Aeronautical Terms beginning with A
AAWU
Alaskan Aviation Weather Unit
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.
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).
Adiabatic cooling
A process of cooling the air through expansion. For example, as air moves up slope it expands with the reduction of atmospheric pressure and cools as it expands.
Adiabatic Cooling
A process by which a parcel of air cools. When a parcel of air is lifted, pressure is reduced due to the elevation increase. This reduction in pressure causes the parcel of air to expand in volume and, in turn, the parcel cools to maintain an energy balance because no energy is added to the parcel.
Adiabatic heating
A process of heating dry air through compression. For example, as air moves down a slope it is compressed, which results in an increase in temperature.
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.
Advection
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.
Airmen’s Meteorological Information (AIRMET)
In-flight weather advisories concerning weather phenomena of operational interest to all pilots and especially to pilots of aircraft not approved for flight in icing conditions. An AIRMET concerns weather of lesser severity than that covered by an advisory of significant meteorological information (SIGMET) or a convective SIGMET. AIRMETs may include advisories of moderate icing.
AIRMET
Inflight weather advisory issued as an amendment to the area forecast, concerning weather phenomena of operational interest to all aircraft and that is potentially hazardous to aircraft with limited capability due to lack of equipment, instrumentation, or pilot qualifications.
Albedo
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.
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.
Altocumulus
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.
Anemometer
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.
Angel
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.
Anticyclone
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.
APOB
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.
Area chart
Part of the low-altitude en route chart series, this chart furnishes terminal data at a larger scale for congested areas.
Area forecast (FA)
A report that gives a picture of clouds, general weather conditions, and visual meteorological conditions (VMC) expected over a large area encompassing several states. (Discontinued, replaced by Graphical Forecasts for Aviation)
ASOS
Automated Surface Observing System.
Astronomical twilight
The period of time before sunrise and after sunset when the sun is not more than 18° below the horizon.
ATIS
Automatic Terminal Information Service
Atmosphere
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.
Atmospherics
Disturbing effects produced in radio receiving apparatus by atmospheric electrical phenomena such as an electrical storm. Static.
Attenuation
In radar meteorology, any process which reduces power density in radar signals.
Aurora
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.
Automated Surface Observing System (ASOS)
Weather reporting system which provides surface observations every minute via digitized voice broadcasts and printed reports.
Automated Surface Observing System (ASOS)
A suite of sensors that measure, collect, and disseminate weather data to help meteorologists, pilots, and flight dispatchers prepare and monitor weather forecasts, plan flight routes, and provide necessary information for correct takeoffs and landings. There are many differences between an ASOS and an All Weather Operations Specialist (AWOS) (see subparagraph 1-3d). It is important for pilots to understand the strengths and limitations of the various configurations. The ASOS is comprised of a standardized suite of weather sensors and is a product of a National Weather Service (NWS), Department of Defense (DOD), and FAA joint venture. One of ASOS’s most important features is its ability to detect precipitation, including intensity of rain, snow, and freezing rain. One current ASOS limitation is its inability to simultaneously detect and report freezing drizzle, ice pellets, or any other freezing precipitation without human augmentation when other forms of precipitation are present. A detailed description of ASOS’s capabilities can be found at the NWS ASOS homepage: http://www.nws.noaa.gov/asos/index.html.
Automated Weather Observation System (AWOS)
A suite of weather sensors that are procured by the FAA or purchased by individuals, groups, airports, etc. It is important to note that the absence of reported precipitation does not mean that such conditions do not exist. The AWOS may not be configured to report this information or have precipitation reporting capability. A detailed description of AWOS’s capabilities can be found in the AIM.
Automated Weather Observing System (AWOS)
Automated weather reporting system consisting of various sensors, a processor, a computer-generated voice subsystem, and a transmitter to broadcast weather data.
Aviation Routine Weather Report (METAR)
Observation of current surface weather reported in a standard international format.
Aviation Weather Service Program
Aviation weather service provided by the NWS and the FAA that collects and disseminates pertinent weather information for pilots, aircraft operators, and air traffic control (ATC).
AWOS
Automated Weather Observing System
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