Thunderstorms are the most common forms of violent change in the weather. They usually occur in the late spring and summer in the United States. When the air is warm and humid there is a good chance that a thunderstorm will develop. Thunderstorms develop by the same process that forms cumulus clouds. The warm air rises continuously making for an unstable atmosphere which is favorable for the development of thunderstorms.
Thunderstorms develop in three stages: the cumulus stage, the mature stage, and the dissipation stage. Starting in the cumulus stage the storm advances in severity until it reaches its peak at the mature stage then begins gradually to die in the dissipation stage.
The cumulus stage
This first stage of the formation of any thunderstorm begins when the sun heats the earth’s surface during the day. The heat on the surface warms the air around it. The warm air, being lighter than cool air, rises in what is called an updraft. As it rises it expands and cools. If the air is moist, then the warm air condenses into a cumulus cloud. This continues as long as the warm air below continues to rise.
The moisture in the clouds that are formed begins to cool off creating a low pressure zone beneath the thunderstorm. The rising and cooling continues until the mature stage of the storm has been reached.
The mature stage
The cumulus cloud that has formed becomes very large and the water in it is heavy. The clouds then are transformed into cumulonimbus clouds. At the same time, cool dry air enters the cloud and since cool air is heavier than warm air, it starts to descend in the cloud.
The raindrops fall through the clouds when the rising air is no longer able to hold them up. At this point a downdraft is formed which pulls the heavy water downward. Now the cumulonimbus is a thunderstorm cell.
The dissipation stage
Due to the mixture of updrafts and downdrafts within the clouds, the thunderstorms produce thunder and lightning. This internal turbulence within the thunderstorm gives rise to strong winds and the possibility of tornadoes.
When the downdrafts become dominate over the updrafts then the thunderstorm begins to dissipate. The warm moist air can no longer rise so cloud droplets can not form. The storm dies out as the cloud disappears from bottom to top.
The process in the development of a thunderstorm from beginning to end, is usually about one hour. If a more powerful supercell thunderstorm develops, it can last for several hours.
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