Hurricane Katrina started in the Bahamas initially as an area of thunderstorms. On August 23, 2005, this was upgraded to a tropical depression, meaning that it had become more organized as a storm.
As it moved east of Nassau, the storms became more concentrated. On August 24, the classification was upgraded again, this time to a tropical storm, and it was given the name Katrina.
It was by now moving towards the coast of Florida and was due to strike land on August 25. Two hours before it hit it was upgraded to a hurricane. Over land, it weakened and was downgraded again to a tropical storm, but then it moved into the Gulf of Mexico where the water temperatures were high. This fed the storm and it was soon upgraded again to a hurricane, reaching category three hurricane status. By August 27, it had grown in both size and intensity, so was upgraded again, this time to a category five hurricane.
At this point, surface wind speeds were being measured at almost 175 miles per hour, and Hurricane Katrina was heading for Louisiana. According to a report by the Hurricane Center, when it hit land it was a category three hurricane, but the winds were still huge, and the storm surge was very high.