Thunderstorms are localized atmospheric phenomena that produce heavy rain, thunder and lightning, and sometimes hail. They are formed in cumulonimbus clouds (big and bulbous) that rise many miles into the sky. Most of the southeastern United States has over 40 days of thunderstorm activity each year, and there are about 100,000 thunderstorms across the country annually. Thunderstorms are different from typical rainstorms because of their lightning, thunder, and occasional hail.