AP IndexIn solar-terrestrial terms, an averaged planetary A Index based on data from a set of specific stations.
APDOn a buoy report, the average wave period (seconds) of all waves during the 20-minute period.
AphelionThe 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…
API MethodA statistical method to estimate the amount of surface runoff which will occur from a basin from a given rainstorm based on the antecedent precipitation…
ApogeeThe farthest distance between the moon and earth or the earth and sun.
Apparent TemperatureA measure of human discomfort due to combined heat and humidity (e.g., heat index).
Apparent WindThe speed and true direction from which the wind appears to blow with reference to a moving point. Sometimes called RELATIVE WIND.
AquicludeA layer of clay which limits the movement of ground water.
AquiferA water-bearing geologic formation, sometimes confined between clay layers and sometimes on the surface.
AquifugeA geologic formation which has no interconnected openings and cannot hold or transmit water.
Arable landLand which when farmed in adequate size units for the prevailing climatic and economic setting, and provided with the essential on-farm improvements of…
Arch DamA concrete or masonry dam which is curved upstream in plan so as to transmit the major part of the water load to the abutments and to keep the dam in…
Arch Filament System (AFS)In solar-terrestrial terms, a bright, compact plage crossed by a system of small, arched filaments, which is often a sign of rapid or continued growth in…
ArcticThe region within the Arctic Circle, or, loosely, northern regions in general, characterized by very low temperatures.
arctic airA very cold and dry air mass that forms primarily in winter and the northern interior of North America.
Arctic air massArctic air mass Mass of very cold air in the Arctic regions which invades lower latitudes at irregular intervals.
Arctic CircleArctic Circle The line of latitude 66° 32' N (often taken as 66.5° N). Along this line the Sun does not set on the day of the summer solstice, and does…
Arctic frontThe boundary or front separating deep, cold arctic air from shallower, relatively less cold polar air.
Arctic hazeArctic haze Haze in Arctic regions which reduces horizontal and slant visibility and which may extend to a height of about 10 km.
Arctic highArctic high A weak high which appears over the Arctic Basin during late spring, summer and early autumn, one of the centers of action in the atmosphere.
Arctic mistArctic mist A mist of ice crystals; a very light ice fog.
Arctic Oscillation(abbrev. AO)- The Arctic Oscillation is a pattern in which atmospheric pressure at polar and middle latitudes fluctuates between negative and positive…
Arctic Sea SmokeSteam fog, but often specifically applied to steam fog rising from small open water within sea ice.
ArcusA low, horizontal cloud formation associated with the leading edge of thunderstorm outflow (i.e., the gust front).
Area Forecast DiscussionThis National Weather Service product is intended to provide a well-reasoned discussion of the meteorological thinking which went into the preparation of…
Area of InfluenceThe area covered by the drawdown curves of a given pumping well or combination of wells at a particular time.
Area SourceAn array of pollutant sources, so widely dispersed and uniform in strength that they can be treated in a dispersion model as an aggregate pollutant…
Area Wide Hydrologic Prediction System(Abbrev. AWHPS) - A computer system which automatically ingests areal flash flood guidance values and WSR-88D products and displays this data and other…
Area-Capacity CurveA graph showing the relation between the surface area of the water in a reservoir, the corresponding volume, and elevation.
AridA region that receives too little water to support agriculture without irrigation. Less than ten inches of rainfall a year is typically considered arid.
ARM Climate Research Facility (ACRF)A U.S. Department of Energy designated national user facility that provides a unique asset for the study of global change to the broader national and…
Artesian WellWater held under pressure in porous rock or soil confined by impermeable geologic formations. An artesian well is free flowing. See confined aquifer.
Artificial ControlA weir or other man-made structure which serves as the control for a stream-gaging station.
ASOSThe ASOS program is a joint effort of the National Weather Service (NWS), the Federal Aviation Administration (FAA), and the Department of Defense (DOD).
AsphaltA dark-brown-to-black cement-like material containing bitumen as the predominant constituent. It is obtained by petroleum processing.
Astronomical DawnThe time at which the sun is 18 degrees below the horizon in the morning. Astronomical dawn is that point in time at which the sun starts lightening the…
Astronomical DuskThis is the time at which the sun is 18 degrees below the horizon in the evening. At this time the sun no longer illuminates the sky.
Astronomical Unit(abbrev. AU)- The mean earth-sun distance, equal to 1.496x1013 cm, or 214.94 solar radii.
ATDTDCSAutomated Tone Dial Telephone Data Collection System - Data collection system where cooperative observers collect precipitation, stage, and temperature…
AtmosphereThe gaseous envelope surrounding the Earth, which consists almost entirely of nitrogen (78.1% volume mixing ratio) and oxygen (20.
Atmospheric Boundary LayerSame as Boundary Layer - in general, a layer of air adjacent to a bounding surface.
Atmospheric circulationAtmospheric circulation System of atmospheric motions over the Earth on the scale of the whole globe (general atmospheric circulation), or over a certain…
Atmospheric Circulation ModelA mathematical model for quantitatively describing, simulating, and analyzing the structure of the circulation in the atmosphere and the underlying causes.
Atmospheric LifetimeThe lifetime of a greenhouse gas refers to the approximate amount of time it would take for the anthropogenic increment to an atmospheric pollutant…
atmospheric moistureThe moisture content of the air as indicated by several measurements including relative humidity, specific humidity, dewpoint, vapor pressure, water vapor…
Atmospheric phenomenonAtmospheric phenomenon (phenomena) As commonly used in weather observing practice, an observable occurrence of particular physical significance within the…
Atmospheric PressureThe pressure of the air at sea level; one standard atmosphere at zero degrees centigrade is equal to 14.695 pounds per square inch (1.
Atmospheric RadiationInfrared radiation (energy in the wavelength interval of 3- 80 micrometer) emitted by or being propagated through the atmosphere.
atmospheric temperatureThe temperature indicated by a thermometer exposed to the air in a place sheltered from direct solar radiation.
atmospheric turbulenceHigh frequency velocity fluctuations that lead to turbulent transport of momentum, heat, mositure, and passive scalars, and often expressed in terms of…
atmospheric windowA region of the electromagnetic spectrum, from 8 to 12 micrometers, where the atmosphere is transparent to radiation.
ATMPOn a buoy report, the air temperature (Celsius).
Atomic weightThe average weight (or mass) of all the isotopes of an element, as determined from the proportions in which they are present in a given element, compared…
AtomsThe smallest unit of matter that possesses chemical properties. All atoms have the same basic structure: a nucleus containing positively charged protons…
AttenuationA mechanism for controlling gene expression in prokaryotes that involves premature termination of transcription.
Augmented reportA meteorological report prepared by an automated surface weather observing system for transmission with certified observers signed on to the system to add…
AuroraA display of colored light in the atmosphere, caused by collisions between charged particles from a planet’s magnetosphere and atmospheric gases near…
Aurora AustralisSame as Aurora Borealis, but in the Southern Hemisphere. Also known as the southern lights; the luminous, radiant emission from the upper atmosphere over…
Aurora BorealisAlso known as the northern lights; the luminous, radiant emission from the upper atmosphere over middle and high latitudes, and centred around the earth's…
Auroral OvalThe ring-shaped belts of auroral activity around the north and south magnetic poles.
Automated Event Reporting Gage(see Tipping Bucket Rain Gage); for river stage gages, IFLOWS pressure transducer type gages can be programmed to report if water surface rises or falls…
Automated ReportA meteorological report prepared by an automated surface weather observing system for transmission, and with no certified weather observers signed on to…
Automated Surface Observing SystemThe ASOS program is a joint effort of the National Weather Service (NWS), the Federal Aviation Administration (FAA), and the Department of Defense (DOD).
AUTOMATIC GAIN CONTROLAny method of automatically controlling the gain of a receiver, particularly one that holds the output level constant regardless of the input level.
AutumnThe season of the year that is the transition period from summer to winter, occurring as the sun approaches the winter solstice.
Autumnal EquinoxThe equinox at which the sun approaches the Southern Hemisphere, marking the start of astronomical autumn in the Northern Hemisphere.
AvalancheA mass of snow, rock, and/or ice falling down a mountain or incline. In practice, it usually refers to the snow avalanche.
Avalanche AdvisoryA preliminary notification that conditions may be favorable for the development of avalanches in mountain regions.