The updraft of humid, warm air into the atmosphere starts the cumulus stage. The air cools as it rises and condenses into one cumulus (small puffy) cloud, or cluster of cumulus clouds. At first, these clouds cannot get very tall because the air surrounding the cloud is very dry, and causes the water droplets to evaporate quickly. But, as these water droplets evaporate, they add moisture to the surrounding air, eventually allowing the rising moist air to condense at higher levels, and the cumulus cloud begins to grow. As the water vapor in the air condenses into water droplets and ice crystals, it releases a lot of heat. This heat keeps the air inside the cloud warmer than the air outside it, allowing for even further vertical growth.
As the cloud gets very tall, the water molecules in it become larger and heavier. Eventually they get too heavy to be pushed up by the rising warm air anymore, and begin the fall back down. Some of the cold, dry air in the atmosphere may also be drawn into the cloud, in a process called entrainment. Since cold air is heavier than warm air, this air begins to descend in the cloud as a downdraft, pulling the heavy water molecules with it, creating rain. Once the downdraft forms, the cloud become as cumulonimbus, and we enter into the second stage of a thunderstorm, the mature stage. The presence of the updraft and downdraft mark a cell of the thunderstorm. Most thunderstorms have more than one cell in them. During the mature stage there is rain, thunder and lightning. Also, during the mature stage the cumulonimbus cloud gets so high that it reaches the stratosphere. Strong winds in the stratosphere blow horizontally on the top of the cloud, producing the characteristic anvil.
After about 30 minutes, the thunderstorm begins to dissipate. This occurs when the updrafts weaken and the downdrafts begin to dominate. The cloud runs out of its supply of warm, moist air since there is no longer any blowing upward into it, and cloud droplets can no longer form. The cloud begins to disappear from bottom to top, and the rain becomes lighter and eventually stops. The entire process takes about an hour.
In most thunderstorms, however, the cold downdraft of air will force another updraft of warn to occur out in front of it, causing a new thunderstorm cell to form. It is typical for a thunderstorm to grow in a line this way, one cell next to the other, each in a different stage of development. For example, you could have a thunderstorm made of three cells, the left one in the dissipating stage, the middle one in the mature stage, and the right one in the cumulus stage. Thunderstorms that form in this manner are called multicell storms. Most ordinary air mass thunderstorms are multicelled, as well as most severe thunderstorms.
Cumulus Stage
Mature Stage
Dissipating Stage