Tropical Weather Glossary

CYCLONE: An atmospheric closed circulation rotating counter-clockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere.

EYE: The relatively calm center of the tropical cyclone that is more than one half surrounded by a wall cloud.  This clear, calm area is the result of sinking air in the center of a tropical cyclone.

EYE WALL/WALL CLOUD: An organized band of cumuliform clouds immediately surrounding the center of a tropical cyclone.  Eye wall and wall cloud are used synonymously.  The eye wall typically contains the strongest winds and most violent weather within a tropical system.

EXTRATROPICAL: A term used in advisories and tropical summaries to indicate that a cyclone has lost its “tropical” characteristics.  This usually happens once a system moves from mid latitudes to high latitudes and changes from a “warm core” to a “cold core” system.

FUJIWHARA EFFECT: An interaction where tropical cyclones within a certain distance (300–750 nautical miles, depending on the sizes of the cyclones) of each other begin to rotate about a common midpoint.  Typically, one of the cyclones becomes dominant while the other weakens.

GALE WARNING: A warning of 1-minute sustained winds in the range of 34–47 knots (39–54 mph).  The warning can be issued either for predicted winds in this range or winds that are already occurring.  It is not necessarily associated with tropical cyclones.

HIGH WIND WARNING: A high wind warning is defined as 1-minute average surface winds of 35 knots or greater (40 mph or greater) lasting for 1 hour or longer, or winds gusting to 50 knots or greater (58 mph or greater), regardless of duration, that are either expected or observed over land.

HURRICANE/TYPHOON: A warm-core tropical cyclone in which the maximum sustained surface wind is 64 knots or greater (74 mph or greater).  The term hurricane is used for Northern Hemisphere cyclones east of the International Dateline to the Greenwich Meridian.  The term typhoon is used for Pacific cyclones north of the Equator, west of the International Dateline.

HURRICANE LOCAL STATEMENT: A public release prepared by local National Weather Service offices in or near a threatened area, giving specific details for its county/parish warning area, about (1) weather conditions, (2) evacuation decisions made by local officials, and (3) other precautions necessary to protect life and property.

HURRICANE SEASON: The portion of the year having a relatively high incidence of hurricanes.  The hurricane season in the Atlantic, Caribbean, and Gulf of Mexico runs from June 1 to November 30.  The hurricane season in the Eastern Pacific basin runs from May 15 to November 30.  The hurricane season in the Central Pacific basin runs from June 1 to November 30.

HURRICANE WARNING: A warning that sustained winds 64 knots or higher (74 mph or higher) associated with a hurricane are expected in a specified coastal area in 24 hours or less.  A hurricane warning can remain in effect when dangerously high water or a combination of dangerously high water and exceptionally high waves continue, even though winds may be less than hurricane force.

HURRICANE WATCH: Issued when hurricane conditions are possible or expected in a given area with the next 36 hours.

STORM SURGE: An abnormal rise in sea level accompanying a hurricane or other intense storm, and whose height is the difference between the observed level of the sea surface and the level that would have occurred in the absence of the cyclone.  Storm surge is usually estimated by subtracting the normal or astronomic high tide from the observed storm tide.

STORM TIDE: The actual level of sea water resulting from the astronomic tide combined with the storm surge.

STORM WARNING: A warning of 1-minute sustained surface winds of 48 knots or greater (55 mph or greater), either predicted or occurring, not directly associated with tropical cyclones.

SUBTROPICAL CYCLONE: A low pressure system that develops over subtropical waters that initially has a non-tropical circulation but in which some elements of tropical cyclone cloud structure are present.  Subtropical cyclones can evolve into tropical cyclones.

SUBTROPICAL DEPRESSION: A subtropical cyclone in which the maximum sustained surface wind speed is 33 knots or less (38 mph or less).

SUBTROPICAL STORM: A subtropical cyclone in which the maximum sustained surface wind speed is 34 knots or more (39 mph or more).

TROPICAL CYCLONE: A warm-core, nonfrontal low pressure system of synoptic scale that develops over tropical or subtropical waters and having a definite organized surface circulation.

TROPICAL DEPRESSION: A tropical cyclone in which the maximum sustained surface wind speed is 33 knots or less (38 mph or less).

TROPICAL DISTURBANCE: A discrete tropical weather system of apparently organized convection—generally 100–300 nautical miles in diameter—originating in the tropics or subtropics, having a nonfrontal migratory character, and maintaining its identity for 24 hours or more.

TROPICAL STORM: A tropical cyclone in which the maximum sustained surface wind speed ranges from 34–63 knots (39–73 mph).

TROPICAL STORM WARNING: A warning for tropical storm conditions, including sustained winds within the range of 34–63 knots (39–73 mph), that are expected in a specified coastal area within 24 hours or less.

TROPICAL STORM WATCH: Tropical storm conditions are expected in a given area within the next 36 hours.  A tropical storm watch normally is not issued if the system is forecast to attain hurricane strength.