Weather Glossary: A's

Ablation
Depletion of snow and ice by melting and evaporation.
Abutment
The part of a valley or canyon wall against which a dam is constructed. Right and left abutments are those on respective sides of an observer looking downstream.
Abutment Seepage
Reservoir water that moves through seams or pores in the natural abutment material and exits as seepage.
ACCAS
An acronym for Altocumulus Castellanus.
Accessory Cloud
A cloud which is dependent on a larger cloud system for development and continuance. Roll clouds, shelf clouds, and wall clouds are examples of accessory clouds.
Accuracy
Degree of conformity of a measure to a standard or true value.
Acid rain
Cloud or rain droplets containing pollutants, such as oxides of sulfur and nitrogen, to make them acidic.
Acre-foot
The amount of water required to cover one acre to a depth of one foot. An acre-foot equals 326,851 gallons, or 43,560 cubic feet.
Active Conservation Storage
The portion of water stored in a reservoir that can be released for all useful purposes such as municipal water supply, power, irrigation, recreation, fish, wildlife, etc. Conservation storage is the volume of water stored between the inactive pool elevation and flood control stage.
Active (Usable) Storage Capacity
The total amount of reservoir capacity normally available for release from a reservoir below the maximum storage level. It is total or reservoir capacity minus inactive storage capacity. More specifically, it is the volume of water between the outlet works and the spillway crest.
Adiabatic Process
The change of temperature of air without transferring heat. In an adiabatic process compression results in warming, and expansion results in cooling.
Adirondack Type Snow Sampling Set
A snow sampler consisting of a 5-foot (1.5 m) fiberglass tube, 3" (76 mm) in diameter, with a serrated-edge steel cutter at one end and a twisting handle at the other. This sampler has a 60-inch (152 cm) snow depth capacity.
Advection
The horizontal movement of an air mass that causes changes in the physical properties of the air such as temperature and moisture. Commonly used with temperatures, i.e., "warm air advection", or moisture, i.e., "moisture advection".
Advection Fog
Fog that forms as warmer, moist air moves over a cold ground. The air is cooled to saturation by the loss of heat to the cold ground. Unlike radiation fog, advection fog may form under cloudy skies and with moderate to strong winds. Initial stability is relatively unimportant since low level cooling makes the air unstable near the ground.
ADVIS
A program which combines the Antecedent Precipitation Index (API) method of estimating runoff with unit hydrograph theory to estimate streamflow for a headwater basin.
Advisory
1. Highlights special weather conditions that are less serious than a warning. They are for events that may cause significant inconvenience, and if caution is not exercised, it could lead to situations that may threaten life and/or property.
2. Official information issued by tropical cyclone warning centers describing all tropical cyclone watches and warnings in effect along with details concerning tropical cyclone locations, intensity and movement, and precautions that should be taken. Advisories are also issued to describe tropical cyclones prior to issuance of watches and warnings, and subtropical cyclones.
Aeration Zone
A portion of the lithosphere in which the functional interstices of permeable rock or earth are not filled with water under hydrostatic pressure. The interstices either are not filled with water or are filled with water that is no held by capillarity.
Afterbay
The tail race of a hydroelectric power plant at the outlet of the turbines. The term may be applied to a short stretch of stream or conduit, or to a pond or reservoir.
Agglomerate
An ice cover of floe formed by the freezing together of various forms of ice.
AGL
Above ground level.
Airborne Snow Survey Program
Center (NOHRSC) program that makes airborne snow water equivalent and soil moisture measurements over large areas of the country that are subject to severe and chronic snowmelt flooding.
Airborne Snow Water Equivalent Measurement Theory
A theory based on the fact that natural terrestrial gamma radiation is emitted from the potassium, uranium, and thorium radioisotopes in the upper eight inches (20 cm) of the soil. The radiation is sensed from low flying aircraft 500 feet (152 m) above the ground. Water mass in the snow cover attenuates the terrestrial radiation signal. The difference between airborne radiation measurements made over bare ground and snow-covered ground can be used to calculate a mean areal snow water equivalent value with a root mean square error of less than a half inch (13 mm).
Air Mass
A large body of air that has nearly uniform conditions of temperature and humidity.
Air Mass Thunderstorm
Generally, a thunderstorm not associated with a front or other type of synoptic-scale forcing mechanism. Air mass thunderstorms typically are associated with warm, humid air in the summer months; they develop during the afternoon in response to insolation, and dissipate rather quickly after sunset. They generally are less likely to be severe than other types of thunderstorms, but they still are capable of producing downbursts, brief heavy rain, and (in extreme cases) hail over 3/4" (19 mm) in diameter. Also, see Popcorn Convection and Single Cell Thunderstorm.
AIRMET (AIRman's METeorological Information)
This NWS aviation product advises of weather that maybe hazardous, other than convective activity, to single engine, other light aircraft, and Visual Flight Rule (VFR) pilots. However, operators of large aircraft may also be concerned with these phenomena.
Air Parcel
An imaginary small body of air that is used to explain the behavior of air. A parcel is large enough to contain a very great number of molecules, but small enough so that the properties assigned to it are approximately uniform throughout.
Air Pressure
The force exerted on a surface by the weight of the air above it. The internationally recognized unit for measuring this pressure is the kilopascal.
Air Quality Statement (AQI)
This National Weather Service product is issued to relay air pollution information and issue Ozone Action Days.
Air Stagnation
A meteorological situation in which there is a major buildup of air pollution in the atmosphere. This usually occurs when the same air mass is parked over the same area for several days. During this time, the light winds cannot "cleanse" the buildup of smoke, dust, gases, and other industrial air pollution.
Air Transportable Mobile Unit
A modularized transportable unit containing communications and observational equipment necessary to support a meteorologist preparing on-site forecasts at a wildfire or other incident.
Albedo
The portion of incoming radiation which is reflected by a surface.
ALERT Flood Warning System
A cooperative, community-operated flood warning system; the acronym stands for Automated Local Evaluation (in) Real Time.
Alberta Clipper
A fast-moving low-pressure system that moves southeast out of Canadian Province of Alberta (southwest Canada) through the Plains, Midwest, and Great Lakes region usually during the winter. This low pressure area is usually accompanied by light snow, strong winds, and colder temperatures. Another variation of the same system is called a "Saskatchewan Screamer".
Aleutian Low
A semi-permanent, subpolar area of low pressure located in the Gulf of Alaska near the Aleutian Islands. It is a generating area for storms and migratory lows often reach maximum intensity in this area. It is most active during the late fall to late spring. During the summer, it is weaker, retreating towards the North Pole and becoming almost nonexistent. During this time, the North Pacific High pressure system dominates.
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.
Aliasing
The process by which frequencies too high to be analyzed with the given sampling interval appear at a frequency less than the Nyquist frequency.
Altimeter
An instrument that indicates the altitude of an object above a fixed level. Pressure altimeters use an aneroid barometer with a scale graduated in altitude instead of pressure.
Altimeter Setting
That pressure value to which an aircraft altimeter scale is set so that it will indicate the altitude above mean sea-level of an aircraft on the ground at the location for which the value was determined.
Altitude
Height expressed as the distance above a reference point, which is normally sea level or ground level.
Altocumulus (Ac)
These clouds are composed of mainly water. They appear as white or gray colored roll like elements or bands. The individual elements are large and darker than in cirrocumulus clouds. These clouds from between 6,500 and 23,000 feet (2,000 and 7,000 meters).
Altocumulus Castellanus (ACCAS)
They are middle level convective clouds and possibly they should be classified as clouds with extensive vertical development. They are composed of mainly water vapor. They are characterized by their billowing tops and comparatively high bases. These clouds from between 6,500 and 23,000 feet (2,000 and 7,000 meters). These clouds are a sign of instability aloft, and may precede the rapid development of thunderstorms.
Altocumulus Standing Lenticular (ACSL)
These clouds are formed on the crests of waves crested by barriers in the wind flow. The clouds show little movement hence the name standing. Wind, however, can be quite strong blowing through the cloud. They are characterized by their smooth, polished edges. These may also form on wave crests. They are composed of mainly water vapor and they are generally duller than Cirrocumulus Standing Lenticular (CCSL). These clouds from between 6,500 and 23,000 feet (2,000 and 7,000 meters).
Altostratus (As)
It is a bluish veil or layer of clouds having a fibrous appearance. The outline of the sun may show dimly as through frosted glass. It often merges gradually into cirrostratus. As with cirrostratus, it often is part of a cloud shield associated with a front. This type of cloud is composed of mainly water vapor and result from lifting a layer. These clouds form between 6,500 and 23,000 feet (2,000 and 7,000 meters).
Alluvium
Sediments deposited by erosional processes, usual by streams.
Anabranch
A diverging branch of a river which re-enters the main stream.
Ancraophobia
The fear of the wind. See Anemophobia.
Anchor Ice
Submerged Frazil ice attached or anchored to the river bottom, irrespective of its formation.
Anchor Ice Dam
An accumulation of anchor ice which acts as a dam and raises the water level.
Anemometer
An instrument used for measuring the speed of the wind.
Anemophobia
The fear of air drafts or wind. See Ancraophobia.
Aneroid Barometer
An instrument designed to measure atmospheric pressure. It contains no mercury (Hg).
Angels
Radar echoes caused by birds, insects, and localized refractive index discontinuities.
Annual Flood
The maximum discharge peak during a given water year (October 1 - September 30).
Anomalous Propagation (AP)
Radio wave propagation that occurs due to non-standard atmospheric conditions. Most common usage refers to ground returns detected by downward bending of the radar beam.
Antecedent Precipitation Index (API)
A measure of how much moisture in the top layer of soil within a drainage basin. In other words, this is a soil moisture index.
Antecedent Precipitation Index (API) Method
A statistical method to estimate the amount of surface runoff which will occur from a basin from a given rainstorm based on the Antecedent Precipitation Index, physical characteristics of the basin, time of year, storm duration, rainfall amount, and rainfall intensity.
Attenuation
Any process in which the flux density (power) of a beam of energy is dissipated.
Anticipated Convection (AC)
The Day 1, Day 2, and Day 3 Convective outlooks issued by the Storm Prediction Center in Norman, Oklahoma.
Anticyclone
An area of high pressure around which the wind blows clockwise in the Northern Hemisphere and counterclockwise in the Southern Hemisphere. At the center of the circulation, there is sinking air. Generally, this sinking air provides clear skies.
Anticyclonic Rotation
Rotation in the opposite sense as the Earth's rotation. In the Northern Hemisphere, this would be clockwise as would be seen from above.
Antlophobia
The fear of floods.
Anvil
The flat, spreading top of a Cumulonimbus Cloud (Cb). Thunderstorm anvils may spread hundreds of miles downwind from the thunderstorm itself, and sometimes may spread upwind. See Back-Sheared Anvil.
Anvil Crawler
Slang for a lightning discharge occurring within the anvil of a thunderstorm, 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 the parent thunderstorm, or during an active Mesoscale Convective System (MCS).
Anvil Dome
A large overshooting top or penetrating top on the top of a Cumulonimbus Cloud (Cb).
Anvil Rollover
Slang for a circular or semicircular lip of clouds along the underside of the upwind part of a back-sheared anvil, indicating rapid expansion of the anvil. See cumuliform anvil, knuckles, and mushroom.
Anvil Zits
Slang for frequent (often continuous or nearly continuous), localized lightning discharges occurring from within a thunderstorm anvil.
AOA
An acronym for "At or Above".
AOB
An acronym for "At or Below".
Apparent Temperature

A measure of human discomfort due to combined heat and humidity. It measures the increased physiological heat stress and discomfort associated with higher than comfortable humidities.

The apparent temperature is less than the actual air temperature when the humidity is relatively low and that the apparent temperature indicates the reduced stress and increased comfort associated with the higher rate of evaporative cooling of the skin.

Apparent temperatures greater than 80°F (27°C) are likely to produce some discomfort. Values in excess of 105°F (41°C) may be dangerous and even life-threatening, with severe heat exhaustion or heat stroke possible if the exposure is prolonged or physical activity is high. The degree of stress may vary with age, health, and body characteristics.

The apparent temperature does not consider the effects of air movement (wind speed) or exposure to sunshine on the degree of discomfort or stress. The apparent temperature is determined by the following Quasi-empirical equation:

Apparent Temperature = 1.03T + T (exp((DP-59)/17)-1)/19-3
where T is the Observed Temperature (°F) and DP is the Dew Point (°F)
Approaching Severe Levels
A thunderstorm which contains winds of 40 to 57 mph (35 to 49 kt / 64 to 92 km/h), or hail 1/2" (13 mm) or larger but less than 1" in diameter. See Severe Thunderstorm.
Apogee
The farthest distance between the moon and earth or the earth and sun.
Aquiclude
A formation which contains water but cannot transmit it rapidly enough to furnish a significant supply to a well or spring.
Aquifer
Permeable layers of underground rock, or sand that hold or transmit groundwater below the water table that will yield water to a well in sufficient quantities to produce water for beneficial use.
Aquifuge
A geologic formation which has no interconnected openings and cannot hold or transmit water.
Arch Dam
A concrete arch dam is used in sites where the ratio of width between abutments to height is not great and where the foundation at the abutments is solid rock capable of resisting great forces. The arch provides resistance to movement. When combined with the weight of concrete (arch-gravity dam), both the weight and shape of the structure provide great resistance to the pressure of water.
Arctic Air
A mass of very cold, dry air that usually originates over the Arctic Ocean north of Canada and Alaska.
Arctic High
A very cold high pressure that originates over the Arctic Ocean.
Arcus (Arc Cloud)
A dense, arched-shaped, menacing-looking accessory cloud to a cumulonimbus that can occur along the leading edge of a thunderstorm's gust front as the consequence of uplift of stable warm air. Same as a shelf cloud.
Area Forecast Discussion (AFD)
This National Weather Service product is intended to provide a well-reasoned discussion of the meteorological thinking which went into the preparation of the Zone Forecast Product. The forecaster will try to focus on the most particular challenges of the forecast. The text will be written in plain language or in proper contractions. At the end of the discussion, there will be a list of all advisories, non-convective watches, and non-convective warnings. The term non-convective refers to weather that is not caused by thunderstorms. Intermediate Area Forecast Discussion will be issued when either significant forecast updates are being made or if interesting weather is expected to occur.
Area of Influence
The area covered by the drawdown curves of a given pumping well or combination of wells at a particular time.
Area-Capacity Curve
A graph showing the relation between the surface area of the water in a reservoir, the corresponding volume, and elevation.
Arid
An adjunctive applied to regions where precipitation is so deficient in quantity, or occurs at such times, that agriculture is impracticable without irrigation.
Arroyo
A water-carved channel or gully in arid country, usually rather small with steep banks, dry most of the time, due to infrequent rainfall and the shallowness of the cut which does not penetrate below the level of permanent ground water.
Artesian Well
A well drilled into a confined aquifer with enough hydraulic pressure for the water to flow to the surface without pumping. Also called a flowing well.
Artificial Control
A weir or other man-made structure which serves as the control for a stream-gauging station.
ASOS
An acronym for Automated Surface Observing System.
Astraphobia
Fear of being struck by lightning. See Astraphobia, Astrapophobia, Brontophobia, Ceraunophobia, and Tonitrophobia..
Astrapophobia
The fear of thunder and lightning. See  Astraphobia, Astrapophobia, Brontophobia, Ceraunophobia, and Tonitrophobia..
Atmosphere
The gaseous envelope surrounding the earth, composed primarily of nitrogen and oxygen.
Atmospheric Pressure
The pressure asserted by the mass of the column of air directly above any specific point (also called air pressure or barometric pressure).
Atmospheric Stability
An indication of how easily a parcel of air is lifted. If the air is very stable it is difficult to make the parcel rise. If the air is very unstable the parcel may rise on its own once started.
Attenuation
Radar definition - It refers to the reduction of the radar beam power due to the reflection or absorption of energy when it strikes a target. The greatest attenuation occurs when the radar beam goes through very heavy rain.
Hydrological definition - The process where the flood crest is reduced as it progresses downstream.
ATTM
An acronym for "at this time".
Aurora
A glowing light display in the nighttime sky cause by excited gases in the upper atmosphere giving off light. In the Northern Hemisphere, it is called the Aurora Borealis (northern lights). In the Southern Hemisphere, it is called Aurora Australis (southern lights).
Auroraphobia
The fear of the Northern Lights.
Automated Local Evaluation in Real Time (ALERT)
A local flood warning system where river and rainfall data area collected via radio signals in real-time at an ALERT base station.
Automatic Radio Theodolite Master Control Unit (ART)
This unit is used to automatically track radiosondes.
Automatic Surface Observing System (ASOS)
ASOS serves as the nation's primary surface weather observing network. It is designed to support weather forecast activities and aviation operations and, at the same time, support the needs of the meteorological, hydrological, and climatological research communities.
Automatic Weather Observing System (AWOS)
A suite of sensors, which 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.
Avalanche
A mass of snow, rock, and/or ice falling down a mountain or incline. In practice, it usually refers to the snow avalanche. In the United States, the term snow slide is commonly used to mean a snow avalanche.
Avalanche Advisory
A preliminary notification that conditions may be favorable for the development of avalanches in mountain regions.
Aviation Area Forecast (FA) - Most recent FA here.
This NWS aviation product is a forecast of clouds and weather conditions over an area as large as the size of several states. It must be used in conjunction with Airmet bulletins for the same area in order to get a complete picture of the weather. The area forecast together with the Airmet Sierra bulletin are used to determine forecast enroute weather and to interpolate conditions at airports which do not have terminal forecasts (FT's) issued.
Aviation Low Level Significant Weather Forecasts
A low-level graphics product is a forecast of aviation weather hazards, primarily intended to be used as a guidance product for briefing the VFR pilot. The forecast domain covers the 48 contiguous states, southern Canada and the coastal waters for flight levels below 24,000 ft.
Aviation High Altitude Significant Weather Forecasts
High Altitude Significant Weather (SIGWX) forecasts are provided for the en-route portion of international civil aeronautical operations. The AWC fulfills the role of the Washington Regional Area Forecast Center (RAFC) by providing a suite of SIGWX forecast products. These products are used directly by airline dispatchers for flight planning and weather briefing before departure, and by flight crew members during the flight.
AViatioN model (AVN)
One of the operational forecast models run at National Centers for Environmental Prediction. It was replaced by the Global Forecast System (GFS).
Aviation Weather Center (AWC)
One of the National Centers for Environmental Prediction. The Aviation Weather Center (AWC), located in Kansas City, Mo., enhances aviation safety by issuing accurate warnings, forecasts and analyses of hazardous weather for aviation interests. The Center identifies existing or imminent weather hazards to aircraft in flight and creates warnings for transmission to the aviation community. The Center also originates operational forecasts of weather conditions that will affect domestic and international aviation interests.
AWOS
An acronym for Automated Weather Observation System.
Azimuth
A direction in terms of a 360° compass. North is at 0°. East is at 90°. South is at 180°. West is at 270°.