Difference between revisions of "Hurricane"

Revision as of 00:38, September 2, 2008

Hurricane Katrina approaching New Orleans.

A tropical cyclone is a powerful, rotating storm that begins over a warm ocean. 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 when they happen over the North Atlantic Ocean, the Caribbean Sea, the Gulf of Mexico, or the Northeast Pacific Ocean. Such storms are known as typhoons if they occur in the Pacific Ocean west of an imaginary line called the International Date Line, and cyclones if originating in the Indian Ocean.

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.

Classification

Tropical cyclones are classified according to maximum sustained wind speed.^{[1][2][3][4][5]}

Hurricane conditions

Hurricanes require a special set of conditions, including 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 sea 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, lowering 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 relatively 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 farther 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 closer to the equator. For a hurricane to develop, there must be little wind shear, or in other words, 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, changed direction, or both. 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.

Bibliography

Footnotes

1. ↑ http://www.nhc.noaa.gov/aboutgloss.shtml
2. ↑ http://www.nhc.noaa.gov/aboutsshs.shtml
3. ↑ http://www.aoml.noaa.gov/hrd/tcfaq/A1.html
4. ↑ http://www.nhc.noaa.gov/2007atlan.shtml
5. ↑ http://www.aoml.noaa.gov/hrd/tcfaq/D1.html

See also

• Hurricane Katrina

External links

• National Hurricane Center