Ask the Weather Guys: Why do hurricanes form over tropical oceans?
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Ask the Weather Guys: Why do hurricanes form over tropical oceans?

Angel Marshman wades through floodwaters from Tropical Depression Imelda after trying to start his flooded car Wednesday, Sept. 18, 2019, in Galveston, Texas.

The air temperature is fairly uniform throughout a hurricane. This is the case even though the air sinks and is warmed by compression on the outer edge of the hurricane, while near its eyewall the air rises vigorously and cools by expansion.

In contrast, the organized storms experienced in fall and winter in middle latitudes, such as the Midwest, are known as mid-latitude cyclones. One of the most notable characteristics of these storms is the presence of strong temperature, humidity and wind contrasts at what are known as fronts. In fact, fronts are such an integral part of the structure of these storms that they are often referred to as frontal cyclones.

In a hurricane, even though some of the cooling of air is counteracted by the enormous amount of latent heat release that occurs in the eyewall convection as water vapor is condensed into liquid water, one might still reasonably expect the eyewall region to be cooler than the periphery of the storm. This is not the case because the low-level air that is directed toward the center of the storm is greatly modified during that journey, making constant contact with a wildly windswept and very warm ocean surface.

As a consequence of the warm sea-surface temperature (SST), the inflow air is both warmed and moistened as it flows toward the eyewall — evaporation from the ocean being enhanced by the strong winds and disturbed ocean surface. For this reason, very warm, very moist air is the optimal fuel for the sustenance of a hurricane. This explains why warm, tropical ocean basins are where hurricanes form.

"Weather Guys" Steve Ackerman and Jonathan Martin are professors in the University of Wisconsin-Madison department of atmospheric and oceanic sciences.

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