Several conditions are required for the development of tornadoes and the thunderstorm clouds with which most tornadoes are associated. Once the air begins to rise and becomes saturated, it will continue rising to great heights and produce a thunderstorm cloud, if the atmosphere is unstable. An unstable atmosphere is one where the temperature decreases rapidly with height. Amospheric instability can also occur when dry air overlays moist air near the earth's surface. Finally, tornadoes usually form in areas where winds at all levels of the atmosphere are not only strong, but also turn with height in a clockwise, or veering, direction.
Tornadoes can appear as a traditional funnel shape, or in a slender rope-like form.
Some have a churning, smoky look to them, and others contain multiple vortices . E ven others may be nearly invisible, with only swirling dust or debris at ground level as the only indication of the tornado's presence. Some of the most violent tornadoes develop from supercell thunderstorms. These storms are the most impressive of all thunderstorms and can often produce large hail and tornadoes, though less thanhalf of all supercell thunderstorms produce tornadoes. Supercell thunderstorms, and the tornadoes they sometimes produce, are most common in the central part of the United States.