Hurricanes (called typhoons in the western Pacific and tropical cyclones in the Indian Ocean) can propel themselves to an altitude of 50,000 feet (15,000 meters) or more, where the rising air finally vents itself in spiraling exhaust jets of cirrus clouds. The largest ever, the 1979 Pacific typhoon Tip, sent gale-force winds across more than 650 miles (1,000 kilometers). Even an average hurricane packs some 1.5 trillion watts of power in its winds—equivalent to about half the world's entire electrical generating capacity.
Starting this great weather engine requires surface waters of 80ºF (27ºC) or more, moist air, and little wind shear—a difference in wind speed at the surface and aloft that can tear apart a developing hurricane. But those ingredients often produce nothing more than a tropical disturbance—an unremarkable cluster of thunderstorms. "Disturbances look very similar day to day," says David Nolan of the Rosenstiel School, "and then all of a sudden you get a big burst of convection, then within six hours it becomes a depression, then it becomes a hurricane, then it's flooding my apartment." Katrina soaked Nolan's 14th-floor Miami Beach home as the storm crossed Florida on its fateful course to New Orleans and the Gulf Coast. "It would be really nice to say what you need to make a hurricane," he adds. "And we really can't do that yet."
One thing was clear in 2005: Conditions were ideal for making hurricanes. From June through November—the official Atlantic hurricane season—bulletins and warnings streamed from the National Hurricane Center in Miami. But the most telling moment of the season came on November 29, one day before its official end, when NHC director Max Mayfield and other officials gave a summary report. Even as the officials recited a sobering roll call of power and destruction, the NHC duty forecaster was charting tropical storm Epsilon, just then getting ready to spin itself into yet another hurricane.
Yet 2005 was just a continuation of the upward trend that began in 1995. Because of a tropical climate shift that brought warmer waters and reduced wind shear, the Atlantic has spawned unusual numbers of hurricanes for nine of the past eleven seasons. "We're 11 years into the cycle of high activity and landfall," NOAA meteorologist Gerry Bell says, "but I can't tell you if it will last another ten years, or thirty."