Thunderstorms form when rapidly rising warm air brings moisture to higher parts of the atmosphere, then water vapor cools and condenses into droplets that fall to the Earth as rain. The process of rising warm air is called convection, which occurs when the atmosphere above the surface of the planet is unstable due to sudden changes in air pressure and temperature.
Thunderstorms are created in one of three basic ways. Orographic thunderstorms form when warm air rushes up the side of a mountain or hill, and the moisture continues into the atmosphere, where it cools. Air mass thunderstorms occur in localized areas of unstable air. Frontal thunderstorms are caused by the boundaries between warm air and cold fronts that destabilize the atmosphere over widespread areas.
Supercell thunderstorms require very strong, warm updrafts and quick, cool downdrafts of circulating air in lower levels of the atmosphere. Water releases heat energy into the cloud when it condenses, and this warm heat rises along updrafts again to cool. This latent heat feeds supercell thunderstorms for hours due to constant warmth in the cloud.
Eventually, thunderclouds become saturated with water and moisture falls to the ground as rain. The National Weather Service explains there are as many as 2,000 thunderstorms occurring on Earth at any given time. Worldwide, nearly 16 million thunderstorms occur annually. Roughly 100,000 thunderstorms occur in the United States every year, and 10 percent of these reach severe levels.