Reported September 2006

BACKGROUND: Since 1984, Dr. William Gray has been the "official" source for predicting the annual hurricane season, which was a record-breaker in 2005. At 76, he is passing the torch to 25-year-old Phil Klotzbach, a member of Gray's ongoing research project to continue improving hurricane probability forecasts.

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 condensed into water vapor to form storm clouds, releasing heat in the process. This heat causes the condensation process to continue, so 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 according to 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 show by strong winds and combines with normal tides to create hurricane storm tides -- and the resulting flooding. The worst devastation from hurricane Katrina, for example, occurred when flooding caused the New Orleans levees to fail.

WHAT IS DOPPLER RADAR? Doppler radar uses a well-known effect of light called the Doppler shift. Just as a train whistle will sound higher as it approaches a platform and then become lower in pitch as it moves away, light emitted by a moving object is perceived to increase in frequency (a blue shift) if it is moving toward the observer; if the object is moving away from us, it will be shifted toward the red end of the spectrum. Doppler radar sends out radio waves that bounce off objects in the air, such as raindrops or snow crystals, and then measures how much the frequency changes in returning radio waves to better determine wind direction and speed.

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

If you would like more information, please contact:

Phil Klotzbach
Department of Atmospheric Science
Colorado State University
Fort Collins, CO 80523
(970) 491-8605

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

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