The first stage of cloud formation leading to a tornado is the cumulus cloud. Cumulus clouds are the puffy, white clouds that appear often even in calm weather. They occur when warm, moist air rises and meets colder air, at which point the moisture condenses into clouds. There are several types of cumulus clouds, and cumulus congestus, or tower-like cumulus clouds, are the most likely to become cumulonimbus clouds.
Cumulonimbus clouds form from cumulus clouds, frequently along a cold front. They are low- to middle-level clouds, tall and dense, and they are often associated with thunderstorms and lightning. Cumulonimbus clouds have a characteristic anvil shape, with a flat top due to wind shear. They can look like a huge mushroom. Lightning can descend from the flat top; this is called anvil lightning. These anvil-shaped clouds are the largest clouds in the atmosphere.
The wind shear at the top of an anvil-shaped cumulonimbus cloud can cause a convective loop of air to start spinning within the cloud. Then the updraft causes the spinning air to tilt upwards, vertically. This vertical updraft then rotates within the cloud and a mesocyclone forms. At this point, the cloud is part of a supercell thunderstorm, with high winds and frequently a lot of rain and hail.
This cloud, with a mesocyclone within it, can become a tornado because of a phenomenon called the rear flank downdraft, or RFD. This downdraft occurs because the rain from the cloud drags the mesocyclone within the cloud down toward the ground. At this point, you can see a funnel cloud descending from the cumulonimbus cloud. When the funnel cloud touches the ground, it becomes a tornado within a few minutes. The tornado can last from a few minutes to over an hour. Eventually, the RFD cools off the warm air powering the tornado, and it diminishes in power and speed.