A AMS - see Arctic Air Mass

ABNDT - abundant

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.

Absolute Zero - the lowest theoretically possible temperature in the universe, 0 Kelvin (-273.15°C or -459.7°F). All motion (including molecular vibration) stops at this temperature.

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

ABT - about

ABV - above

AC - see Altocumulus

AC - Anticipated Convection - see Convective Outlook

ACCAS (usually pronounced ACK-kis) - see Altocumulus Castellanus

Accretion - when ice crystals collide with supercooled droplets, freezing on contact and sticking together. This process is also known as riming (see Rime Ice) and can produce graupel (snow pellets).

ACCUM or ACCUMS - accumulation(s)

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

ACLD - above cloud level

ACPY - accompany

ACRS - across

ACTV - active

ACYC - see Anticyclone

Adiabatic - Changes in temperature caused by the expansion (cooling) or compression (warming) of a body of air as it rises or descends in the atmosphere, with no exchange of heat with the surrounding air.

ADJ - adjacent

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

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

ADVCTN - see Advection

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

Advection Fog - a fog that forms when warm air flows over a cold surface and cools from below until saturation is reached.

ADVN - advance

ADVY - advisory

AFB - Air Force Base

AFCT - affect

AFD - see Area Forecast Discussion

AFOS - Automation of Field Operations and Services

AFT - after

AFTN - afternoon

AFTR - after

AFWA - Air Force Weather Agency

Aggregation - when falling ice crystals collide and stick to one another. This is how many snowflakes are formed.

AGL - above ground level

AGN - again

AHD - ahead

Air - the mixture of gases that surrounds the earth

Air Mass (AMS) - 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)

Air-Mass Thunderstorm - a thunderstorm that forms from localized convection within an unstable air mass (e.g., not along a frontal boundary); 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 Parcel - an imaginary body of air a few meters in diameter that possesses nearly uniform properties within it. Also referred to as a Parcel.

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

Air Pressure - the pressure exerted by the air

AK - Alaska

AL - Alabama

Albedo - the fraction of radiation that reflects off a body

Alberta Clipper - a fast-moving low pressure system that moves southeast out of the Canadian province of Alberta through the Plains, Midwest, and Great Lakes region usually during the winter. It is usually accompanied by light snow, strong winds, and colder temperatures.

ALF - aloft

ALG - along

Algorithm - a computer program (or set of programs) that 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 (AC) - a mid-level cloud that occurs as a layer or patch with a wavy appearance and is typically white and/or gray

Altocumulus Castellanus (ACCAS) - 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.

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

ALQDS - all quadrants

ALTHO - although

Ambient Air - the surrounding undisturbed outside air

AMD - amend

American Meteorological Society (AMS) - the American Meteorological Society promotes the development and distribution of information and education on the atmospheric (and related oceanic and hydrologic) sciences and the advancement of their professional applications.

Amplitude - the distance from rest to crest in a wave. In other words, it is the displacement of a particle from its rest position. Let’s say that we have a rope at rest and it is lying straight on the ground. If someone comes along and wiggles the rope from side to side like a snake, we see a wave. The distance between a crest (or peak) and the original rest position is the amplitude. The amplitude can also be measured from the trough to the rest position.

AMS - see Air Mass

AMS - see American Meteorological Society

AMT - amount

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.

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

Anomaly - the departure or deviation from normal. For example, let’s say that the normal temperature for a month is 50 °F. If the actual temperature average for the month is 55 °F, the monthly temperature is 5 °F above normal, which is a positive anomaly of 5 °F. A negative anomaly occurs when a temperature is below the normal value.

Anticyclone (ACYC) - an atmospheric circulation that rotates clockwise in the Northern Hemisphere and counter-clockwise in the Southern Hemisphere, which 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

Anticyclonic Vorticity Advection (AVA) - see Negative Vorticity Advection

Anvil (or 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

Anvil Crawler - a lightning discharge that occurs in the anvil of a thunderstorm and is characterized by one or more channels that appear to crawl along the underside of the anvil. They typically appear during the weakening or dissipating stage of a parent thunderstorm, or during an active MCS.

AO - see Arctic Oscillation

AOA - at or above

AOB - at or below

AP - see Anomalous Propagation

Apparent Force - an imaginary force. Newton’s First Law of Motion needs a coordinate system that does not move (inertial motion). However, the earth moves underneath us, so we use apparent forces to explain what we observe. Apparent forces include Coriolis Force and Centrifugal Force.

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.

APRCH - approach

APRCHG - approaching

APRCHS - approaches

AR - Arkansas

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.

Arctic Air Mass (A AMS) - see Continental Arctic Air Mass.

Arctic Oscillation (AO) - a pattern in which pressure at polar and middle latitudes fluctuates, leading to a difference in storm paths in the Northern Hemisphere. For example, in the "negative phase", cold air plunges into the Midwestern United States and western Europe, and storms bring rain to the Mediterranean. In the "positive phase", the opposite is true, so storms follow a more northern path, leaving areas such as California, Spain, and the Middle East drier.

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

Area Forecast Discussion (AFD) - a detailed NWS product that discusses the meteorological thinking behind a forecast. This product is useful in learning more about how weather works.

ARND - around

ARR - arrive/arrival

AS - see Altostratus

ASAP - as soon as possible

ASL - above sea level

ASOS - see Automated Surface Observing Systems

ASSCD - associated

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 to 80 micrometer) emitted by or being propagated through the atmosphere. It consists of both upwelling and downwelling components. Compare with terrestrial radiation.

Atmospheric Stability - the amount of resistance of a layer of air to vertical motion (how likely is it that a parcel will rise or sink?). It is also the condition of equilibrium. Stable equilibrium occurs when a parcel or object will return to its original position, while unstable equilibrium occurs when a parcel moves away from its original position. For example, if one pushes a rock at the bottom of a valley up a hill, the rock will keep rolling back down the hill. This is stable equilibrium. If a rock is at the top of the mountain and gets nudged, it will roll down and away from its original position. This is unstable equilibrium.

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 waves, long wave, shortwave).

Atmospheric Window - the wavelength range between 8 and 11 micrometers in which little absorption of infrared radiation takes place. These windows allow certain types of radiation to easily pass through the atmosphere to Earth’s surface and from the surface to space.

Attenuation - any decrease in amplitude, density, or energy as a 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.

ATTM - at this time

Aurora - a luminous phenomenon in the night sky that 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

Automated Surface Observing System (ASOS) - a system designed to provide automated meteorological measurements of several parameters at selected airports and other locations. The system is operated and controlled by the Federal Aviation Administration (FAA), NWS, and Department of Defense (DOD). In addition to the parameters measured by AWOS, ASOS can also report present weather, icing, lightning, sea level pressure, and precipitation accumulation.

Automated Weather Observation System (AWOS) - operated and controlled by the Federal Aviation Administration (FAA), these weather stations observe temperature, dew point, wind speed and wind direction, visibility, sky cover and ceiling up to twelve thousand feet, and altimeter setting.

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 12 hours everywhere. Compare with vernal equinox, offset by six months.

AVA - see Anticyclonic Vorticity Advection

AVBL - available

Average (AVG) - the "middle" of a data set. For example, if your test scores are 75, 92, 90, 83, and 89, your average score is 85.8 [(75+92+90+83+89)/5]. An average is also known as a mean.

AVG - see Average

AVHRR - see Advanced Very High Resolution Radiometer

Aviation Model (AVN) - see Global Forecast System (GFS) Model.

Aviation Weather Center (AWC) - one of 9 NCEP centers. Provides aviation warnings and forecasts of hazardous flight conditions for domestic and international air space. It is located in Kansas City, Missouri.

AVN - see Aviation Model (now part of the GFS Model)

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 Amedeo Avogadro (1776 -1856), an Italian chemist who identified this relationship.

AWC - see Aviation Weather Center

AWIPS - see Advanced Weather Interactive Processing System

AWOS - see Automated Weather Observation System

AZ - Arizona

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°.