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Answers archive: Hurricane science and forecasting
Q: Why do hurricanes strengthen over warm water and weaken over cooler water?
A: Hurricanes do not always strengthen over warmer water or weaken over cooler water. The atmospheric environment in which the hurricane is embedded has a profound effect on whether a hurricane weakens or strengthens. This is what makes forecasting hurricane intensity change so difficult.
However, sea water temperatures can cause changes in intensity. Cooler water puts less moisture and energy into the atmosphere, typically reducing its ability to make thunderstorms. Conversely, warmer water puts more moisture and energy into the atmosphere, making it more amenable to thunderstorm development.
Since thunderstorms are one important conduit for quickly transferring ocean energy into the atmosphere, their absence prevents strengthening and/or causes weakening.
Hence, cold water is unfavorable for hurricane development. In general, the ocean water temperature necessary to form a hurricane is around 80°F. But once a hurricane forms, it can persist in water below 80°F, sometimes for many days.
This USATODAY.com resource page shows how hurricanes develop.
(Answered by Steve Lyons, tropical weather expert at the Weather Channel, November 28, 2005)
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Q: Were the Gulf of Mexico's water temperatures unusually warm this year? If so, is that why there were so many major hurricanes?
A: The Gulf of Mexico is shallower than the open ocean, so much like a backyard kiddie pool, it heats up quickly under the blazing summer sun. While preliminary data indicate that the Gulf water temperatures were slightly above normal this year, warm water is only one factor that can strengthen storms. In the cases of Katrina and Rita, and to a lesser extent, Wilma, these hurricanes strengthened rapidly in the Gulf not only because of warm Gulf waters, but also because of very favorable atmospheric conditions, such as low wind shear and upper-level high pressure.
A NOAA Climate Diagnostics Center map shows that sea-surface temperatures across much of the Gulf of Mexico were about 1°C above average between from early August to early November, 2005.
An article on global warming and its possible relation to Hurricane Katrina is on this USA TODAY page.
(Answered by Bob Swanson, USA TODAY's assistant weather editor, November 10, 2005)
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Q: I know typhoons and hurricanes are the same type of storm, but it seems like most typhoons are usually stronger than hurricanes. Is this true?
A: Hurricanes form in limited areas of the Atlantic and northeast Pacific, but typhoons in the northwest Pacific draw on the world’s largest pool of deep, warm seawater. The journal Science reported in September that, from 1990 to 2004, 41% of northwest Pacific typhoons and southwest Pacific tropical cyclones reached Category 4 or 5 strength, while only 25% of Atlantic hurricanes did.
This USATODAY.com resource page has more about typhoons.
(Answered by meteorologist Bob Henson, a writer at the University Corporation for Atmospheric Research and the author of The Rough Guide to Weather, November 7, 2005.)
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Q: How is a hurricane's path predicted?
A: It should be recognized that, even for the experts at the National Hurricane Center, predicting a hurricane's path often comes with great uncertainty.
The forecast of a hurricane's path depends largely upon the accuracy of the predicted winds from computer forecast models. The speed and direction of these steering winds typically vary with altitude. Weak tropical cyclones tend to be steered more by lower-level winds, while upper-level winds usually influence the paths of stronger hurricanes.
So not only does an accurate path forecast depend on the accuracy of the computer models, but it is also tied into the forecast intensity of the storm as well.
Learn more about hurricane forecasting on this USA TODAY resource page.
(Answered by Bob Swanson, USA TODAY's assistant weather editor, November 6, 2005)
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Q: What is meant by the "Atlantic basin?"
A: It's a geographic term used to describe where Atlantic tropical storms and hurricanes form. The Atlantic basin includes a part of the North Atlantic Ocean, the Gulf of Mexico and the Caribbean Sea. There are seven separate basins in oceans around the world, each of which uses a different list of names chosen by the countries affected in that particular basin.
