Q: What is the difference between a hurricane, a typhoon, and a cyclone?

A: All are names for intense tropical cyclones. Hurucan is what the ancient Carib people called the God of Evil. Today as then the Caribbean Sea is the spawning ground for hurricanes in the Atlantic and Eastern Pacific Oceans. They are called typhoons In the western Pacific and cyclones in the Indian Ocean, but the physics is the same. A cyclone is any mass of air that spirals around a low pressure center. The tropical cyclone is an organized collection of thunderstorms embedded in a swirling mass of air hundreds of miles across that behaves like an upside down drain to carry air to the top of the troposphere eight miles up. It is a good example of a positive feedback heat engine. As low pressure forces warm, moist surface air upwards, water vapor condenses and releases heat. This creates more uplift, causing more condensation which releases more heat and causes more uplift which causes more condensation, and so on. The cyclone will intensify as long as conditions are favorable. At the center of the spiral is the eye; the more intense the strom, the smaller the eye. The eye is a vertical tube of clear skies and calm winds a few tens of miles across within which air sinks back to the surface. Although the main upwards flow of air is around the eye, the entire air mass is unstable. Spiraling rows of thunderstorms with relatively clear areas between them extend outwards for up to two thousand miles from the center, gradually diminishing in intensity. A tropical cyclone begins as a disturbance in the steady westerly flow of the tradewinds. If the water is warm enough, if the rising column of air is not sheared by strong winds aloft or snuffed by descending air, and if it is far enough from the equator for the Coriolis effect to twist the air into a spiral, it may intensify to become a hurricane. Even the strongest hurricanes eventually die as they move over land where the supply of water vapor is limited, or drift into higher latitudes where cold air and water quickly shut down the heat engine.

Richard Brill is assistant professor of science at Honolulu Community College where he teaches earth and physical science. Send questions to him at Honolulu Community College, 874 Dillingham Blvd., Honolulu, HI 96817 or email o rickb@hcc.hawaii.edu

The Hurricane ©1995 Richard C. Brill