Instability
The first meteorological component of a tornado is an unstable atmosphere. This instability is required to produce thunderstorms, also known as supercells. Atmospheric instability can be introduced in two ways. First, the most common cause of atmospheric instability is a cold front. Here, cold dry air circulates around an area of low pressure. This circulation causes it to collide with warm, moist air, triggering vertical instability. A dry line, separating warm dry air from warm moist air, can produce the same results.
Wind Shear
The second meteorological component of a tornado is wind shear. This condition occurs when strong winds rotate in direction with height. This rotation is typically clockwise; however, counter-clockwise wind shear can occasionally occur. These rotating winds create a large cylinder of air that is horizontally rotating. This rotating column of air can be likened to a baker's rolling pin.
Mesocyclone
The third meteorological component of a tornado is a mesocyclone. The atmospheric instability triggers the development of thunderstorms. During their initial development, they form a strong updraft. As air is pumped through this updraft, it begins to circulate. When this area of vertical circulation, called a mesocyclone, crosses paths with the area of horizontal wind shear, it lifts it into a vertical position as well. Like a figure skater drawing in her arms during a spin, the mesocyclone's rotation becomes much faster. Typical mesocyclones range in width from two to six miles.
Wall Cloud
The final meteorological component of a tornado is a wall cloud. As the thunderstorm matures, it develops a downdraft. This region of the storm is responsible for the precipitation. However, it also feeds back into the mesocyclone at the surface. This feeding strengthens the developing area of low pressure under the updraft. The injection of moisture-rich air produces condensation that lowers from the base of the thunderstorm. This descending wall of circulating clouds is called a wall cloud. It signifies the strengthening of the mesocyclone, and is the area in which tornadoes are produced.
Tornado
If the mesocyclone strengthens enough, the strong circulation will reach the ground. When this occurs, it is called a tornado. The tornado itself is invisible, composed only of very strong, circulating winds. However, once it makes contact with the ground, debris begins to circulate with the winds. This debris is what makes the tornado visible. Tornadoes typically occur in the southwest quadrant of a thunderstorm, but only about 30 percent of mesocyclones produce one.