Reported February 2008

HOW DOES A HURRICANE FORM? A hurricane is a type of tropical cyclone, a low-pressure system that usually forms in the tropics and has winds that circulate counterclockwise near the earth's surface. Storms are considered hurricanes when their wind speeds surpass 74 MPH. Every hurricane arises from the combination of warm water and moist warm air. Tropical thunderstorms drift out over warm ocean waters and encounter winds coming in from near the equator. Warm, moist air from the ocean surface rises rapidly, encounters cooler air, and condenses into water vapor to form storm clouds, releasing heat in the process. This heat causes the condensation process to continue, so that more and more warm moist air is drawn into the developing storm, creating a wind pattern that spirals around the relatively calm center, or eye, of the storm, much like water swirling down a drain. The winds keep circling and accelerating to form a classic cyclone pattern.

The American Meteorological Society and the American Geophysical Union contributed to the information contained in the TV portion of this report.

If you would like more information, please contact:

Dr. Kevin Trenberth
Head Climate Analysis Section, NCAR
(303) 497 1318
trenbert@ucar.edu

American Meteorological Society
Boston, MA 02108-3693
(617) 227-2425
http://www.ametsoc.org

American Geophysical Union
Washington, DC 20009-1277
1-800-966-2481
http://www.agu.org

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