It's Different in the Tropics

Most American meteorologists grew up in the mid-latitudes, so that's where they went to school. For many forecasters, tropical weather seems to come from another planet. It's far different from mid-latitude weather. It's within these tropics that hurricanes are born.

The tropics are the area between the Tropic of Cancer and the Tropic of Capricorn—the latitude lines 23.5 degrees north and south of the equator. Within this zone, the excess heat and moisture contribute to tremendous potential instability. Often that instability will be capped by sinking motions at altitudes near 10,000 feet. Clouds form in the heat, but they aren't allowed to reach great heights because of that subsidence, that sinking air.

Weather Words

"June—too soon;
July—stand by;
August—look out you must;
October—all over."

Hurricanes in the West, by Admiral Nares

The weather usually stays under control. But if some mechanism removes that sinking trend and replaces it with rising currents, the weather goes absolutely wild. The process is similar to shaking a soda bottle and removing the cap. Those lifting mechanisms might simply be air running into a mountain range, or zones of convergence where winds at the surface run into each other, forcing the air to rise. Among the heavy lifters are those tropical low-pressure systems. The rising motions within these systems will cause torrential downpours.

Unlike frontal cyclones, tropical cyclones don't require temperature contrasts for development. There aren't any significant contrasts in the tropics. The temperature is relatively uniform. The standard forecasting techniques that hinge on those contrasts just don't work in the tropics. The big book of forecast methods was written for fronts and their associated weather. In the tropics, fronts do not exist, often pushing mid-latitude forecasters out of the game.

The source of energy for tropical weather is found in the tremendous heat release that occurs when clouds form. Heat is required to evaporate water off the ocean surface. When that rising plume of water vapor goes back into its liquid phase during cloud formation, that absorbed heat is released. The heat has been stored and is called latent heat. A warm ocean surface is required to release enough latent heat for storm formation.


Latent heat is the heat that is absorbed or released during a substance's change of state, such as during evaporation or condensation.


In July 1861, Cherrapunji, India, received 366 inches of rain— about 10 times the normal rainfall for an entire year in most of the world. In 1972, Hurricane Agnes flooded much of the eastern United States with an estimated 28 trillion gallons of water pouring out of the sky.

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Excerpted from The Complete Idiot's Guide to Weather © 2002 by Mel Goldstein, Ph.D.. All rights reserved including the right of reproduction in whole or in part in any form. Used by arrangement with Alpha Books, a member of Penguin Group (USA) Inc.