ADVECTION: Generally the horizontal or isobaric transport of an atmospheric property (such as temperature or moisture) by the wind. AFOS: Automation of Field Operations and Services. Standard computer system used by National Weather Service Offices. AIR MASS: A widespread body of air, with consistent temperature and moisture characteristics. ALBERTA CLIPPER: A storm system that develops near the Province of Alberta region in Canada and moves rapidly east and southeast into the Great Lakes and then into the Northeast. ALTOCUMULUS: A principal cloud type, white and/or gray in color, present in the mid-altitudes with a cumuliform-like shape. ALTOSTRATUS: A principal cloud type, gray or bluish in color, present in the mid-altitudes with a sheet or fiborous appearance. ANEMOMETER: The general name for instruments which measure speed or force of the wind. ANTICYCLONE: An area of high pressure around which the winds circulate in a clockwise direction in the Northern Hemisphere. Usually responsible for fair-dry weather. ASOS: Automated Surface Observing System. An automated device which observes sky conditions, temperature, dewpoint, wind direction, wind speed, and barometric pressure and other meteorological parameters. AVN: A numerical weather prediction model, the Aviation Model that is run every 12 hours at the National Meteorological Center. AWIPS: Advanced Weather Information Processing System. The planned National Weather Service computer system to replace the current AFOS system. It will integrate computer guidance from National Meteorological Center, surface and upper air observations, satellite and radar graphics on one computer network. BACKDOOR COLD FRONT: A cold front that approaches from the north to northeast instead of the more usuall west to northwest direction. BAROMETER: An instrument for measuring atmospheric pressure. BERMUDA HIGH: An area of high pressure centered over the western Atlantic Ocean, that usually develops over the summer months. This weather system produces a southerly wind and often moves warm and humid air into the Northeast. BLIZZARD: A severe weather condition characterized by snow with winds in excess of 35 mph and visibilities frequently 1/4 mile or less, for an extended period of time usually 3 hours or greater. BLOWING DUST: Dust picked up locally from the surface and blown about. BROKEN CLOUDS: Clouds which cover between 6/10 and 9/10 of the sky is covered by clouds. CEILING: The height of the lowest layer of clouds that is reported as broken or overcast and which is not thin. CIRRUS: A principal cloud type, present a high altitudes, usually above 18,000 feet, usually composed of ice crystals. CIRROCUMULUS: A principal cloud type, they are cirrus clouds with vertical development. CIRROSTRATUS: A principal cloud type, they are cirrus clouds with a flat veil or sheet like appearance. CLEAR: Sky condition of less than 1/10 cloud coverage. CLIMATE: The statistical collective of weather records during a specified period of time. COLD FRONT: The front edge of a cold air mass. Often produces showery precipitation and sometimes severe weather. CONDENSATION: The change of phase in which gas changes into a liquid. CORIOLIS FORCE: An apparent force on moving particles produced by the rotation of the Earth. In the Northern Hemisphere the wind is deflected to the right by the coriolis force. CONVECTION: An atmospheric motion that is predominately vertical and is driven by some sort of heat source. CUMULONIMBUS CLOUD: A principal cloud type, exceptionally dense and vertically developed. Produces showery preciptitation that is often heavy. Usually capped by an anvil on top and a very dark base. CUMULUS CLOUD: A principal cloud type, usually in the shape of individual vertical elements with a flat base and a bulging upper portion resembling cauliflower. CYCLONE: An area of low pressure around which the wind circulates in a counterclockwise direction in the Northern Hemisphere. Usually responsible for wet-disturbed Also the term used for a hurricane in the Indian Ocean and in the Western weather. DEW: Moisture that has condensed on objects near the ground, after the temperature has fallen to or below the the dewpoint temperature near that surface. DEWPOINT: The temperature to which the air must be cooled for water vapor to condense at a constant temperature and at a constant amount of water vapor. DOPPLER RADAR: A very sensitive type of weather radar which can determine some atmospheric motion. It utilizes the Doppler Effect to measure the velocity of particles in the atmosphere. DRIZZLE: Very small, numerous, slowly falling water droplets, with diameters less than 0.5 millimeters. ECMF: A numerical weather prediction model, from the European Center for Meteorology Forecast center. ETA: A numerical weather prediction model, the ETA forecast model is generated every 12 hours by the National Meteorological Center. FAIR: Less than 4/10 opaque cloud cover, no precipitation, and no extremes in temperature, visibility or wind. FOG: The visible aggregate of minute water droplets suspended in the atmosphere near the earth's surface. FREEZING RAIN: Rain which falls as liquid then freezes upon impact of a surface, resulting in a coating of ice on exposed objects. FRONT: The leading edge of a defined airmass. Frontal types include cold fronts, warm fronts, occluded fronts and stationary fronts. FROST: Moisture that has frozen on a surface near the ground , after the temperature has fallen near or below freezing near that surface. FUNNEL CLOUD: A violent,rotating, funnel-shaped cloud extending below the base of a thunderstorm. When it reaches the ground it is called a tornado. GALE: Wind speeds from 39 to 54 mph (34 to 47 knots). GOES: Geostationary Operational Environmental Satellite. GUST: A brief sudden increase in wind speed, a fluctuation greater than 10 knots during a period less than 30 seconds. HAIL: Frozen precipitation in the form of balls or irregular lumps of ice produced by convective clouds such as cumulonimbus. HAZE: Fine dust or salt particles present in a portion of the atmosphere. HEAT INDEX: Also known as the Apparent temperature, it is an non-physical value which combines the effect of the air temperature and amount of moisture in the air to illustrate how it "feels." HUMIDITY: Generally the measure of the water vapor content of the air. HURRICANE: An intense tropical cyclone In North Atlantic, Caribbean Sea, Gulf of Mexico and the Eastern Pacific Ocean with sustained wind speeds in excess of 74 mph (64 knots). JET STREAM: Relatively strong winds concentrated within a narrow band in the atmosphere. LFM: A numerical weather prediction model, the Limited Fine Mesh model is generated every 12 hours by the National Meteorological Center. LIGHTNING: All of the various forms of electrical discharge produced by thunderstorms. METEOROLOGY: The study of atmospheric phenomena. MICROMETEOROLOGY: The portion of the science of meteorology that deals with the smallest-scale physical and dynamical processes. MOS: Model Output Statistics. MRF: A numerical weather prediction model, the Medium Range Forecast model is generated every 12 hours by the National Meteorological Center. NCDC: National Climatic Data Center. The archive center for climate, observational and forecast data from the National Weather Service. NGM: Nested Grid Model generated every 12 hours by National Meteorological Center. NHC: National Hurricane Center. The office of the National Weather Service responsible for tracking and forecasting tropical cyclones. NATIONAL METEOROLOGICAL CENTER (NMC): The central processing, Computer, forecasting and communications facility location of the National Weather Service. NOAA: National Oceanic and Atmospheric Administration. A branch of the US Department of Commerce, NOAA is the parent organization of the National Weather Service. NOAA WEATHER RADIO (NWR): The voice of the National Weather Service. Continuous, 24 hour a day broadcast of weather information, forecasts and warnings. It is programmed from local National Weather Service offices. NOR'EASTER: An intense low pressure system that tracks along the east coast of the United States, usually during the fall, winter, and spring months. These storms can produce strong northeast winds, large waves, and intense precipitation. NSSFC: National Severe Storms Forecast Center. The office responsible for monitoring and forecasting severe convective weather. OCCLUDED FRONT: A composite of two fronts that usually occurs when a cold front overtakes a warm front. OVERCAST: Sky condition when greater than 9/10 of the sky is covered by clouds. PARTLY CLOUDY: Sky condition when between 3/10 and 7/10 of the sky is covered by clouds. PRECIPITATION: Any form of water particles, liquid or frozen, that fall from the atmosphere and reach the ground. RADAR: A device used to detect precipitation by measuring emitting an electromagnetic signal and measuring the intensity of the reflected energy. RADIOSONDE: An instrument that is connected to a weather balloon that collects meteorological data such as temperature, pressure, moisture and wind information as it ascends through the atmosphere. RAIN: Liquid water precipitation generally with a diameter greater than 0.5 mm. RELATIVE HUMIDITY: The percent of the amount of water vapor in the air compared to the capacity for water vapor of the air. RIDGE: An elongated area of high pressure in the atmosphere. RUC: A numerical weather prediction model, the Rapid Update Cycle model is generated every 3 hours by the National Meteorological Center. SCATTERED CLOUDS: Sky condition when between 1/10 and 5/10 of the sky is covered by clouds. SEA BREEZE: A local wind that blows from a sea or ocean towards land, its caused by the temperature difference between cool air above the water and the warmer land. SEVERE THUNDERSTORM: A thunderstorm with wind gusts in excess of 58 mph (50 knots) and/or hail with a diameter of 3/4" or more. SHOWER: Liquid precipitation with frequent changes in intensity or suddenly stops or starts. SLEET: A type of frozen precipitation, consisting of small transparent pellets produced by the freezing of liquid or semi-liquid precipitation aloft. SNOW: A type of frozen precipitation composed white translucent ice crystals in complex hexagonal forms. SNOW FLURRY: Also referred to as a snow shower, usually very light and brief in nature. SNOW SQUALL: Intense showers or bands of locally heavy snow produced by the lake effect. SQUALL LINE: A non-frontal band, or line, of thunderstorms. STATIONARY FRONT: A transition zone between airmasses, with neither advancing upon the other. STRATUS: A principal cloud type, gray in color, present at low altitudes with a uniform base. THUNDER: The sound produced by a stroke of lightning as it rapidly heats the air surrounding the bolt. THUNDERSTORM: A local storm accompanied by lightning and thunder, usually produced by a cumulonimbus cloud, often accompanied by heavy rain, gusty winds, and sometimes hail. TORNADO: A violent rotating column of air, extending from the base of a cumulonimbus cloud to the ground, producing extraordinary damage and winds up to 300 mph. TROUGH: An elongated area of low pressure. VIRGA: Precipitation falling from the base of a cloud that does not reach the ground because of evaporation. WARM FRONT: A leading boundary of a warm airmass that is often moving into an area influenced by a cooler airmass. WARNING: A notice that is produced when a certain hazard (tornado, severe thunderstorm, flood or winter storm)is occurring or is imminent. WATCH: A notice issued preceding a certain hazard (tornado, severe thunderstorm, flood or winter storm) to warn of the possibility of the particular hazard. WIND CHILL FACTOR: A non-physical value which combines the effect of air temperature and the wind speed to illustrate how it "feels."
This list of definitions was compiled by the National Weather Service Forecast Center in Albany, NY.
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Last Modified 7/20/96