Absolute Temperature - the temperature of a body or substance using the Kelvin temperature scale. It is calculated by adding 273.15 to the Celsius temperature of the body.

Absorption - the process of retaining incident radiant energy in a substance

AC - a shorthand name for the convective outlook issued by the SPC; abbreviation for Anticipated Convection.

ACCAS (usually pronounced ACK-kis) - AltoCumulus CAStellanus; mid-level clouds (bases generally 8,000 to 15,000 feet), of which at least a fraction of their upper parts show cumulus-type development. These clouds often are taller than they are wide, giving them a turret-shaped appearance. ACCAS clouds are a sign of instability aloft, and may precede the rapid development of thunderstorms.

Acid Rain - falling rain (or snow) which has become acidic as a result of its combination with gaseous pollutants, such as oxides of sulfur and nitrogen

Advection - the transport of an atmospheric property (e.g., temperature) by the wind

Air - the mixture of gases that surrounds the earth

Air Mass - a body of air that extends hundreds or thousands of kilometers horizontally and is relatively uniform in temperature and moisture content (see continental arctic, continental polar, continental tropical, maritime polar, and maritime tropical air masses); because all thunderstorms are associated with some type of forcing mechanism, synoptic-scale or otherwise, the term is somewhat controversial and should be used with discretion

Air-Mass Thunderstorm- a thunderstorm which forms from localized convection within an unstable air mass (e.g., not along a frontal boundary)

Air Parcel - an imaginary body of air a few meters in diameter which possesses nearly uniform properties within it

Air Pollution - airborne gaseous, chemical, or organic matter which pollutes the atmosphere

Air Pressure - the pressure exerted by the air

Albedo - the fraction of radiation which reflects off a body

Algorithm - a computer program (or set of programs) which is designed to systematically solve a certain kind of problem. WSR-88D radars (NEXRAD) employ algorithms to analyze radar data and automatically determine storm motion, probability of hail, VIL, accumulated rainfall, and several other parameters.

Altimeter - an instrument for measuring the altitude with respect to a fixed level

Altocumulus - a mid-level cloud which occurs as a layer or patch with a wavy appearance and is typically white and/or gray

Altostratus - a mid-level cloud which occurs as a sheet or layer with a striated, fibrous, or uniform appearance and is gray or bluish (never white)

Ambient Air- the surrounding undisturbed outside air

Anemometer - an instrument for measuring the speed of the wind

Aneroid Barometer - a barometer in which the action of atmospheric pressure in bending a metallic surface is made to move a pointer

Angle of Incidence - the angle at which a ray of light (or radiation) strikes a surface. It is measured between the incoming ray and a perpendicular to the surface at the point of incidence (i.e., where the ray strikes).

Angle of Reflection - the angle at which a reflected ray of energy leaves a reflecting surface. It is measured between the outgoing ray and a perpendicular to the surface at the point of incidence (i.e., where the ray strikes).

Angle of Refraction - the angle at which a refracted ray of energy leaves the interface at which the refraction occurred. It is measured between the direction of the refracted ray and a perpendicular to the interface at the point of refraction.

Ångstrom - a unit of length used in the measurement of the wavelength of short electromagnetic radiation, like X-rays. Named for A.J. Ångstrom (1814-1874), a Swedish physicist.

Anticyclone - an atmospheric circulation that rotates clockwise in the Northern Hemisphere and counter-clockwise in the Southern Hemisphere, that usually has a diameter of 2000 to 3000 kilometers

Anticyclonic Rotation - rotation in the opposite sense as the earth's rotation (i.e., clockwise in the Northern Hemisphere as would be seen from above); the opposite of cyclonic rotation

Anvil (Anvil Cloud) - a supplementary cloud feature viewed as the spreading of the upper portion of a cumulonimbus cloud; thunderstorm anvils may spread hundreds of miles downwind from the storm's updraft

AP (Anomalous Propagation) - a radar term for false (non-precipitation) echoes resulting from nonstandard propagation of the radar beam under certain atmospheric conditions

Aphelion - The point on the annual orbit of a body (about the sun) that is farthest from the sun; at present, the earth reaches this point (152 million kilometer from the sun) on about 5 July. Opposite of perihelion.

Apogee - The point on the orbit of the moon (or any other earth satellite) that is farthest from the earth. Opposite of perigee.

Archimedes' Principle - A net upward or buoyant force, equal in magnitude to the weight of the displaced fluid, acts upon a body either partly or wholly submerged in a fluid at rest under the influence of gravity. Named for Archimedes (287-212 BC), a Greek mathematician who discovered the principle.

Arcus - a low, horizontal cloud formation associated with the leading edge of the thunderstorm outflow (i.e., the gust front); roll clouds and shelf clouds both are types of arcus clouds

ASOS - acronym for Automated Surface Observing Systems; a system designed to provide automated meteorological measurements of several parameters at selected airports

Atmosphere - the air surrounding and bound to the earth

Atmospheric Greenhouse Effect - the warming of the atmosphere by absorption and reemission of infrared radiation by molecules in the atmosphere

Atmospheric Pressure - the pressure exerted on an object as a result of the weight of the column of air above the object

Atmospheric Radiation - Infrared radiation (energy in the wavelength interval of 3- 80 micrometer) emitted by or being propagated through the atmosphere. It consists of both upwelling and downwelling components. Compare with terrestrial radiation.

Atmospheric Wave - Any pattern with some roughly identifiable periodicity in time and/or space. In meteorology, waves in the horizontal flow pattern (e.g., Rossby wave, long wave, short wave).

Attenuation - Any decrease in amplitude, density, or energy as result of an effect such as scattering, absorption, or friction. In physical meteorology, a reduction in radiation flow, especially solar radiation by atmospheric gases and aerosols. In radar meteorology, the decrease in the magnitude of current, voltage, power, or intensity of a signal in transmission between points. Attenuation may be caused by interference such as rain or clouds.

Aurora - a luminous phenomenon in the night sky which results from a radiation emission in the upper atmosphere over middle and high latitudes

Aurora Australis - the name for the aurora of the southern latitudes

Aurora Borealis - the name for the aurora of the northern latitudes

Autumn - The season of the year that is the transition period from summer to winter, occurring as the sun approaches the winter solstice.

Autumnal Equinox - The equinox at which the sun approaches the Southern Hemispheres, marking the start of astronomical autumn in the Northern Hemisphere. The time of this occurrence is approximately 22 September. On that day, daylight is everywhere 12 hours. Compare with vernal equinox, offset by six months.

AVHRR (Advanced Very High Resolution Radiometer) - an instrument that flies on NOAA polar orbiting satellites. It measures radiation at five different wavelengths: one visible, one reflective near-infrared, one middle infrared, and two thermal infrared.

Avogadro's Number - The number of molecules in one mole of gas (6.022.169 X 10²³ per mole). According to Avogadro's law, this number is a constant for permanent gases under normal conditions - that is, pressure of one standard atmosphere and temperature of 0°C (32°F) - the volume occupied by one mole of gas is the same for all permanent gases (22,421 cubic centimeters or 22.42 liters). Named for Amedo Avogadro (1776 -1856), an Italian chemist who identified this relationship.

AVN (AViatioN model) - one of the operational forecast models run at NCEP. The AVN is run four times daily, at 0000, 0600, 1200, and 1800 GMT.

AWIPS - acronym for Advanced Weather Interactive Processing System; a computerized system that processes data received at a NWS Forecast Office from various weather observing systems

Azimuth or Azimuth Angle - The length of arc measured clockwise along the astronomical horizon (in degrees of arc) from the adopted reference direction, usually true north, to that point on the horizon where the particular object or its projection is located; north is defined as 0° (or 360°), east is 90°, south is 180° and west is 270°.