Hurricanes and typhoons are large and sometimes intensely violent storm systems. In meteorological terms, they are tropical cyclones that have maximum sustained winds of at least 120 km/h (75 mph). Atlantic and eastern Pacific storms are called hurricanes, from the West Indian huracan ("big wind"), whereas western Pacific storms are called typhoons, from the Chinese taifun, "great wind."

The primary energy source for a tropical cyclone is the latent heat released when water vapor condenses. Only extremely moist air can supply the energy necessary to spawn and maintain tropical storms, and only very warm air contains enough moisture. Tropical cyclones, therefore, form only over oceans with water temperatures of at least 27° C (80° F). After they have formed, such storms tend to intensify when passing over warmer water and weaken over colder water.

The rate of condensation heating that results from the intense rainfall associated with tropical cyclones is about 100 billion kW.

Structure of the Storm
The mature tropical cyclone is characterized by a circular pattern of stormclouds and torrential rains, whipped by winds that may reach velocities of 160 to 300 km/h (100 to 180 miles per hour) within a radius of 10 to 100 km (6 to 60 mi) from the storm center. The winds diminish rapidly with increasing distance. At a radius of 500 km (300 mi), wind speed is usually less than 30 km/h (18 mph). (The winds rotate in a counterclockwise direction in the Northern Hemisphere and in a clockwise direction in the Southern Hemisphere.) The heaviest precipitation occurs in this region of intense convection. Thunderstorms may produce rainfall rates of 250 mm (10 in) a day. The release of latent heat associated with this rain maintains low pressure and strong winds. The total cloud system of a large tropical cyclone may have a diameter of up to about 3,200 km (2,000 mi).

At the center of the storm, within a "wall" of powerful winds, there is an "eye" — a cloud-free circular region of relatively light winds that has a diameter of 10 to 100 km (6 to 60 mi). Surface pressure reaches its minimum in the eye. Typical values are 950 millibars, but values of less than 900 have been recorded. The sinking motion in the eye, which causes the clearing, also produces adiabatic warming and drying. Temperatures at 5 km (3 mi) above sea level are typically 10° C (18° F) warmer than the tropical storm's environment.

The very-high-velocity winds surrounding the eye are maintained in strength by the large differences in horizontal pressure between the eye and the outer region of the storm. Although the winds themselves are responsible for much of the storm damage, the waves and tides generated by the wind often cause most of the damage to coastal areas. Because much of human activity near the coast is concentrated within a few meters above mean sea level, storm surges can result in considerable loss of life and property.

Speed of Rotation
The rapidly whirling tangential circulation of winds in a tropical cyclone can be explained by the conservation of angular momentum. Just as ice skaters spin faster as they bring their arms down closer to the axis of rotation, so the air rotates faster as it is pulled in toward the center of the storm by the low pressure. Without friction, the wind would increase as the inverse of the distance from the center. Thus, a wind rotating at 5 km/h (3 mph) at a radius of 500 km (300 mi.) would have a velocity of 250 km/h (160 mph) if it reached a radius of only 10 km (6 mi.). Friction reduces the predicted speed, but the basic principle explains the high rotational velocities near the center.

The air that spirals toward the center and rises in the intense convection in the wall of the eye turns outward in the upper troposphere (about 15 km/10 mi. above sea level). As the air moves away from the center, its counterclockwise rotation slows, in accord with conservation of angular momentum. At a distance of about 300 km (190 miles) from the center, the air acquires an anticyclonic (clockwise) rotation.

Occurrence and Movement
Tropical cyclones move with the large-scale wind currents in which they are embedded. The typical speed is 25 km/h (16 mph), but some storms may race along at twice this speed. Others can remain stalled in the same location for several days. This is what happened in 1998 with the powerful Hurricane Mitch, which spent time hovering off the shores of Honduras and Nicaragua. Although it eventually weakened and moved northeastward, the flooding caused by its torrential rains was particularly deadly to these two countries. The full death toll may never be certain, but many thousands of people perished in the resulting floods and mudslides.

In the north Atlantic Ocean, tropical storms tend to develop primarily during the summer months of highest humidity and warmest water-surface temperatures and often appear on into October. Occasional storms develop just before or after this period but only rarely in other months. Usually about five of these tropical cyclones become strong enough to be categorized as hurricanes. Typically these storms track from east to west at low latitudes, moving with the eastern winds of the large subtropical anticyclone that dominates that ocean area. As the storms approach the North American continental landmass, however, they often begin to take a more northerly tack as they curve around the western rim of the anticyclone. (Storms that do not curve in this way enter the Gulf of Mexico or cross over Central America.) As they reach higher latitudes and come under the influence of the westerlies, they usually turn toward the northeast, often missing the continent. This turn to the northeast is called recurvature.

Typhoons of the western Pacific Ocean develop almost exclusively in a band between latitudes 6° and 35°, both north and south of the equator. Those in the Northern Hemisphere occur most frequently in the period from July to November. Once developed, such a typhoon generally tracks northwestwardly while it remains in the zone of the trade winds. Thereafter the storms most commonly recurve in a northeastward direction, generally picking up speed as they enter the wind zone of prevailing westerlies. Typhoons are observed most often in the general vicinity of the South China Sea, but devastating storms have also frequently occurred in the Bay of Bengal.

Surveillance Systems
Until 1944, when reconnaissance of tropical cyclones by aircraft became common, detection of such storms was based entirely on surface reports of land stations or ships, and some small storms probably went unnoticed. Although aircraft reconnaissance has considerably improved the ability to detect and monitor tropical cyclones, the greatest advance in early detection over the remote oceans has been the continuous surveillance maintained by the geosynchronous satellites stationed over fixed points on the equator. Hurricanes and typhoons began to be given names by the U.S. Weather Bureau in 1953.

by Richard A. Anthes

Bibliography: American Meteorological Society, Early American Hurricanes (1963; repr. 1990); Barnes, J., and Frank, N., Florida's Hurricane History (1998); Baumann, Bette, Hurricane Andrew (1992); Beatley, T., and Berke, P. R., After the Hurricane (1998); Carpenter, S. and T., The Hurricane Handbook (1993); Diaz, H., and Pulwarty, R., eds., Hurricanes (1997); Fisher, D. E., The Scariest Place on Earth: Eye to Eye with Hurricanes (1995); Kahl, J. D., Storm Warning (1993); Longshore, David, Encyclopedia of Hurricanes, Typhoons and Cyclones (1998); Pielke, Roger, The Hurricane (1990) and Hurricane Impact and Societal Responses in the USA (1997).