Tropical cyclone

tropical cyclone

The term tropical cyclone (TC) encompasses hurricanes of the Western Hemisphere and their typhoon and cyclone equivalents elsewhere. At maturity, the TC is one of the most intense and feared storms in the world; winds exceeding 90 m s⁻¹ (175 knots; kt) have been measured, and both its rains and storm surge can cause great loss of life and damage.

TCs are often classified by their intensity as follows: 1) tropical depression, with winds up to 17 m s⁻¹ (34 kt); 2) tropical storm, with winds of 18–32 m s⁻¹ (35–64 kt); and 3) hurricane or equivalent, with winds of 33 m s⁻¹ (65 kt) or higher.¹

TCs occur over several oceans and the adjacent land areas of the world: the tropical North Atlantic (including the Caribbean Sea and Gulf of Mexico), the North Pacific off the western coast of Mexico and occasionally as far west as Hawaii, the western North Pacific (including the Philippine Islands and the China Sea), the Bay of Bengal and the Arabian Sea, the southern Indian Ocean off the coasts of Madagascar and the northwestern coast of Australia, and the South Pacific Ocean from the eastern coast of Australia to about 140°W. TCs usually move initially to the west and generally slightly poleward but then "recurve" toward the east if they enter the midlatitude westerlies. Most dissipate upon moving over a large land mass or cool waters, both of which deprive the storm of its primary source of energy as described below. TCs are initiated by a large variety of disturbances, including easterly waves and monsoon troughs. Once formed, they are maintained by the extraction of heat from the ocean at high temperature and heat export at the low temperatures of the upper troposphere. Persistent organized deep convection (thunderstorms) is a defining and essential component. A well-defined eyewall of intense thunderstorms often develops to encircle the more quiescent eye of up to typically 10–100-km diameter as the system attains hurricane strength. TCs are called "warm core" because their convection produces a relatively warm structure compared to their environments.

Mature TCs are more nearly circularly symmetric than are frontal cyclones. Fully mature tropical cyclones range in diameter from 100 km to well over 1000 km.

The warm-core temperature structure leads to a TC usually generating its strongest winds just above the boundary layer. Near the surface, winds spiral inward cyclonically. They do not converge toward a point but, in a hurricane, become roughly tangent to a circle at the eyewall bounding the eye of the storm. Maximum pressure gradients and surface winds of hurricanes are usually much stronger than those of extratropical storms. Winds in the middle troposphere are more nearly cyclonically circular, with anticyclonic outflow typical at the top of the storm.

The cloud and rain patterns vary from storm to storm, but in general there are spiral bands in the outer vortex, while the most intense rain and winds occur in the eyewall. Occasionally, a second, concentric, eyewall forms around the first eyewall.

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¹ The wind speeds are based on winds averaged over a 1- or 10-min period (depending on the region and/or the weather agency).

Term edited 29 May 2020.

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