Storm Impacts | Lightning Impacts | Storm Stories | How Thunderstorms Form
How Lightning Forms | Activities | Storm Safety

How Do Thunderstorms Form?

On a hot summer day the surface of the Earth is heated by the sun. The Earth's surface heats the air just above the surface through the process of conduction.

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. In addition, it will transfer heat from the land surface to upper levels of the atmosphere through the process of convection.

Two of the most important ingredients for thunderstorm formation are instability (unstable air) and moisture.

There are three main types of thunderstorms; orographic, air mass, and frontal.

Orographic thunderstorms are caused by air that is forced up by a mountain or hillside. Air mass thunderstorms are the result of localized convection in an unstable air mass. Frontal thunderstorms occur along the boundaries of weather fronts (e.g. cold front).

 

Supercell thunderstorms occur when very strong updrafts are balanced by downdrafts. This can allow the storm to persist for many hours. In a supercell, a moist, unstable body of 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.

At some point, condensation high in the cloud (now in the form of water droplets and ice) falls to the ground as rain. A cold downdraft forms as the rain falls.