ABOUT HURRICANES: 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.
RATING HURRICANES: Hurricanes are categorized by the strength of their winds according to the Saffir-Simpson Hurricane scale. They are rated from lowest wind speeds (Category 1) to highest (Category 5). But even lower-category storms can cause a great deal of damage, mostly from storm surges, when water is pushed towards the shore by strong winds and combines with normal tides to create hurricane storm tides and flooding. The worst devastation from hurricane Katrina, for example, occurred when flooding caused the New Orleans levees to fail.
The American Geophysical Union and American Meteorological Society contributed to the information contained in the TV portion of this report.