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Weather Term Definitions
- Ablation: The process of being removed. Snow ablation usually refers to removal by melting.
- Accretion: Growth of precipitation particles by collision of ice crystals with supercooled liquid droplets which freeze on impact.
- Acid Rain: Cloud or rain droplets containing pollutants, such as oxides of sulfur and nitrogen, to make them acidic (e.g. pH < 5.6).
- 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.
- Advection: The horizontal transport of air, moisture or other atmospheric properties. Commonly used with temperatures, i.e., "warm air advection".
- Advection Fog: a type of fog that results from the advection of moist air over a cold surface and the cooling of the air to its dew point that follows; this type of fog is most common in coastal regions.
- Advisory: Advisories are issued for weather situations that cause significant inconveniences but do not meet warning criteria and, if caution is not exercised, could lead to life-threatening situations. Advisories are issued for significant events that are occurring, are imminent, or have a very high probability of occurrence.
- Aerosol: Particles of matter, solid or liquid, larger than a molecule but small enough to remain suspended in the atmosphere (up to 100µm diameter). Natural origins include salt particles from sea spray and clay particles as a result of weathering of rocks. Aerosols can also originate as a result of man's activities and in this case are often considered pollutants.
- Aerovane: Aerovanes are commonly used at many weather stations and airports to measure both wind direction and speed. They are similar to wind vanes and cup anemometers except have three-bladed propellers attached to the end of the vane.
- AGL: above ground level.
- Airmass: A large body of air that has nearly uniform conditions of temperature and humidity.
- 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 Pollution: The existence in the air of substances in concentrations that are determined unacceptable. Contaminants in the air we breathe come mainly from manufacturing industries, electric power plants, automobiles, buses, and trucks.
- Air Pressure: Air pressure (atmospheric pressure) is 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.
- Airstream: A significant body of air flowing in the same general circulation.
- Albedo: The percentage of light reflected by an object. Snow covered areas have a high albedo (0.9 or 90%) due to their white color.
- Aeutian Low: A semi-permanent 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.
- Altimeter: An active instrument used to measure the altitude of an object above a fixed level.
- 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.
- Altocumulus: Mid-altitude clouds with a cumuliform shape.
- Altostratus: Mid-altitude clouds with a flat sheet-like shape.
- Anabatic Wind: wind flowing up an incline, such as up a hillside due to local daytime heating; upslope wind. The opposite of a katabatic wind
- Anemometer: An instrument that measures wind speed.
- Aneroid barometer: An instrument built around a metal structure that bends with changing air pressure. These changes are recorded on a pointer that moves back and forth across a printed scale.
- Anomaly: The deviation of (usually) temperature or precipitation in a given region over a specified period from the normal value for the same region.
- Anticyclone: A large body of air in which the atmospheric pressure is higher than the pressure in the surrounding air. The winds blow clockwise around an anticyclone in in the Northern Hemisphere.
- Anticyclonic: describes the movement of air around a high pressure. This is clockwise in the Northern Hemisphere.
- Anvil Cloud: The flat, spreading top of a Cb (cumulonimbus), often shaped like an anvil. Thunderstorm anvils may spread hundreds of miles downwind from the thunderstorm itself, and sometimes may spread upwind (see back-sheared anvil).
- Arctic Air: a mass of very cold, dry air that usually originates over the Arctic Ocean north of Canada and Alaska.
- Arctic Front:The semipermanent, semi-continuous front between the deep, cold arctic air and the shallower, basically less cold polar air of northern latitudes.
- Arctic High: A very cold high pressure that originates over the Arctic Ocean.
- Arctic Jet:The jet stream that is situated high in the stratosphere in and around the Arctic or Antarctic Circles. It marks the boundary of polar and arctic air masses.
- ASOS: Automated Surface Observing System. This system observes sky conditions, temperature and dewpoint, wind direction and speed, and barometric pressure, and precipitation.
- ASTRONOMICAL TWILIGHT: The time after nautical twilight has commenced and when the sky is dark enough, away from the sun's location, to allow astronomical work to proceed. It ends when the center of the sun is 18° below the horizon.
- Atmosphere: The mass of air surrounding the earth and bound to it more or less permanently by the earth's gravitational attraction.
- 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.
- AVHRR: Advanced Very High Resolution Radiometer. It is the main sensor on the polar orbiting satellites.
- AWIPS: Advanced Weather Interactive Processing System is the main visualization and forecasting software package used by National Weather Service forecasters.
