A tropical cyclone is a powerful, rotating storm that begins over a warm body of water. Tropical cyclones form in waters near the equator, and then they move north. The term hurricane denotes a tropical cyclone whose maximum sustained wind speed is at least 74 miles per hour. Less severe tropical cyclones are known as tropical storms or tropical depressions, depending on intensity.
Hurricane winds swirl about the eye, a calm area in the center of the storm. The band of tall, dark clouds surrounding the eye is called the eyewall. The eye is usually 10 to 40 miles in diameter and is free of rain and large clouds. In the eyewall, large changes in pressure create the hurricane's strongest winds. These winds can reach nearly 200 mph. Damaging winds may extend 250 miles from the eye.
Hurricanes are referred to by different labels, depending on where they occur. They are called hurricanes if they occur over the North Atlantic Ocean, Caribbean Sea, Gulf of Mexico, or Northeast Pacific Ocean; typhoons if they occur in the Pacific Ocean west of the International Date Line; and tropical cyclones if they occur in the Indian Ocean or southwest Pacific.
Hurricanes are most common during the summer and early fall. In the Atlantic and the Northeast Pacific, for example, August and September are the peak hurricane months. Typhoons occur throughout the year in the Northwest Pacific but are most frequent in summer. Approximately 85 hurricanes, typhoons, and tropical cyclones occur in a year throughout the world.
Names are assigned to hurricanes, typhoons, and cyclones in order to differentiate between each year's storms. However, there is no universal naming convention. Hurricanes and Atlantic tropical storms are assigned male and female names, whilst the name of a typhoon may be a number (as in Japan), or it may be assigned an Asian name, as in China and the Philippines.
Tropical cyclones are classified according to maximum sustained wind speed.
|miles per hour||meters per second|
|Tropical depression||< 39 m.p.h.||< 17 m/s|
|Tropical storm||39–73 m.p.h.||17–32 m/s||Tropical Storm Erin (2007)|
|Category 1 hurricane||74–95 m.p.h.||33–42 m/s||Hurricane Earl (1998)|
|Category 2 hurricane||96–110 m.p.h.||43–49 m/s||Hurricane Georges (1998)|
|Category 3 hurricane||111–130 m.p.h.||50–58 m/s||Hurricane Fran (1996)|
|Category 4 hurricane||131–155 m.p.h.||59–69 m/s||Hurricane Betsy (1965)|
|Category 5 hurricane||≥ 156 m.p.h.||≥ 70 m/s||Hurricane Camille (1969)|
Hurricanes require a special set of conditions, such as ample heat and moisture, that exist primarily over warm tropical oceans. For a hurricane to form, there must be a warm layer of water at the top of the body of water with a surface temperature greater than 80 degrees Fahrenheit. Warm seawater evaporates and is absorbed by the surrounding air. The warmer the ocean, the more water evaporates. The warm, moist air rises, which lowers the atmospheric pressure of the air beneath. In any area of low atmospheric pressure, the column of air that extends from the surface of the water or land to the top of the atmosphere is relatively less dense and therefore weighs less.
Since air tends to move from areas of high pressure to areas of low pressure, wind is produced. In the Northern Hemisphere, the earth's rotation causes the wind to swirl into a low-pressure area in a counterclockwise direction. This effect of the rotating earth on wind flow is called the Coriolis effect. The Coriolis effect increases in intensity further from the equator. To produce a hurricane, a low-pressure area must be more than 5 degrees of latitude north of the equator. Hurricanes seldom occur close to the equator. For a hurricane to develop, there must be little wind shear— little difference in speed and direction between winds at upper and lower elevations. Uniform winds enable the warm inner core of the storm to stay intact. The storm would break up if the winds at higher elevations increased markedly in speed and/or changed direction. The wind shear would disrupt the budding hurricane by tipping it over or by blowing the top of the storm in one direction while the bottom moved in another direction.
Hurricane Andrew, a Category 5 hurricane, smashed into the area south of Miami, Florida, in August 1992, leaving forty people dead, 100,000 homes damaged or destroyed, more than a million people left without electricity, and damages of $20–30 billion. Much of South Florida's sensitive vegetation was severely damaged. The region had not seen a storm of such awesome power in decades. Andrew destroyed complacency and erased any sense of benign ignorance toward hurricanes among South Florida residents.
Hurricane information for Earth's oceans
- Barnes, Jay. Florida's Hurricane History. (1998). 330 pp.
- Provenzo, Eugene F., and Asterie Baker Provenzo. In the Eye of Hurricane Andrew. (2002). 184 pp. online review
- Williams, John M. and Duedall, Iver W. Florida Hurricanes and Tropical Storms, 1871-2001. (2002). 176 pp. online review