For a tornado to form, a layer of warm, moist air must be present near the surface. This air is less dense than the colder air farthest from the surface and creates a condition referred to as "instability." When combined with strong south winds, the moist air is pushed higher into the atmosphere, creating the initial thunderstorm that gives birth to the tornado.
As warm air rises higher and farther from the surface, it meets the cold, drier air above. The colder air at these altitudes moves in a different direction than the rising warm air. "Wind shear" describes the change in wind speed and direction that occurs when these types of air meet. Wind shear is one of the main contributing factors to the tornado's rotation.
A third layer composed of hot, dry air forms between the warm, humid air at the bottom and the cold, dry air at the top.This layer forms a cap over the bottom layer and enables the warmer air below to become warmer and more unstable. At this point, any pre-existing storm moving in an eastward direction can lift the layers, producing the strong updrafts that combine with wind shear to spawn a tornado.
High winds resulting from the spinning column of air have been clocked at speeds of between 65 to as high as more than 300 miles per hour. These dangerous winds produce the greatest damage, ripping homes and buildings to pieces and sending the resulting debris flying through the air at deadly velocities. People, cars and entire buildings have been found far from where they were picked up and thrown.
Hail is another weather condition associated with tornadoes. Large hailstones the size of grapefruits can accompany tornadoes. In addition to the high winds, the damage from such large hailstones can run into hundreds of millions of dollars, both to property and crops in the area.
In addition to all these weather conditions, it still takes a strong thunderstorm to create a tornado. Because of this, the lightning, thunder and other related conditions of these thunderstorms can occur. Often the resulting flash floods and lightning strikes can be as deadly as the tornadoes.
