TM	Weather Research Center
5104 Caroline Houston, Texas 77004 Phone: 713-529-3076 Fax: 713-528-3538 E-mail: wrc@wxresearch.org

DEFINITIONS:

ANEMOMETER: An instrument which measures wind speed.

BAROMETER: An instrument which measures air pressure.

CENTRAL PRESSURE: The minimum surface pressure at the storms center or maximum surface pressure at the center of high pressure system.

CUMULONIMBUS: A storm cloud with vertical development, often has an anvil shape.

CYCLONE: A low pressure system in which the winds move counter-clockwise around a low pressure in the northern hemisphere.

DISTURBANCE: A disruption of the normal flow, usually associated with cloudiness and precipitation.

DOLDRUMS: A nautical term for the equatorial region of light winds between the trade winds of the two hemispheres.

EASTERLY WAVE: A trough of low pressure embedded in easterly winds at the south of subtropical high pressure areas. The wave moves from east to west.

EXTRA-TROPICAL CYCLONE: An atmospheric disturbance that either originates outside of the tropics or leaves the tropics and loses its tropical storm characteristics.

EYE: The center of a hurricane, typhoon or cyclone where the winds are light and variable. The average diameter of the eye of a hurricane is 10 to 30 miles.

EYE WALL: Cloudy area around the eye of a hurricane, typhoon, or cyclone with large vertical extent and associated with heavy rain and strong winds.

FETCH: The distance which the wind blows over water in nearly a straight line to generate waves and swells.

FILLING: The increase of pressure at the center of a storm system.

FORWARD SPEED: The direction and speed of movement of a storm.

GALE: A wind greater than 34 knots.

GALE FORCE WINDS: Area of winds over 34 knots.

GUST : A sudden brief increase in wind speed. See also peak gust.

HIGH : A pressure system characterized by high pressure at the center, usually accompanied by good or fair weather. These systems spin clockwise in the northern hemisphere.

HURRICANE: A tropical counter-clockwise circulation in the Atlantic and eastern Pacific oceans with maximum winds over 63 knots (74 mph).

HURRICANE FORCE WINDS: Area of storm where sustained winds are over 63 knots (74 mph).

HURRICANE WARNING: Notice issued by National Hurricane Center in Miami. Within 24 hours or less, the coastal area may be subjected to sustained winds of 74 mph or higher.

HURRICANE WATCH: The first alert when a hurricane poses a possible threat issued by National Hurricane Center in Miami.

HYGROMETER: An instrument which measures humidity.

INTERTROPICAL CONVERGENCE ZONE: The zone near the equator along which opposing trade winds of two hemispheres meet.

ISOBAR: A line of constant pressure.

ISOTHERM: A line of constant temperature.

JET STREAM: A narrow band of strong winds usually found at elevations from 20,000 to 50,000 feet.

KNOT: A unit of wind speed equal to one nautical mile (6,080 feet) per hour.

LAND SUBSIDENCE: Slow sinking or settling of ground level.

LANDFALL: The location on the coast where the center of a hurricane passes.

LATITUDE: East-West oriented lines indicating the number of degrees away from the Equator.

LONGITUDE: North-South oriented lines indicating the number of degrees away from the Greenwich Meridian.

MAXIMUM SUSTAINED WIND: Strongest hourly averaged windspeed.

MILLIBAR: A unit of pressure equal to 100 Pascals, or 1,000 dynes per square centimeter. One inch of mercury is equal to 33.86 millibars.

PEAK GUST: Highest instantaneous wind speed recorded at a station during a specified period.

PRECIPITATION: Any or all forms of water particles, whether liquid or solid, that fall from the atmosphere and reach the ground.

RADAR: A device used in meteorology to detect and display areas of intensity of precipitation.

RECURVATURE: The poleward turning of the path of a tropical storm or hurricane.

RIDGE: a line of maximum pressure embedded in anticyclonic flow.

SPIRAL BANDS: Bands of cumulonimbus clouds spiraling in toward the center a tropical cyclone or disturbance.

SQUALL LINE: An intense line of cumulonimbus clouds which frequently precede cold fronts but sometimes are present on the outer edges of hurricanes.

STEERING: The process that occurs when the direction of movement of surface pressure systems are influenced by the circulation in the upper atmosphere.

STORM SURGE: A rise in water level along coastal oceans and adjacent bays which usually occurs with the approach of a hurricane near its right front quadrant. This is the deadliest effect of a hurricane.

THERMOMETER: An instrument which measures temperature.

TORNADO: a severe rotating windstorm of small diameter and great destructive power.

TRACK: Time history of a storm path usually determined by the position of the eye (center).

TRADE WINDS: Steady northeasterly winds blowing across the tropical oceans.

TROPICAL CYCLONE: A low pressure area originating in the tropics which has a warm central core and cyclonic rotation (counter-clockwise in the northern hemisphere).

TROPICAL DEPRESSION: A tropical cyclone with winds up to 34 knots.

TROPICAL DISTURBANCE: A tropical cyclone having slight surface circulation and one closed isobar.

TROPICAL STORM: A tropical cyclone with winds 35 to 64 knots.

TROPICS: The zone usually confined to 23 degrees north and 23 degrees south latitudes.

TROUGH: A line of minimum pressure embedded in cyclonic flow.

TYPHOON: The regional term for a tropical cyclone in the western north Pacific.

WIND FORCE: Air pressure exerted on an object due to the motion of air hitting the object.