A Category One hurricane has winds of 75 to 95 miles an hour (120 to 155 kilometers an hour). Hurricane Katrina's strongest winds were blowing at about 80 miles an hour (130 kilometers an hour) when it made landfall in South Florida on August 25.
But the storm started gaining strength almost from the moment it emerged from the peninsula into the Gulf of Mexico. Residents on the Florida Keys were surprised by the power of the storm as it passed them well offshore.
Hurricane Katrina became a Category Two storm Friday, with winds of 100 miles an hour (160 kilometers an hour). Only 18 hours later, its winds had climbed to 115 miles an hour (185 kilometers an hour), and it reached Category Three.
At this point, Katrina began rapid intensificationunusual for a storm already this powerful, Kaplan said.
"Storms that strong typically don't undergo rapid intensification," Kaplan said. "Starting from 115 miles an hour, as Katrina was, it's pretty unusual for a storm to intensify that rapidly."
Kaplan said this strengthening occurred for several reasons.
Hurricanes draw their power from ocean water that's been heated to at least 80ºF (27ºC). The waters in the Gulf were about 90º:F (32ºC). Also, upper level windsknown as wind shearcan tear apart developing hurricanes. But there were no contrary winds to impede Katrina, Kaplan said.
A third factor may also have been that Katrina possibly crossed paths with an eddy of deep, warm water as it neared the coast of Louisiana, Kaplan said. Meteorologists think such unusually deep eddies sometimes develop and drift across the Gulf of Mexico, which could provide passing hurricanes with additional fuel.
In Katrina's case, such an eddy could have allowed the hurricane to continue its dramatic intensification when it might otherwise have lost strength.
During the early morning of August 28, Hurricane Katrina became a Category Four storm with winds of 145 miles an hour (235 kilometers an hour).
Only six hours later, it became a Category Five storm when its winds reached 160 miles an hour (255 kilometers an hour). And six hours after that, its strongest winds were 175 miles an hour (280 kilometers an hour), making it one of the most powerful storms to ever form in the Atlantic Ocean.
Hurricane Katrina lost some of its power just as it was about to come ashore, but still made landfall near the mouth of the Mississippi River with winds of about 140 miles an hour (225 kilometers an hour) and a storm surge of at least 20 feet (6 meters).
History Repeats Itself
Katrina's rapid intensification was similar to the Labor Day hurricane of 1935, which exploded from a tropical storm with winds of about 40 miles an hour (65 kilometers an hour) to a killer hurricane in about 30 hours. The storm's eye struck Long Key, Florida with winds of perhaps 200 miles an hour (320 kilometers an hour) and a storm surge that completely submerged parts of the island chain.
More than 400 people were killed, including about 260 World War I veterans who were working on a highway construction project. The veterans were being housed in flimsy beachfront work camps on the low-lying islands.
Hurricane Andrew struck southern Dade County, Florida, with winds exceeding 155 miles an hour (250 kilometers an hour) in August 1992. It too erupted from a tropical storm as it drew near Florida. And Hurricane Charley rapidly intensified to a Category Four storm just before striking Punta Gorda, Florida in August 2004.
Willie Drye is the author of Storm of the Century: The Labor Day Hurricane of 1935, published by National Geographic.
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