Florida residents will long remember the hurricane season of 2004. From early August to late September, six major hurricanes (category 3 or above, in which maximum wind speeds hit at least 178 kilometers per hour) formed in the North Atlantic basin. Four of them--Charley, Frances, Ivan and Jeanne--slammed into the Sunshine State. (Ivan's eye actually made landfall in Alabama, but the hurricane's winds roughed up Florida's panhandle.) Although the targeting of Florida seems to be mostly a case of bad luck--the tracks of Atlantic hurricanes depend on the chaotic vagaries of pressure highs and lows along the eastern seaboard--many researchers are convinced that overall hurricane activity in the Atlantic is on the upswing.

Since 1995 the annual number of major Atlantic hurricanes has averaged 3.8, significantly higher than the 60-year average of 2.3. In fact, the occurrence of these hurricanes seems to be oscillating on a decades-long cycle, with activity high from the late 1920s to the 1960s, low from 1970 to 1994 and then rebounding about 10 years ago. The oscillation is by no means smooth; hurricane activity in the Atlantic also swings sharply from year to year. (Overall, however, global hurricane activity is remarkably stable--busy seasons in one ocean are typically counterbalanced by calm seasons in another.)

This article was originally published with the title Stormy Weather.

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