Hurricanes start as areas of low air pressure over warm ocean water. These are known as tropical depressions.

Katrina began as a tropical depression just north of the Bahamas that was formed when the remains of an earlier tropical depression and a tropical wave (a different kind of low air pressure zone moving across the ocean) merged. Tropical storm Katrina became strong enough to be a hurricane just before it passed over Florida.

Hurricanes become weaker over land, and Hurricane Katrina briefly became a tropical storm again. Then it arrived at the Gulf of Mexico where the water was very warm. The heat and humidity allowed the storm to strengthen and become an even stronger hurricane. The storm was a category 1 when it hit Florida, but by the time it reached Louisiana, it was a category 3. The storm reorganized itself at the edge of Louisiana, which used up some of its energy and made it weaker.

Katrina kept continuing north across the United States, growing gradually weaker but doing a lot of damage. It stopped being a hurricane over Mississippi, but storms related to Katrina made it as far as Canada.