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Correction to This Article
A Sept. 13 article on the physics of hurricane formation incorrectly said that the transformation of water from liquid to gas releases heat. That "phase change" consumes heat. The reason it consumes heat, and the implications that has for hurricanes, was correctly described in the article.
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Recipe for a Hurricane Relies on Happenstance

But it not only continues. It intensifies.

As more heat is released into the column of air, the column rises faster, pulling in still more air at the bottom of the system -- the sea surface -- with increasing speed. The faster air moves, the more rapidly it evaporates water. This is why a person getting out of a swimming pool into a breeze dries off and cools off much faster than someone getting out into still air.

This fast-moving, inwardly spiraling air picks up only about 10 percent more water vapor than it would normally carry. But it is a crucial 10 percent. It represents the extra energy that is continually being moved from ocean into air, driving the storm.


In a satellite image taken at 1:15 p.m. Friday, Hurricane Ivan churns toward Jamaica. (National Oceanic And Atmospheric Administration Via AP)

_____Hurricane Explainer_____
Why a Hurricane: Detailed information on what constitutes a hurricane.

"You really need this energy source beneath the storm. It seems small in magnitude, but it's vital," said Robert Tuleya, an adjunct professor at the Center for Coastal Physical Oceanography at Old Dominion University in Norfolk. He used to work with the National Oceanic and Atmospheric Administration.

The energy is so vital that when hurricanes go over land, they tend to fall apart, eventually if not immediately. There are numerous reasons for this, among them the increased friction created by land compared with water. A big difference, however, is simply the loss of potential evaporation that comes with landfall. In addition, when the water on land (generally, in the forest canopy or on the ground) does evaporate, it cools the underlying surface much more dramatically than the ocean cools when water is evaporated from it. That cooling, over time, quenches a hurricane's power.

Of course, a lot more than that goes into making a hurricane. Otherwise, there would be more hurricanes.

"To get the heat engine to go, it doesn't happen spontaneously. Hurricanes definitely need to be triggered by some independent disturbance that comes along in the tropics," said Kerry A. Emanuel, a professor of atmospheric science at the Massachusetts Institute of Technology in Cambridge.

Among the other necessary ingredients is the initial low-pressure system that pulls air to a particular spot of tropical ocean. The Coriolis effect, in which Earth's rotation bends a straight-line wind into an arc, helps get the cyclonic motion going. The presence of very cold air 10 miles up creates a temperature gradient that contributes to a nascent storm's intensity.

There also cannot be large differences in wind speed and direction between the lower and upper parts of the atmosphere -- "shear forces" -- or the storm will not organize.

But if all goes "right," the storm can form and for a precarious stretch lasting hours or days is able to concentrate its newfound energy and use it to recruit more.


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