In terms of world weather, a hurricane is an unusual phenomenon that depends on just the right combination of ingredients. For example, hurricanes are always formed over tropical oceans.

"You could think of it as a 'heat engine,'" says Chris Landsea, a meteorologist at the National Hurricane Center in Miami, Florida. "It takes warm, mostly tropical air, squeezes it out in thunderstorms, and a portion of that heat that is released when the rain that is produced warms the air, lowers the pressure, and the winds spin up to try to fill that low pressure. So, in general, the warmer the waters and the colder the air aloft, the more energy that the hurricane can tap into."

Mr. Landsea says that a hurricane is really a large collection of thunderstorms "that are all spinning together. And as it gets stronger they actually form a ring called the 'eye wall,' and that's where the strongest rain is and the strongest winds reside."

Meteorologists rank the strength of individual hurricanes by numerical categories, ranging from one through five. Category One hurricanes are the weakest and Category Five hurricanes are the most severe. Until 2005, only three Category Five hurricanes have ever been recorded in the continental United States.

Katrina was a Category Five hurricane when it was over the Gulf of Mexico. Rita also reached Category Five status on Thursday, and in fact surpassed it to become one of the strongest storms in U.S. history. But experts hope Rita, like Katrina, will weaken before it hits land.

Meterologist Chris Landsea says scientists have yet to prove that the increased number of powerful hurricanes that have formed in recent years has any direct connection with the phenomenon of global warming. He says Atlantic Basin hurricanes like Katrina and Rita can be expected to occur every 25 to 40 years.

"We had been in a quiet period from 1970 to the mid 1990s, but the last ten years in particular have been very active," he says. Historical records indicate that what is happening now is very similar to what happened in the 1920s to the late 1960s. But, Mr. Landsea says, "The coastline is a lot different. There are a lot of people - many, many more people - living in harm's way than what it was like in the middle part of the 20th century. So when a hurricane hits today there is not a coast in the U.S. that won't get impacted severely because of all the people living there."

Chris Landsea says that with the Gulf Coast region still literally picking up the pieces from Hurricane Katrina's strike on August 29th, the menace posed by Hurricane Rita is particularly troubling.

"And in particular the Gulf of Mexico is frightening because it's fairly shallow waters offshore," he notes. "When a hurricane hits, the hurricane winds drive that water on shore in something called a 'storm surge.' And in the worst hurricanes, you can get storm surges that are five, six, seven or eight meters, where the ocean rises up. And if you're a town or your home is only a few meters above sea level, you are at risk of being completely inundated."