On August 26, 2005, Katrina changed from a tropical storm to a hurricane because it moved over very warm water in the Gulf of Mexico, which helped it get stronger. There wasn't much wind going in different directions high up in the sky, which can stop storms from growing, so Katrina could keep building up. Lastly, the air around Katrina was very moist, which is good for making storms bigger, helping it turn into a hurricane.
