Weather Glossary and Terms

We have posted a glossary of weather terms, both common and uncommon, at our company informational site windwardinstruments.com.

The entire list is available here Windward Instruments Weather Glossary.

Below are the terms A - C:

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 ).

ACCURACY: The degree with which an instrument measures a variable in terms of an accepted standard value or true value; usually measured in terms of inaccuracy but expressed as accuracy; often expressed as a percentage of full-scale range.

AIR DENSITY: The mass density of a parcel of air expressed in units of mass per volume.

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 wind shield used around the mouth of a precipitation gage to reduce the effect of wind on catch.

ANALOG: Pertaining to measurements or devices in which the output varies continuously (e.g., voltage or rotation signals); compare to digital.

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

ANEROID BAROMETER: A barometer which measures atmospheric pressure using one or a series of aneroid capsules.

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

AWOS: Automated Weather Observing System. A Federal Aviation Administration specification describing an automatic weather station capable of making aviation weather observations without operator involvement.

BAR: A unit of pressure equal to 100 kilopascals.The millibar (1/1000 bar) is commonly used in aviation and meteorology. The pascal (Newton/meter² ) is the S.I. unit for pressure.

BAROGRAPH: A continuous-recording barometer.

BAROMETER: An instrument for measuring the pressure of the atmosphere; the two principle types are aneroid and mercurial.

BAROMETRIC PRESSURE: The atmospheric pressure at a given point due to the gravitational force on the column of air above it.

BAUD: A unit used in describing the rate of character transmission and equal to approximately one bit per second.

BCD: Binary Coded Decimal. A code format in which decimal digits (0-9) are expressed as four digit binary numbers.

BIMETALLIC THERMOMETER: A thermometer, the sensitive element of which consists of two metal strips which have different coefficients of expansion and are brazed together. The distortions of the system in response to temperature variations are used as a measure of temperature.

BOURDON TUBE: Closed, curved, flexible tube of elliptic cross section which is deformed, according to type, by variations of atmospheric pressure or temperature and so provides a measurement of the particular parameter.

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.

CEILOMETER: Instrument used to measure cloud base height.

CELSIUS: A temperature scale having the freezing point of pure water at 0° and the boiling point at 100° under standard sea level pressure. Also called Centigrade scale.

CENTIGRADE TEMPERATURE SCALE: The older name for the Celsius temperature scale. The use of this name was officially abandoned by international agreement in 1948.

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.

COMPASS POINTS: The cardinal points of the compass: N, NNE, NE, etc.

CONFORMAL COATING: A protective coating applied to circuits usually by spray deposition.

CROSSWIND: A wind blowing perpendicular to the course of a moving object. Often used when referring to winds affecting ballistics.

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.

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