BARBARA KLEIN: The National Oceanic and Atmospheric Administration said Hurricane Katrina was a strong category three hurricane when it hit land in Louisiana. But researchers say other forces than its wind speed helped cause Katrina’s extensive destruction. NOAA scientists say Katrina’s air pressure was very low. The lower the air pressure, the stronger the storm. And Katrina was also an unusually wide storm.

Katrina’s most damaging power, however, came from the water it brought. The storm surge was estimated at more than six meters, and may have been as high as nine meters.

BOB DOUGHTY: All this water poured into Lake Pontchartrain on the north side of New Orleans. It also flooded into the Mississippi River to the south. New Orleans was built below sea level. The city is surrounded by levees made of earth and walls made of concrete.

The water and wind pressure from Katrina broke through the flood dams and destroyed many areas of New Orleans. The surge washed away large areas of the coastal cities of Biloxi and Gulfport, Mississippi. There was also heavy damage in Alabama.

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A man walks trough a flooded street in the Mexican town of Ciudad Anahuac in July. About 18,000 people were evacuated from the area after rains that accompanied Hurricane Alex

BARBARA KLEIN: Some scientists believe climate change affects major storms. Some say the warming of Earth’s atmosphere is already