Hurricanes need warmth to develop, which is why they usually form during the summer and fall, when the ocean waters are warm enough to provide fuel for the storm. Ocean water cools and warms slowly, so in the spring the water is not yet warm enough to support a storm; once the oceans warm during the summer, the water stays warm throughout the fall season.
Hurricanes are most common during the early summer and late fall, with 97 percent of hurricanes occurring between June 1 and November 30, the official dates for the Atlantic hurricane season. Although most tropical storms develop during this six-month period, hurricanes can and have occurred outside this time frame. Worldwide, September is the most active month, and May is the quietest, but storm activity varies depending on the region. The following list outlines the most active periods for various regions.
Atlantic Basin: Storm activity is highest from August to October, with 78 percent of the tropical storm days, 87 percent of the minor hurricane days and 96 percent of the major hurricane days originating during this period. Activity peaks around early to mid-September.
Northeast Pacific Basin: This region has a longer storm season than many, extending from late May or early June until late October or early November. Storm frequency is greatest from late August or early September.
Northwest Pacific Basin: In this region, storms occur throughout the entire year, although storms are most frequent between July and November. Activity peaks in late August or early September, and is less frequent in February and the first half of March.
North Indian Basin: Here, tropical storms are most common between April and December, with activity peaking in both May and November. Storms are most severe between April and June and between late September and early December.
The Southwest Indian and Australian/Southeast Indian basins: Storm seasons are similar for these two regions, extending from late November or early December until May. Peak activity occurs in mid-January and again in mid-February or early March.
Australian/Southwest Pacific Basin: In this region, activity begins in late October or early November to May, peaking in late February or early March.
Hurricane seasons seem to vary in intensity, occurring in cycles alternating between high storm activity and significantly quieter periods. For example, between 1944 and 1969, there was an average of 2.7 major hurricanes each year, but between 1970 and 1994, there was only an average of about 1.5 major hurricanes a year. In 1995, intensity began to increase again, with an average of 3.55 hurricanes every year between 1995 and 2003. The cause for these cycles is not known, but it is believed they may be due to long-term temperature changes in the Atlantic Ocean.