The Tornado:

Tornadoes form when three very different types of air come together in a particular way. Near the ground lies a layer of warm and humid air, along with strong south winds. Colder air and strong west or southwest winds lie in the upper atmosphere. The warm, humid air near the surface is much less dense than the cold, dry air aloft. This condition, known as instability, is a necessary ingredient for tornado formation.

Instability means that if the warm, moist air can be given an initial push to move upwards, the air will keep on rising, delivering its moisture and energy to the formation of the tornado's parent thunderstorm. The change in wind speed and direction with height (wind shear) is also a necessary ingredient of tornado formation — it is linked to the eventual development of rotation from which a tornado may form.

The third ingredient is a layer of hot, dry air between the warm, moist air at low levels and the cool, dry air aloft. This hot layer acts as a cap and allows the warm air underneath to warm further, making the atmosphere even more unstable. Explosive development of the severe thunderstorms that spawn tornadoes begins to occur when a storm system high in the atmosphere moves east and begins to lift the various layers. Through this lifting process the cap is removed, thereby setting the stage for explosive thunderstorm development as strong updrafts develop. As the rising air encounters wind shear, it may cause the updraft to begin rotating - and a tornado is born.