Thunderstorms begin to develop when the atmosphere becomes sufficiently unstable.
This instability can arise from daytime heating, an inflow of higher dew points in the lower atmosphere, or cooling in the upper atmosphere. There must also be sufficient moisture in the air to form the clouds and rain, and enough lifting in the atmosphere to lift the parcels of air to form the clouds. When enough lifting is present, the parcels of air rise into the atmosphere and are cooled until the air eventually reaches saturation, causing cumulus clouds to form. If there is enough instability and lifting in the atmosphere, the cumulus cloud will continue to grow vertically into a cumulonimbus cloud or thunderstorm. Over 100,000 thunderstorms occur across the United States every year.