southern hemisphere, thus having an effect on wind direction.

Coronagraph

An instrument for photographing the corona and prominences of the sun at times other than at solar eclipse.

Cotton-region shelter

A medium-sized instrument shelter. It is a white louvered box with a flat double roof and is mounted four feet above the ground on a four-legged stand.

Counterradiation

The downward flux of atmospheric radiation passing through a given level surface, usually taken as the earth’s surface. This result of infrared (long-wave) absorption and re-emission by the atmosphere is the principal factor in the greenhouse effect.

CPU

Central Processing Unit. The part of a computer which controls and directs all functions.

Creeping

Defect in the action of an aneroid barometer resulting in a sluggish adjustment of the index toward the correct reading when the barometer is subjected to a large and rapid change in pressure.

Crosswind

A wind blowing in a direction perpendicular to the course of a moving object.

CRT

Cathode Ray Tube. A display element, consisting of a vacuum tube and screen, used with computers.

Cryopedometer

Instrument for measuring the depth to which the soil is frozen.

Cup anemometer

Anemometer which measures wind speed by the speed of rotation of 3 or 4 hemispherical or conical cups, each fixed to the end of a horizontal arm projecting from a vertical axis. Invented about 1845 by Thomas Romney Robinson (1792-1882), an Irish astronomer and physicist. See condenser-discharge anemometer, contact anemometer. Compare to bridled-cup anemometer.

Current meter

Any one of numerous devices for the measurement of either speed alone or of both direction and speed (set and drift) in flowing water.

Cyanometer

Generally, an instrument designed to measure or estimate the blueness of the sky. See Linke-scale.

Cyclone

An area of low atmospheric pressure which has a closed circulation that is cyclonic (counterclockwise in northern hemisphere and clockwise in southern hemisphere).