Global warming is believed to be more of a phenomenon

Over the past 50 years, according to scientists at the Hurricane Research Center of the National Oceanic and Atmospheric Administration, the average May-to-December season sees 10 storms grow large enough to be given names. Six of those will reach sustained winds of at least 75 mph and thus become hurricanes.

The frequency of hurricanes appears to go in cycles that last from 25 to 30 years. From 1950 to 1969, for example, hurricanes were frequent occurrences in the Atlantic Ocean, Caribbean Sea and Gulf of Mexico. Then followed 25 years of relative calm. Since 1995, however, the region has been buffeted by an average of slightly more than eight hurricanes each season, the most active period on record, although all of them don't make landfall.

The reason, scientists say, is the confluence of ingredients needed to stir up a storm. Those include warmer water temperatures in the Atlantic Ocean off the coast of Africa, low-velocity winds at high altitudes (high winds in high places tend to knock downhurricanes before they get started, a phenomenon known as wind shear); and higher humidity levels.

The warm water heats the air; the hot, moist air rises; the pressure drops; the prevailing east-to-west winds push into the area; and even Earth's rotation helps start the clouds moving counterclockwise. How big a role global warming may play in hurricanes' frequency is in question. In a letter sent two weeks ago to U.S. senators conducting a hearing on global warming, a group of 10 researchers said it is unlikely the phenomenon has any role in the increased number of hurricanes in the past few years. One reason is that global warming is believed to be more of a phenomenon near Earth's polar regions than near the equator.