Hurricane Ernesto dumped flooding rains along the east coast of the U.S., Hurricane John lashed the Pacific coast of Mexico. Typhoon Saomai killed at least 436 people in China and injured thousands when it stormed ashore a few weeks back--the strongest tropical cyclone to hit that nation in half a century--and a new typhoon, dubbed Ioke, slammed Wake Island with 50-foot waves.
Variously called hurricanes, typhoons or cyclones, depending on which ocean they form in, these storms rank among nature's fiercest. Huge, whirling tempests that form out at sea, tropical cyclones are assessed based on their strength in wind power. Those in the Atlantic are ranked on the Saffir-Simpson scale, developed in 1969 by scientists at the U.S. National Hurricane Center. The scale rates hurricanes by category [click here for the scale]. Even at their weakest, hurricanes generate winds in excess of 74 miles an hour. And stronger storms--such as last year's catastrophic Hurricane Katrina--wallop with winds greater than 131 miles an hour.
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But despite the fact that tropical cyclones can release as much energy as 10,000 nuclear bombs, they spring from the same humble beginnings as any storm: a disturbance caused by converging winds. Atlantic hurricanes--named for Huracan, an evil deity of Central America's Tainos people--typically form when a thunderstorm blows off the coast of Africa, travels out to sea and gathers power over the eastern Caribbean. The storms, however, require high humidity, light winds in the upper atmosphere and warm seas to spin up to cyclonic strength. If one of these ingredients is missing, the storm will peter out.
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These conditions prevail most strongly in the North Atlantic between August and October, but that is a relatively brief season. Conditions off the Pacific coast of Asia, in contrast, are ripe for typhoon formation between June and December. As a tropical disturbance grows in strength, surface pressure in the area around the storm falls. And the characteristic swirling around the storm's eye--actually a low-pressure vortex that compels higher pressure air downward, where it is warmed by the sea, fueling storm clouds that produce driving rain--intensifies.
Because warm seas and even warmer air in the lower atmosphere power these storms, some researchers have predicted that more and stronger tropical cyclones will result from increasing temperatures worldwide. The most recent addition to this growing body of evidence links climate change and increasing storms by statistically comparing near-surface air and Atlantic sea-surface temperatures with hurricane intensity over the last 50 years. Already, tropical cyclones are forming where they have rarely been seen before: Catarina (pictured above) formed in the South Atlantic in March 2004, the first such storm ever monitored.