How Thunderstorms Form
On a hot summer day the surface of the Earth is heated by energy from the Sun. Then, the warm Earth's surface warms air near the ground. The warm air rises. As it rises, cooler air sinks towards the ground.
The action of warm air rising and cold air sinking (convection) plays a key role in the formation of severe thunderstorms. If the warm surface air is forced to rise, it will continue to rise, because it is less dense than the surrounding air. This is called an updraft.
Supercell thunderstorms occur when very strong updrafts are balanced by downdrafts, which can allow the storm to persist for many hours. In a supercell, moist, warm air may be forced to rise by an approaching cold front. The result is a strong, persistent updraft of warm moist air. Speeds in an updraft can be as fast as 90 miles per hour! The air cools as it rises. Water vapor condenses and forms cumulus clouds. When condensation occurs, heat (latent heat/energy ) is released and helps the thunderstorm grow.