Tornado Life Cycle

Supercell
Tornadoes often develop from a class of thunderstorms known as supercells. Supercells contain a concentrated area of rotation low in the atmosphere, this rotation causes the development of tornadoes. Tornadoes that form from the supercell often follow the same life cycle. The supercells within a thunderstorm begin tornadoes with an area of rapidly descending air follow known as Rear Flank Downdraft (RFD). This RFD accelerates in its decent dragging the rotation in the atmosphere with it, thus creating the start of a Tornado.

Formation
As the rotation of the cyclone approaches the ground, a visible condensation funnel appears to descend from the base of the storm, often from a rotating wall cloud. As the funnel descends, the RFD also reaches the ground, creating a gust front that can cause damage a good distance from the tornado. Usually, the funnel cloud becomes a tornado within minutes of the RFD reaching the ground

Maturity
From the start the tornado has a good source of warm, moist atmosphere to power it, so that it can grow to its mature stage. This can last anywhere from a few minutes to more than an hour. During its mature stage the tornado often causes the most damage. In rare cases the Tornado can be more than one mile across. During the mature stage the RFD becomes a cool surface of winds and begins to wrap around the tornado, cutting off the warm air which feeds the it.

Dismise
As the RFD surrounds and chokes off the tornado's air supply, the vortex begins to weaken, and becomes thin. This stage often lasting no more than a few minutes, after which the tornado fizzles. The shape of the tornado is now determined by the winds of the storm above and can be blown into fantastic patterns. Even though the tornado is dieing, the tornado is still capable of causing damage. The storm is contracting into a thin tube and winds can increase. intense supercells tornadoes can develop cyclically. As the first tornado dissipate, the storm's inflow may be concentrated into a new area closer to the center of the storm. If a new cyclone develops, the cycle could start all over again, producing new tornadoes. Occasionally, the first and second cyclone produce a tornado at the same time.

Though this is a widely-accepted theory for how most tornadoes form, live, and die, it does not explain the formation of smaller tornadoes, such as land spouts, long-lived tornadoes, or tornadoes with multiple vortexes, however, most tornadoes follow a pattern similar to this one.