Hurricane - tropical cyclone in which winds attain speeds greater than 74 mi (119 km) per hr. Wind speeds reach over 190 mi (289 km) per hr in some hurricanes. The term is often restricted to those storms occurring over the N Atlantic Ocean; the identical phenomenon occurring over the W Pacific Ocean is called a typhoon; a tropical cyclone around Australia is called a willy-willy; and over the Indian Ocean, a tropical cyclone. Hurricanes have a life span of 1 to 30 days. They weaken and are transformed into extratropical cyclones after prolonged contact with the colder ocean waters of the middle latitudes, and they rapidly decay after moving over land areas.

Formation of Hurricanes

A cyclone that eventually reaches hurricane intensity first passes through two intermediate stages known as tropical depression and tropical storm. Hurricanes start over the oceans as a collection of storms in the tropics. The deepening low-pressure center takes in moist air and thermal energy from the ocean surface, convection lifts the air, and high pressure higher in the atmosphere pushes it outward. Rotation of the wind currents tends to spin the clouds into a tight curl; as the winds reach gale force, the depression becomes a tropical storm. The mature hurricane is nearly circularly symmetrical, and its influence often extends over an area 500 mi (805 km) in diameter.

As a result of the extremely low central pressure (often around 28.35 in./72 cm but sometimes considerably lower, with a record 25.91 in./65.8 cm registered in a 1958 typhoon) surface air spirals inward cyclonically (counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere), converging on a circle of about 20 mi (30 km) diameter that surrounds the hurricane's "eye." The circumference of this circle defines the so-called eye wall, where the inward-spiraling, moisture-laden air is forced aloft, causing condensation and the concomitant release of latent heat; after reaching altitudes of tens of thousands of feet above the surface, this air is finally expelled toward the storm's periphery and eventually creates the spiral bands of clouds easily identifiable in satellite photographs.

The upward velocity of the air and subsequent condensation make the eye wall the region of heaviest precipitation and highest clouds. Because the outward increase in pressure is greatest there, the eye wall is also the region of maximum wind speed. By contrast, the hurricane eye is almost calm, experiences little or no precipitation, and is often exposed to blue sky. Temperatures in the eye are 10 degrees Fahrenheit to 15 degrees Fahrenheit (5 degrees Celsius–8 degrees Celsius) warmer than those of the surrounding air as a result of sinking currents at the hurricane's core.

Movement and Occurrence of Hurricanes

Hurricanes and typhoons usually move westward at about 10 mph (16 kph) during their early stages and then curve poleward as they approach the western boundaries of the oceans at 20° to 30° lat., although more complex tracks are common. In the Northern Hemisphere, incipient hurricanes usually form over the tropical Atlantic Ocean and mature as they drift westward; hurricanes also form off the west coast of Mexico and move northeastward from that area. An average of 3.5 tropical storms per year eventually mature into hurricanes along the east coast of North America, usually over the Caribbean Sea or the Gulf of Mexico between June and November; one to three of these approach the U.S. coast annually, some changing their direction from west to northeast as they develop. Hurricanes and typhoons of the N Pacific usually develop sometime between May and December; typhoons and tropical cyclones of the Southern Hemisphere favor the period from December through April; Bay of Bengal and Arabian Sea tropical cyclones occur either between April and June or September and December, the times of the onset and retreat of the monsoon winds.

Damage Caused by Hurricanes

High winds are a primary cause of hurricane-inflicted loss of life and property damage. Another cause is the flooding resulting from the coastal storm surge of the ocean and the torrential rains, both of which accompany the storm. The Saffir-Simpson scale is the standard scale for rating the severity of a hurricane as measured by the damage it causes. It classifies hurricanes on a hierarchy from category 1 (minimal), through category 2 (moderate), category 3 (extensive), and category 4 (extreme), to category 5 (catastrophic). Only two category-5 storms have hit the United States since record-keeping began—the 1935 Labor Day hurricane, which devastated the Florida Keys, killing 600, and Hurricane Camille in 1969, which ravaged the Mississippi coast, killing 256 and causing damage estimated at $1.4 billion. Hurricane Mitch in 1998 was a category-5 storm at peak intensity over the western Caribbean, and Hurricane Gilbert in 1988 was a category-5 storm at peak intensity and is the strongest Atlantic tropical cyclone of record. The 1970 Bay of Bengal tropical cyclone killed about 300,000 persons, mainly by drowning. In the United States, Hurricanes Hugo (1989) in Charleston, S.C., and Andrew (1992) in Homestead, Fla., caused billions of dollars worth of damage. Other hurricanes can still cause major flooding and damage, even when downgraded to a tropical storm, as did Hurricane Agnes (1972). To decrease such damage several unsuccessful programs have studied ways to "defuse" hurricanes in their developing stages; more recent hurricane damage-mitigation steps have included better warning systems involving real-time satellite imagery. A hurricane watch is issued when there is a threat of hurricane conditions within 24–36 hours. A hurricane warning is issued when hurricane conditions (winds greater than 74 mph/119 kph or dangerously high water and rough seas) are expected in 24 hours or less.

Bibliography

See B. Tufty, One Thousand One Questions Answered about Hurricanes, Tornados, and Other Natural Air Disasters (1987); R. A. Pielke, The Hurricane (1990); J. Barnes, Florida's Hurricane History (1998); J. Barnes, North Carolina's Hurricane History (1998); D. Longshore, Encyclopedia of Hurricanes, Typhoons, and Cyclones (1998); E. Larson, Isaac's Storm (1999).

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