They develop over tropical or subtropical waters and have definite, organized circulations. In midlatitudes, tropical cyclones take on some of the characteristics of extratropical cyclones. While extratropical cyclones are more frequent in winter, tropical cyclones occur most frequently in summer and autumn.

Classification of Tropical Cyclones

There are three recognized categories of tropical cyclones, all of which must show evidence of a closed circulation at the surface. The three classes are based on observed or estimated surface wind speeds. Their definitions are as follows:

TROPICAL DEPRESSION--A tropical cyclone whose sustained winds are less than 34 knots and whose pressure pattern exhibits one or more closed isobars on the surface

TROPICAL STORM--A tropical cyclone whose pressure pattern results in sustained surface winds of 34 to 63 knots, inclusive

HURRICANE OR TYPHOON--A tropical cyclone whose sustained surface winds exceed 63 knots

Life Cycle

Tropical cyclones are sustained by the energy released through the latent...