Formation of a thunderstorm
The development of a thunderstorm requires high humidity in the troposphere. High temperatures on the ground cause the upwards circulation of air masses within a convective thunderstorm. Warm and moist air is sucked from the basis and the sides. The condensation of the water vapour results in large cumulus clouds. In addition, it releases energy which increases the force of the updraft and accelerates the development of the thunderstorm. The cumulus cloud (cumulus congestus) rises up to 10'000 m.
In the upper troposphere
As soon as the cloud reaches the upper troposphere, where temperatures of less than -20°C occur, the water drops freeze. In those altitudes cold and dry air is sucked. As a consequence the cloud cools down and downward winds develop. The cloud develops into a cumulonimbus cloud.
So far, the convective air masses with velocities up to 40 m/s maintained the water drops at a certain level or even lifted it aloft. With the falling winds the drops are transported downwards. A shower or in extreme cases hail develops. The up- and downward movement of air masses causes electric tension which are discharged by lightning and thunder.
If there are now only downward winds, the cumulonimbus breaks up and evaporates. This is visible on the supreme part of the cloud, which looks like an ambos.Thereby the rainfall decreases.
Air mass thunderstorms are the most frequently form of appearance of thunderstorms. Thunderstorms associated with approaching frontal systems are less often and less severe.