Hurricanes Defined

One of the most powerful natural phenomena on the face of Earth, the hurricane is a member of a broader class of phenomena called cyclones.* The term cyclone refers to any weather system that circulates in a counterclockwise direction in the Northern Hemisphere and in a clockwise direction in the Southern Hemisphere. "Tropical cyclones" typically form over ocean waters of the tropics. The tropics are the area on Earth's surface between the Tropic of Capricorn and the Tropic of Cancer, 23° 27" south and north of the equator, respectively. Extratropical cyclones, for comparison, form as a result of the temperature contrast between the colder air at higher latitudes and warmer air closer to the equator. Extratropical storms form over both the ocean and land.

Tropical cyclones have been given different names depending on their region of origin. In the western north Pacific, they are called typhoons, while in the Bay of Bengal they are referred to as severe cyclonic storms of hurricane intensity. In the Atlantic, Gulf of Mexico, Caribbean, and Pacific north of the equator and east of the international dateline they are hurricanes. Evidence of tropical cyclones has been documented in a variety of other geographic locations including Europe and North Africa at earlier geologic times (Ager, 1993). Figure 1 shows the tracks of all tropical cyclones with winds greater than 39mph for the 10-year period 1979 to 1988.

The meteorological community uses a number of terms to classify the various stages in the life cycle of tropical cyclones. The following are definitions of tropical cyclones used in the Atlantic Ocean basin (Pielke and Pielke, 1997):

*This chapter considers hurricanes as an extreme meteorological event. It first discusses the physical aspects of hurricanes, including their development and impacts on ocean and land. It then overviews societal impacts.

Figure 1 Tracks of all tropical cyclones with winds greater than 39 mph for a 10-year period (Neumann, 1993).


Tropical low Tropical disturbance

Tropical depression

Tropical storm


A surface low-pressure system in the tropical latitudes. A tropical low and an associated cluster of thunderstorms that has, at most, only a weak surface wind circulation. A tropical low with a wind circulation of sustained 1 -min surface winds of less than 34 knots (kt) [39 miles per hour (mph), 18 meters per second [m/s] circulating around the center of the low], [A knot (i.e., a nautical mile per hour) equals about 1.15 mph. A nautical mile is the length of 1 min of arc of latitude.] A tropical cyclone with maximum sustained surface winds of 34 to less than 64 kt (39 to 74 mph, 18 to 33 m/s).

A tropical cyclone with maximum sustained surface winds of 64 kt (74 mph, 33 m/s) or greater. (In the Pacific Ocean west of the international date line, hurricanes are called typhoons. They are the same phenomenon.)

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