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All thunderstorm cells progress through three stages called the lifecycle. These stages are
(1) the cumulus stage, (2) the mature stage, and (3) the dissipating stage. The difference
between a severe and a less severe thunderstorm is related to the length of time and area
covered when it is in the mature stage. Local air mass thunderstorms have a life-cycle from
about 20 minutes to 15 hours, and are one type of thunderstorm. The other type covers
larger areas, such as squall lines in advance of fronts, and may last as long as 24 hours.

4. Cumulus Stage

Although most cumulus clouds do not become thunderstorms, the initial stage of a thunderstorm
is always a cumulus cloud. The main feature of the cumulus or "building" stage is the predominant
updraft which may extend from the earth's surface to several thousand feet above the visible
cloud top. During the early period of this stage the water drops in the cloud are very small
but they grow into raindrops as the cloud builds upward.