Tropical cyclones are powered by heat engines -- "machines" that use heat to do work. In a hurricane, the work comes in the form of driving furious waves and winds. The hurricane sucks in warm, humid air from the lower atmosphere. The air rises and condenses, releasing water vapor and the stupendous amount of heat energy that the moisture absorbed as it evaporated from the ocean. Finally, the storm exhausts this expended air into the upper atmosphere.

Define "stupendous"? The average hurricane releases heat energy equivalent to 200 times the global production of electricity! This released heat drives hurricane winds and powers the upward convection in the storm (convection is the movement of lighter fluids over heavier fluids). The rising air creates a low pressure area near the ocean that draws in more energy-laden air, feeding the continuing storm.

Due to the Coriolis effect, the lower levels of a tropical cyclone start rotating counter-clockwise in the Northern Hemisphere, but clockwise in the Southern. 
 

Hurricane winds whirl around the bizarrely calm "eye," a circular region with little wind, no rain and often a blue sky. The placid eye is surrounded by a circular "eyewall" of furious, thunderstorm-type clouds and the fiercest winds. When Hurricane Camille shredded the U.S. Gulf Coast in 1968, winds in the eyewall reached 200 miles per hour.

Sept. 4, 2004, Freeport, Bahamas: Hurricane Frances snapped trees, tore up roofs and flooded parts of Grand Bahama Island. AP Photo/Andres Leighton

Gargantuan winds, combined with extremely low atmospheric pressure near the eye, cause a catastrophic rise in sea level called a storm surge. This destructive mound of water, topped with furious, wind-whipped waves, can hoist the surface 20 feet above average sea level, causing biblical-scale flooding along coastlines. In 1899, the Bathhurst Bay hurricane produced a record record storm surge of 13 meters!

A storm surge in New Orleans could spell disaster -- the city is well below sea level, and flooding could be catastrophic. As we write, the city has already begun to evacuate in anticipation of Hurricane Ivan.

Although storm surges are the most dangerous element of these storms, water causes another problem: All that condensing moisture eventually falls as torrential rain. Although hurricane winds slow as they move inland and become deprived of energy, rains can still be drenching. The record rainfall inundated Reunion Island in the Indian Ocean in 1966. In 12 hours, a 'cane dumped 45 inches of water!

Hurricanes come, and hurricanes go, but the overall trend is periodic lulls, followed by a series of gangbuster years, like the present.

September 11, 2004, West Melbourne, Fla.: Emergency rescue team member Tom Branco hands out water after Hurricane Frances.Photo: Jocelyn Augustino, FEMA

Even though hurricanes don't seem to be getting more intense, damage is increasing, mainly because of development and torrid population growth in prime hurricane country: which in the United States includes the Carolinas, Florida and the Gulf Coast.

The threat extends further up the Atlantic Coast. A huge storm around New York City could funnel a 30-foot storm surge toward the city, flooding auto and subway tunnels and causing fearsome destruction.