Advanced Weather Interactive Processing System (AWIPS) - A computerized workstation that enables National Weather Service meteorologists to integrate a variety of weather data with displays of graphics and satellite and radar imagery.

advection - Horizontal transport of an atmospheric property (e.g. temperature, or moisture) solely by the wind.

advection fog - A type of fog caused by the movement of mild, humid air over a cold surface, and the consequent cooling of that air to below its initial dewpoint.

advisory - 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.

air mass - A large body of air in which the temperature, humidity, and other properties are nearly constant. 

air-mass thunderstorm - A type of thunderstorm that forms in a warm, moist, unstable air mass and is not associated with frontal or associated synoptic-scale lifting mechanisms.

aloft - A broad description of a location in the atmosphere at some height above the earth's surface.

altocumulus - Mid-altitude clouds with a cumuliform shape.

altostratus - Mid-altitude clouds with a flat, sheet like shape.

anemometer - An instrument used for measuring wind speed.

anticyclone - A large area of high pressure around which the winds blow clockwise in the Northern Hemisphere.

anvil cloud - A flat, elongated cloud formation at the top of a thunderstorm.

atmosphere - The envelope of air surrounding the earth, composed primarily of nitrogen and oxygen.

B

back-door cold front - A cold front along the eastern U.S. that moves from the northeast, rather than from the west or northwest.

barometer - An instrument that measure atmospheric pressure.

blizzard - A severe weather condition consisting of low temperatures, sustained wind speeds of 35 mph or more, and visibilities reduced to a less than a quarter mile by blowing and/or falling snow.

bow echo - An accelerated portion of a squall line of thunderstorms, taking on a bow configuration, created by strong downburst winds.

C

ceiling - The height above the Earth's surface ascribed to the lowest cloud layer, when the sky cover is reported as broken or overcast.

Celsius - Temperature scale named after Swedish astronomer Anders Celsius. The scale defines the boiling point of water as 100^°C and the freezing point of water as 0^°C at sea level pressure.

cirrocumulus - High altitude clouds with a cumuliform shape.

cirrostratus - High altitude clouds appearing as a whitish veil, and usually thin.

cirrus - High altitude clouds composed of ice crystals.

climate - The weather conditions of a region averaged over an extended period of time.

cold air damming - The phenomenon in which a low-level cold air mass is trapped topographically. Often, this cold air is entrenched on the east side of mountainous terrain...such as the Appalachian mountains. 

cold front - A boundary separating two air masses in which colder air displaces warmer air. Represented on weather maps with a blue line studded with triangles pointing in the direction of motion.

condensation - Phase transition from gas to liquid, usually referring to the method by which water vapor becomes a liquid.

contrail - A narrow white trail of ice crystal clouds produced by flying aircraft.

convection - The transport and mixing of heat by way of vertical air motions.

Coriolis force - An apparent force caused by the rotation of the earth. In the Northern Hemisphere winds are deflected to the right, and in the Southern Hemisphere to the left.

cumulonimbus - A cumulus cloud of significant vertical extent. This cloud often has an anvil shaped appearance on top. This cloud is also known as a thunderhead, and tends to produce heavy showers, accompanied at times by lightning, gusty winds and hail.

cumulus - Know as fair weather clouds, they are in the shape of individual detached domes, with a flat base and a bulging upper portion resembling cauliflower, and  less vertically-developed than a cumulonimbus cloud.

cyclogenesis - The process by which a localized low pressure forms and/or strengthens.

cyclone - Same as a low; a weather system that is characterized by  relatively low surface air pressure, with winds rotating inward and counter-clockwise in the Northern Hemisphere. 

D

dense fog advisory - Issued by the National Weather Service for widespread fog which reduces visibilities to less than 1/4 mile.

derecho - A widespread windstorm consisting of a complex of thunderstorms that develop into a long-lived squall line or a mesoscale convective complex.

dew - Liquid water condensed directly onto grass and other objects near the ground. This occurs when the temperature falls below the dewpoint temperature of the surrounding air.

dewpoint - The temperature to which air must be cooled in order for saturation to occur.

dog days - The period of greatest heat in summer; typically occurring between late July and early September.

Doppler radar - A weather radar that has the capability to determine the velocity of targets (precipitation) moving directly toward or away from the radar.

downburst - A downdraft that exits the base of a thunderstorm and spreads out at the earth's surface as strong, gusty winds, at time causing damage.

drizzle - Small, numerous, and uniformly dispersed water drops that may appear to float while following air currents. Drop diameter is 0.2 to 0.5 millimeters or 0.01 to 0.02 inches.

drought - An extended interval of abnormally dry weather sufficiently prolonged for the lack of water to cause serious hydrologic imbalance, such as crop damage, and water supply shortages, in the affected area.

dryline - A nonfrontal boundary separating warm, dry air and warm, humid air.

dust devil - A small, but vigorous whirlwind, usually of short duration, rendered visible by dust, sand, or debris picked up from the ground.

E

El Niņo - A major warming of surface ocean waters in the eastern tropical Pacific. These events occur irregularly at 2 to 7 years and may persist for as long as 2 years. They are characterized by shifts in the overall weather pattern.

excessive heat warning - Issued by the National Weather Service to warn the public that heat indices of 115° F or greater are expected for 3 hours or more.

excessive heat watch - Issued by the National Weather Service when heat indices of 115° F or greater may occur in the next few days.

eye - In meteorology, usually the "eye of the storm" i.e. hurricane. It is the roughly circular area of light winds and fair weather found in the center of a hurricane.

F

fair - Refers to a forecast of less than 4/10 opaque sky cover, no precipitation, and no extreme conditions of temperature, visibility and winds.

flash flood - A flood that rises and falls quite rapidly, usually as a result of intense rainfall over a small area, in a short amount of time, usually under 6 hours.

flash flood warning - Issued by the National Weather Service to warn that a dangerous flash flood situation is imminent or already occurring in the warned area.

flood - The condition that occurs when water overflows the natural or artificial confines of a stream, river or other body of water, or accumulates by drainage over low lying areas.

flood stage - That level on a river gauge at which overflow of the natural banks of a stream or river occurs.

flood warning - Issued by the National Weather Service to warn that rivers, creeks, or other bodies of water, will overflow their banks in the next 6 to 24 hours, resulting in a flood of the surrounding area.

flood watch - Issued by the National Weather Service when any type of flooding is expected within the next 12 to 36 hours.

fog - A visible aggregate of minute water droplets suspended in the atmosphere near the earth's surface. Basically, a cloud in contact with the ground, reducing visibility.

forecast - A definite statement or statistical estimate of expected future occurrences.

freeze - The condition that exists when, over a widespread area, the surface temperature of the air remains below freezing 32° F   (0° C) for a sufficient time, usually resulting in damage to vegetation and crops.

freeze warning - Issued by the National Weather Service when temperatures are expected to drop at or below 32° F during the growing season. Usually not issued between mid-November to mid-April.

freezing drizzle - Drizzle that falls as a liquid onto a surface that is at or below 32° F, forming a coat of glaze.

freezing fog - A fog whose droplets freeze upon contact with exposed cold objects and form a coating of rime and/or glaze.

freezing rain - Rain that falls in liquid form, then freezes upon impact with cold surfaces at or below freezing, to form a coating of glaze.

front - The transition zone between two air masses.

frost - A cover of ice crystals deposited directly upon a surface at or below freezing.

frost advisory - Issued by the National Weather Service at the end (fall) or beginning (spring) of the growing season in conjunction with forecast low temperatures of 33° F to 36° F and prime radiational cooling conditions, which results in frost formation, and damage to tender vegetation and crops.

Fujita scale - Tornado intensity scaled developed by T. Theodore Fujita of the University of Chicago. It classifies tornadoes from F0 (gale tornado with wind speeds 40-72 mph) to F5 (incredible tornado with wind speeds 261-318 mph) on the basis of rotational wind speeds estimated from the damage they caused.

funnel cloud - A rapidly rotating column of air extending below the cloud base but not reaching the ground. If it reaches the ground, then it is classified as a tornado.

G

gale - In general, an unusually strong wind. Technically, a wind whose speed is from 39 to 46 mph. This wind can break twigs from trees.

Geostationary Operational Environmental Satellite (GOES) - A satellite designed to monitor weather systems. It rotates at the same speed as the earth, remaining over the same spot above the equator.

glaze - A coating of ice, generally clear and smooth, formed on exposed surfaces by freezing rain or drizzle.

graupel - Also known as soft hail. They are snow pellets, generally 2 to 5 millimeters in diameter.

ground clutter - The pattern of radar echoes from fixed ground targets, such as buildings, mountains, or vegetation near the radar.

growing season - Generally, the portion of the year during which the temperature of cultivated vegetation remains sufficiently high to allow plant growth.

gust - A sudden brief increased in the wind speed, typically of less than a 20 second duration.

gust front - The leading edge of a mass of relatively cool, gusty air that flows out of the base of a thunderstorm.

gustnado - A relatively weak tornado associated with the thunderstorm outflow at the leading edge of a thunderstorm cell; often found along a gust front.

H

hail - Frozen precipitation in the form of balls or irregular lumps of ice.

hard freeze - A freeze in which unprotected seasonal vegetation is destroyed.

haze - Suspension in the atmosphere of extremely small, dry aerosols, which can reduce visibility.

heat advisory - Issued by the National Weather Service when heat indices are expected to range from 105 to less than 115 degrees for at least 3 hours during the day.

heat index - The apparent temperature that describes the combined effect of high air temperatures and high humidity levels.

heat lightning - The luminosity observed from ordinary lightning too far away for its thunder to be heard.

heat wave - An extended time interval of abnormally hot and usually humid weather. 

high - In meteorology, the center of an area of high pressure. Also known as an anticyclone.

high wind warning - Issued by the National Weather Service when sustained wind speeds are expected to be 40 mph or greater, or wind gusts are expected to be 58 mph or greater.

humidity - Generally, some measure of the water vapor content of air.

hurricane - A tropical cyclone, characterized by maximum sustained winds greater than 74 mph, in the North Atlantic Ocean, Caribbean Sea, Gulf of Mexico, and in the Eastern North Pacific off the west coast of Mexico to the International Dateline.

hydrology - The scientific study of the waters of the earth.

I

ice pellets - Also called sleet. Frozen precipitation consisting of transparent or translucent pellets of ice, 5 millimeters or less in diameter.