Tropical storms and hurricanes that affect the United States form in the Atlantic Ocean, the Gulf of Mexico, and the Caribbean Sea. So far this year, there's been one tropical storm, Allison. Tropical season should pick up later this month, and especially in August and September. The increase in activity is tied to warming ocean waters.

Let's take a look, month by month, at where tropical storms have formed over the last 100 years. In June, nearly all develop in the Gulf of Mexico or the Caribbean Sea. These bodies of water are smaller than the open Atlantic and not as deep, so they warm up faster.

In July, the area of development spreads east into the central Atlantic. And in August, it expands all the way into the eastern Atlantic as those waters finally warm up enough to allow weak disturbances coming off of Africa to strengthen into full-fledged tropical storms. By September, ocean waters are as warm as they'll get, and as a result, the area where storms form is as large as it ever gets, and the number of tropical systems reaches its maximum as well.