hurricane
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hurricane
hurricane, tropical cyclone in which winds attain speeds greater than 74 mi (119 km) per hr. Wind speeds gust over 200 mi (320 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; around Australia 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./960 millibars but sometimes considerably lower, with a record 25.69 in./870 millibars registered in a 1979 NW Pacific 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 a clear sky. Temperatures in the eye are 10℉ to 15℉ (5℃–8℃) warmer than those of the surrounding air as a result of sinking currents at the hurricane's core.
Movement and Occurrence of Hurricanes
Damage Caused by Hurricanes
Flooding and high winds are the primary causes of hurricane-inflicted loss of life and property damage. The flooding results 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). A supertyphoon is equivalent to a category 4 or 5 hurricane.
Only four category-5 storms have hit the United States since record-keeping began—the 1935 Labor Day hurricane, which devastated the Florida Keys, killing 600; Hurricane Camille, in 1969, which ravaged the Mississippi coast, killing 256; Andrew, in 1992, which leveled much of Homestead, Fla.; and Michael, in 2018, which devastated parts of the Florida Panhandle. Hurricane Dorian was a category-5 storm when it devastated the N Bahamas in 2019, and other Atlantic hurricanes have been category 5 at landfall in Central America and the Caribbean or while over water. Wilma, in 2005, was the most intense Atlantic tropical cyclone, with a record low pressure of 26.055 in. (882 millibars) and winds of 185 mi (297 km) per hr when it was over the W Carribean; Hurricane Allen, in 1980, had stronger winds (190 mi/305 km per hr) in the Yucatan Channel. The 1935 Labor Day hurricane and Dorian were the strongest hurricanes to come ashore.
Tropical cyclones are deadly and devastating storms. The 1970 Bay of Bengal tropical cyclone killed some 300,000 persons, mainly by drowning, and devastated Chittagong (now in Bangladesh); some 130,000 died when a cyclone struck Myanmar along the Andaman Sea in 2008. The deadliest U.S. hurricane was the 1900 Galveston storm, which killed 8,000–12,000 people and destroyed the city. It also was one of the deadliest Atlantic tropical storms. Hurricane Katrina (2005), one of the worst natural disasters in U.S. history, was economically the most destructive U.S. storm, devastating the SW Mississippi and SE Lousiana coasts, flooding New Orleans, killing some 1,200 people, and leaving hundreds of thousands homeless. Hurricane Ida (2021) was the second-most deadly storm to hit Louisiana, tying Hurricane Laura's (2020) record 150 mph maximum winds, the strongest on record for the state. Ida went on to create considerable damage in the greater NEw York City region due to flooding. Harvey (2017) dropped more than 60 in. (150 cm) of rain in some locations in Texas, with much of the rain falling after it was no longer a hurricane; it was the second costliest U.S. hurricane. Maria (2017) was the most deadly U.S. storm since the Galveston storm, killing an estimated 2,975 people in Puerto Rico, largely in its aftermath due to the storm's devastating effects on infrastructure and the medical system, and it also was the third costliest. Sandy (2012), although technically an extratropical cyclone and no longer a hurricane when it made landfall, was the fourth most destructive storm economically, affecting New Jersey, New York, and 15 other states. Hugo (1989) in South Carolina; Opal (1995), Charley, Ivan, and two others (2004), and Irma (2017) in Florida; and Rita (2005) and Ike (2008) in Louisiana and Texas also caused billions of dollars of damage; earlier devastating storm include the Great Miami hurricane of 1926, the Lake Okeechobee hurricane of 1928, and the Great New England hurricane of 1938. Tropical storms and weak or downgraded hurricanes also can cause major flooding and damage, as did Agnes (1972), Allison (2001), Harvey, and Imelda (2019).
To decrease devastation and casualties several unsuccessful programs 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).
Hurricane
a destructive wind of long duration, with a velocity exceeding 30 m/sec; it measures 12 on the Beaufort scale. Tropical cyclones, especially those arising in the Caribbean Sea, are also called hurricanes.
REFERENCES
Nalivkin, D. V. Uragany, buri i smerchi. Moscow, 1969.Shuleikin, V. V. “Zavisimost’ mezhdu moshchnost’iu tropicheskogo uragana i temperaturoi podstilaiushchei poverkhnosti okeana.” Izv. AN SSSR: Fizika atmosfery i okeana, vol. 6, no. 12, 1970.
Shuleikin, V. V. “Razvitie i zatukhanie tropicheskogo uragana v razlichnykh teplovykh usloviiakh.” Izv. AN SSSR: Fizika atmosfery i okeana, vol. 8, no. 1, 1972.
Shuleikin, V. V. “K raschetu traektorii tropicheskikh uraganov.” Izv. AN SSSR: Fizika atmosfery i okeana, vol. 9, no. 12, 1973.