US: Although the Gulf of Mexico has seen two category five hurricanes this summer - with wind speeds exceeding 249 km/h (155 mph) - such extremely powerful storms are rare beasts that often fade before reaching land.
Since 1928, only 28 category five hurricanes have formed in the Atlantic Ocean and, of those, only eight have struck land, three of them in the United States.
Those three were an unnamed storm which hit the Florida Keys in 1935; Camille, which hit Mississippi in 1969; and Andrew, which struck southern Florida in 1992.
Hurricane Katrina weakened to category four, with winds of 211 km/h (131mph) or higher, before it struck the Gulf Coast last month. Rita is expected to do the same before it encounters the Texas coast tomorrow morning.
Nonetheless, the occurrence of two massive storms within one month has prompted speculation about whether it is linked to global warming or cyclical changes in ocean temperature. The bottom line is: no one knows.
One thing on which most scientists agree is that higher ocean temperatures lead to more intense storms. Hurricanes - or tropical cyclones - cannot form unless the ocean temperature is at least 26 degrees for at least 46m (150ft) below the surface. Otherwise, water does not evaporate from the ocean surface rapidly enough to sustain the huge energy required for a hurricane.
The higher the temperature, the more water vapour and heat energy released into the air, fuelling the storm.
Other factors, such as winds at certain altitudes, can interfere with the formation and stability of a storm, but water temperatures are the ultimate driving force. Currently, waters in the Gulf of Mexico average about 28 degrees, providing a powerful driving force for Rita.
Waters in the tropical Atlantic are about one degree warmer than in the 1970s, according to Kerry Emanuel of the Massachusetts Institute of Technology.
In a report published in the journal Nature at the end of July, Mr Emanuel argues the destructive power of Atlantic hurricanes has nearly doubled since then, though the average number of tropical cyclones worldwide remains at about 90 a year.
Suzana Camargo, a cyclone specialist at Columbia University, supports Mr Emanuel's argument based on her own studies of typhoons in the Pacific. When water temperatures rise, she says, the number of typhoons does not increase, but their intensity does.
Mr Emanuel argues that it is due to global warming caused by the release of carbon dioxide and other gases related to industrial activity that trap the sun's heat and raise the Earth's temperature.
However George Taylor, state climatologist of Oregon, counters that records of past hurricanes reflect a cyclical heating and cooling of ocean waters.
Mr Taylor also notes that global warming models predict increases in ocean temperatures primarily at the most northern and most southern latitudes, not in the mid-ocean regions where hurricanes are spawned. - (Los Angeles Times-Washington Post service)