The general term for cyclones originating in the Tropics or Subtropics. In successive stages of intensification, the tropical cyclone may be classified as tropical disturbance, tropical depression, tropical storm, and hurricane or typhoon. Statistics on the locations and frequency of these storms are given in G193. See also cyclone.
Tropical cyclones are classified by form and intensity as follows:
A tropical disturbance is a discrete system of apparently organized convection — generally 100 to 300 miles in diameter — having a nonfrontal migratory character, having maintained its identity for 24 hours or more. It may or may not be associated with a detectable perturbation of the wind field. It has no strong winds and no closed isobars. i.e., isobars that completely enclose the low
A tropical depression has one or more closed isobars and some rotary circulation at the surface. The highest sustained (1-minute mean) surface wind speed is 33 knots.
A tropical storm has closed isobars and a distinct rotary circulation. The highest sustained (1-minute mean) surface wind speed is 34 to 63 knots.
A hurricane or typhoon has closed isobars, a strong and very pronounced rotary circulation, and a sustained (1-minute mean) surface wind speed of 64 knots or higher. The maximum winds of a cyclone can be roughly estimated from the central pressure (Po) as V (knots) = 15 x square root(1010-Po). Questions in the topic Lows and hurricanes present further discussion."
A super typhoon is a typhoon with sustained winds over 130 knots.
Tropical cyclones occur almost entirely in six rather distinct areas, four in the Northern Hemisphere and two in the Southern Hemisphere. The name by which the tropical cyclone is commonly known varies somewhat with locality as follows (see G193):
North Atlantic: A tropical cyclone with winds of 64 knots or greater is called a hurricane.
Eastern North Pacific: The name hurricane is used as in the North Atlantic.
Western North Pacific: A fully developed storm with winds of 64 knots or greater is called a typhoon or, locally in the Philippines as a Baguio.
North Indian Ocean: A tropical cyclone with winds of 34 knots or greater is called a cyclonic storm.
South Indian Ocean: a tropical storm with winds of 34 knots or greater is called a cyclone.
Southwest Pacific and Australia area: the name cyclone is used as in the South Indian Ocean.
A severe tropical cyclone originating in the Timor sea and moving southwestward and then southeastward across the interior of northwestern Australia is called a willy-willy. Tropical cyclones have not been observed in the south Atlantic Ocean or in the South Pacific Ocean east of longitude 140° W.
See G190 for an illustration of pressure and wind speed within a tropical cyclone. G189 is a comparison of tropical and extratropical cyclones. G191 illustrates the formation of a tropical cyclone by an easterly wave in the Intertropical Convergence Zone.
G193 lists statistics by region and month for hurricane occurrence worldwide. G194 shows the characteristics of cyclone tracks in the Northern Hemisphere. South of the equator, they move west and then curve poleward toward the south.