Aeronautical Terms beginning with A

Absolute instability

A state of a layer within the atmosphere in which the vertical distribution of temperature is such that an air parcel, if given an upward or downward push, wiII move away from its initial level without further outside force being applied.

Absolute temperature scale

Kelvin Temperature Scale. A temperature scale with zero degrees equal to the temperature at which all molecular motion ceases, i.e., absolute zero (0° K = -273° C); the Kelvin degree is identical to the Celsius degree; hence at standard sea level pressure, the melting point is 273° K and the boiling point 373° K.

Adiabatic process

The process by which fixed relationships are maintained during changes in temperature, volume, and pressure in a body of air without heat being added or removed from the body.


The horizontal transport of air or atmospheric properties. In meteorology, sometimes referred to as the horizontal component of convection.

Advection fog

Fog resulting from the transport of warm, humid air over a cold surface.

Air density

The mass density of the air in terms of weight per unit volume.

Air mass

In meteorology, an extensive body of air within which the conditions of temperature and moisture in a horizontal plane are essentially uniform.

Air mass classification

A system used to identify and to characterize the different air masses according to a basic scheme. The system most commonly used classifies air masses primarily according to the thermal properties of their source regions: “tropical” (T); “polar” (P); and “Arctic” or “Antarctic” (A). They are further classified according to moisture characteristics as “continental” (c) or “maritime” (m).

Air parcel

A. small volume of air, small enough to contain uniform distribution of its meteorological properties, and large enough to remain relatively self-contained and respond to all meteorological processes. No specific dimensions have been defined, however, the order of magnitude of 1 cubic foot has been suggested.


The ratio of the amount of electromagnetic radiation reflected by a body to the amount incident upon it, commonly expressed in percentage; in meteorology, usually used in reference to insolation (solar radiation); i.e., the albedo of wet sand is 9, meaning that about 9% of the incident insolation is reflected; albedoes of other surfaces range upward to 80-85 for fresh snow cover; average albedo for the earth and its atmosphere has been calculated to range from 35 to 43.


An instrument which determines the altitude of an object with respect to a fixed level. A pressure altimeter.

Altimeter setting

The value to which the scale of a pressure altimeter is set so as to read true altitude at field elevation.

Altimeter setting indicator

A precision aneroid barometer calibrated to indicate directly the altimeter setting.


Height expressed in units of distance above a reference plane, usually above mean sea level or above ground.


White or gray layers or patches of cloud, often with a waved appearance; cloud elements appear as rounded masses or rolls; composed mostly of liquid water droplets which may be supercooled; may contain ice crystals at subfreezing temperatures.

Altocumulus castellanus

A species of middle cloud of which at least a fraction of its upper part presents some vertically developed, cumuliform protuberances (some of which are taller than they are wide, as castles) and which give the cloud a crenelated or turreted appearance; especially evident when seen from the side; elements usually have a common base arranged in lines. This cloud indicates instability and turbulence at the altitudes of occurrence.


An instrument for measuring wind speed.

Aneroid barometer

A barometer which operates on the principle of having changing atmospheric pressure bend a metallic surface which, in turn, moves a pointer across a scale graduated in units of pressure.


In radar meteorology, an echo caused by physical phenomena not discernible to the eye; they have been observed when abnormally strong temperature and/or moisture gradients were known to exist; sometimes attributed to insects or birds flying in the radar beam.

Anomalous propagation (sometimes called A P )

In radar meteorology, the greater than normal bending of the radar beam such that echoes are received from ground targets at distances greater than normal ground clutter.


An area of high atmospheric pressure which has a closed circulation that is anticyclonic, i.e., as viewed from above, the circulation is clockwise in the Northern Hemisphere, counterclockwise in the Southern Hemisphere, undefined at the Equator.

Anvil cloud

Popular name given to the top portion of a cumulonimbus cloud having an anvil-like form.


A sounding made by an aircraft.

Arctic air

An air mass with characteristics developed mostly in winter over Arctic surfaces of ice and snow. Arctic air extends to great heights, and the surface temperatures are basically, but not always, lower than those of polar air.

Arctic front

The surface of discontinuity between very cold (Arctic) air flowing directly from the Arctic region and another less cold and, consequently, less dense air mass.


The mass of air surrounding the Earth.

Atmospheric pressure (also called 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.


Disturbing effects produced in radio receiving apparatus by atmospheric electrical phenomena such as an electrical storm. Static.


A luminous, radiant emission over middle and high latitudes confined to the thin air of high altitudes and centered over the earth’s magnetic poles. Called “aurora borealis” (northern lights) or “aurora australis” according to its occurrence in the Northern or Southern Hemisphere, respectively.


In radar meteorology, any process which reduces power density in radar signals.

Astronomical twilight

The period of time before sunrise and after sunset when the sun is not more than 18° below the horizon.

Absolute vorticity

The rotation of the Earth imparts vorticity to the atmosphere; absolute vorticity is the combined vorticity due to this rotation and vorticity due to circulation relative to the Earth (relative vorticity).

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