General Weather Glossary

TERM Description
ACID RAIN Cloud or rain droplets containing pollutants, such as oxides of sulfur and nitrogen, to make them acidic.
ADVECTION The horizontal transport of air or atmospheric properties. Commonly used with temperatures, i.e., "warm air advection", or moisture, i.e., "moisture advection".
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 having similar horizontal temperature and moisture characteristics.
ALTOCUMULUS Mid-altitude clouds with a cumuliform shape.
ALTOSTRATUS Mid-altitude clouds with a flat sheet-like shape.
ANEMOMETER An instrument that measures wind speed.
ANTICYCLONE A large area of high pressure around which the winds blow clockwise in the Northern Hemisphere.
ANVIL A flat, elongated cloud formation at the top of a thunderstorm.
ATMOSPHERE The gaseous envelope surrounding the earth, composed primarily of nitrogen and oxygen.
BAROMETER An instrument for measuring atmospheric pressure.
BLOWING DUST Reduction of visibility by winds blowing across dry ground with little or no foliage.
BOW ECHO An accelerated portion of a squall line of thunderstorms, taking on bow configuration, created by strong downburst winds.
BROKEN CLOUDS Clouds which cover between 6/10 and 9/10 of the sky.
CEILING The height of the lowest layer of clouds, when the sky is broken or overcast.
CIRRUS High clouds above 18,000 feet, composed of ice crystals.
CLIMATE The historical record of average daily and seasonal weather events.
COLD FRONT The boundary between a cold air mass that is advancing and a relatively warmer airmass.
CONDENSATION The process of gas changing to liquid. The process by which water vapor changes into water droplets and clouds.
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 vertically developed cumulus cloud, often capped by an anvil shaped cloud. Also called a thunderstorm cloud, it is frequently accompanied by heavy showers, lightning, thunder, and sometimes hail or gusty winds.
CUMULUS CLOUD A cloud in the shape of individual detached domes, with a flat base and a bulging upper portion resembling cauliflower. A cloud less vertically-developed than a cumulonimbus cloud.
CUT OFF LOW An area of low pressure aloft cut off from its associated jet stream.
CYCLONE An area of low pressure around which winds blow counterclockwise in the Northern Hemisphere. Also the term used for a hurricane in the Indian Ocean and in the Western Pacific Ocean.
DEW Moisture that has condensed on objects near the ground, whose temperatures have fallen to the dew point temperature.
DEW POINT The temperature to which the air must be cooled for water vapor to condense.
DOPPLER RADAR A type of weather radar that determines whether atmospheric motion is toward or away from the radar. It uses the Doppler effect to measure the velocity of particles suspended in the atmosphere.
DOWNBURST A severe localized downdraft from a thunderstorm.
DRIZZLE Small, slowly falling water droplets, with diameters between .2 and .5 millimeters.
DRY LINE A line that separates very warm, moist air to the east from hot, dry air to the west.
DUST DEVIL A small, rapidly rotating wind that is made visible by the dust, dirt, or debris it picks up. Also called a whirlwind, it develops best on clear, dry, hot afternoons.
EL NINO A major warming of the equatorial waters in the Pacific Ocean. El Nino events usually occur every 3 to 7 years, and are characterized by shifts in "normal" weather patterns.
FLOOD STAGE The level of a river or stream at which considerable inundation of surrounding areas will occur.
FOG The visible aggregate of minute water droplets suspended in the atmosphere near the earth's surface. Essentially a cloud whose base is at the earth's surface.
FREEZING LEVEL The altitude in the atmosphere where the temperature equals 32F.
FREEZING RAIN Rain which falls as liquid then freezes upon impact, resulting in a coating of ice on exposed objects.
FRONT The transition zone between two distinct air masses. The basic frontal types are cold fronts, warm fronts, occluded fronts, and stationary fronts.
FROST The covering of ice, due to condensed water vapor, that is formed on exposed surfaces whose temperature falls below freezing.
FUNNEL CLOUD A rotating, cone-shaped column of air extending downward from the base of a thunderstorm, but not in contact with the ground. When it reaches the ground it is then called a tornado.
GEOSTATIONARY SATELLITE A satellite that rotates at the same rate as the earth, thus remaining over the same spot above the equator.
GOES Geostationary Operational Environmental Satellite.
GREENHOUSE EFFECT The warming of the atmosphere by the trapping of earth's longwave radiation being radiated to space. The gases most responsible for this effect are water vapor and carbon dioxide.
GROUND FOG Fog produced over the land by the cooling of the lower atmosphere as it comes in contact with the ground. Also known as radiation fog.
GUST A brief sudden increase in wind speed. Generally the duration is less than 20 seconds and the fluctuation greater than 10 mph.
GUST FRONT The leading edge of the downdraft from a thunderstorm.
HAIL Precipitation in the form of circular or irregular-shaped lumps of ice.
HALOS Rings or arcs that seem to encircle the sun or moon. They are caused by the refraction of light through the ice crystals in cirrus clouds.
HAZE Fine dry or wet dust or salt particles in the air that reduce visibility.
HIGH The center of an area of high pressure, accompanied by anticyclonic and outward wind flow in the northern hemisphere. Also known as an anticyclone.
HUMIDITY The amount of water vapor in the atmosphere. (See relative humidity).
HURRICANE A severe tropical cyclone with sustained wind speeds in excess of 74 mph (64 knots).
INDIAN SUMMER An unseasonably warm period near the middle of autumn, usually following a substantial period of cool weather.
INVERSION An increase in temperature with height. The reverse of the normal cooling with height in the atmosphere.
ISOBAR A line of equal barometric pressure on a weather map.
JET STREAM Strong winds concentrated within a narrow band in the atmosphere. The jet stream often "steers" surface features such as fronts and low pressure systems.
KNOT One nautical mile per hour (1.15 mph).
LAPSE RATE The change in temperature with altitude in the atmosphere.
LIGHTNING An electrical discharge from a thunderstorm.
LOW The center of an area of low pressure, accompanied by cyclonic and inward wind flow in the northern hemisphere. Also known as a cyclone.
MACROBURST Large thunderstorm downbursts with a 2.5 mile diameter or greater outflow of damaging winds lasting 5 to 20 minutes.
MEASURABLE Precipitation of 0.01" or more.
MESOCYCLONE The rotating updraft in a supercell thunderstorm.
METEOROLOGY The study of the atmosphere and atmospheric phenomena.
MICROBURST A strong localized downdraft less than 2.5 miles in diameter from a thunderstorm. Peak gusts last from 2 to 5 minutes.
MILLIBAR A unit of atmospheric pressure. 1 mb = 100 Pa (pascal). Normal surface pressure is approximately 1013 millibars.
NEXRAD NEXt Generation RADar. A NWS network of about 160 Doppler radars being installed nationwide.
NOAA WEATHER RADIO (NWR) Continuous, 24 hour-a-day VHF broadcasts of weather observations and forecasts directly from National Weather Service offices. A special tone allows certain receivers to alarm when watches or warnings are issued.
OCCLUDED FRONT A complex frontal system that occurs when a cold front overtakes a warm front. Also known as an occlusion.
OROGRAPHIC UPLIFT The vertical forcing of air by terrain features such as hills or mountains. This can create orographic clouds and/or precipitation.
OUTFLOW Air that flows outward from a thunderstorm.
OVERCAST Sky condition when 9/10 or 10/10 of the sky is covered.
OVERSHOOTING TOP A 'bubble' of cloud sticking up above the anvil of a thunderstorm, due to a vigorous updraft within the storm.
OZONE A form of oxygen containing 3 molecules, usually found in the stratosphere, and responsible for filtering out much of the sun's ultraviolet radiation.
PRECIPITATION Liquid or solid water molecules that fall from the atmosphere and reach the ground.
PRESSURE The force exerted by the interaction of the atmosphere and gravity. Also known as atmospheric pressure.
RADAR An instrument used to detect precipitation by measuring the strength of the electromagnetic signal reflected back. RADAR = RAdio Detection And Ranging.
RADIOSONDE An instrument attached to a weather balloon that transmits pressure, humidity, temperature, and winds as it ascends.
RAIN Liquid water droplets that fall from the atmosphere, having diameters greater than drizzle.
RAINBOW Optical phenomenon when light is refracted and reflected by moisture in the air into concentric arcs of color.
RELATIVE HUMIDITY The amount of water vapor in the air, compared to the amount the air could hold if it was totally saturated. (Expressed as a percentage).
RIDGE An elongated area of high pressure at the surface or aloft.
SCATTERED CLOUDS Sky condition when between 1/10 and 5/10 of the sky is covered.
SEVERE THUNDERSTORM A strong thunderstorm with wind gusts in excess of 58 mph (50 knots) and/or hail with a diameter of 3/4" or more.
SHELF CLOUD Long, wedge-shaped clouds associated with the gust front. Shelf clouds indicate the downdraft or outflow of a thunderstorm.
SHOWER Precipitation that is intermittent, in space, time, or intensity.
SLEET A type of frozen precipitation, consisting of small transparent ice pellets.
SNOW Frozen precipitation composed of ice particles in complex hexagonal patterns.
SNOW FLURRIES Light snow showers, usually of an intermittent nature with no measurable accumulation.
SQUALL LINE A non-frontal band or line of thunderstorms.
STATIONARY FRONT A transition zone between air masses, with neither advancing upon the other.
STRAIGHT LINE WINDS Thunderstorm winds most often found with the gust front. They originate from downdrafts and can cause damage which occurs in a "straight line", as opposed to tornadic wind damage which has circular characteristics.
STRATOSPHERE The layer of the atmosphere above the troposphere, where temperature increases with height.
STRATUS Flat low level clouds.
SUBSIDENCE Sinking air that is associated with warming air and little cloud formation.
SUBTROPICAL JET The branch of the jet stream that is found in the lower latitudes.
SUPERCELL A highly organized thunderstorm with a rotating updraft, known as a mesocyclone. It poses an inordinately high threat to life and property. Often produces large hail, strong winds, and tornadoes.
SUSTAINED WINDS The wind speed obtained by averaging the observed values over a one minute period.
THERMAL Small rising column of air due to surface heating.
THUNDER The sound wave produced as a lightning stroke heats the air causing it to rapidly expand.
THUNDERSTORM A storm with lightning and thunder, produced by a cumulonimbus cloud, and usually associated with gusty winds, heavy rain, and sometimes hail and tornadoes.
TORNADO A violently rotating column of air below the base of a thunderstorm, and in contact with the ground. A tornado does not require the visible presence of a condensation funnel cloud.
TRACE Precipitation amounts less than 0.01".
TRADE WINDS Persistent low-level tropical winds that blow from the subtropical high pressure centers towards the equatorial low.
TROPICAL DEPRESSION Tropical mass of thunderstorms with a cyclonic wind circulation and winds between 20 and 34 knots.
TROPICAL DISTURBANCE An organized mass of tropical thunderstorms, with a slight cyclonic circulation and winds less than 20 knots.
TROPICAL STORM An organized cyclone in the tropics with wind speed between 35 and 64 knots.
TROPOSPHERE The lowest layer of the atmosphere where the temperature decreases with height. Most of earth's weather occurs in this layer.
TROUGH An elongated area of low pressure at the surface or aloft.
TURBULENCE Disrupted flow in the atmosphere that produces gusts and eddies.
VIRGA Precipitation falling from the base of a cloud and evaporating before it reaches the ground.
VISIBILITY The horizontal distance an observer can see and identify a prominent object.
VORTICITY A measure of the amount of "spin" (rotation) and "shear" in the atmosphere.
WALL CLOUD An isolated lowering of a cloud that is attached to the rain-free base of a thunderstorm, generally to the rear of the visible precipitation area. Wall clouds indicate the updraft of or the inflow to a thunderstorm.
WARM FRONT A boundary between a warm air mass that is replacing a cooler air mass.
WARNING Issued when a particular hazard is "imminent" or already occurring (e.g., tornado warning, flash flood warning).
WATCH Forecast issued in advance to alert the public of the possibility of a particular hazard (e.g., tornado watch, flash flood watch).
WATERSPOUT A column of rotating air over a body of water (i.e., a tornado over the water).
WIND SHEAR The change of wind speed or direction with distance or height.
WIND VANE An instrument that determines the direction from which a wind is blowing. is the U.S. government's official web portal to all federal, state and local government web resources and services.