Several conditions are required for the development of tornadoes and the thunderstorm clouds with which most tornadoes are associated. Abundant low level moisture is necessary to contibute to the development of a thunderstorm, and a "trigger" is needed to lift the moist air aloft. Once the air begins to rise and becomes saturated, it will continue rising to great heights to produce a thunderstorm cloud,m it the atmosphere is unstable. An unstable atomosphere is one where the temperature decreases rapidly with height. Atmospheric instability can also occur when dry air overlays mois air near the earth's surface. Finally, tornadoes usually form in areas where winds at all levels of the atmosphere are not only stong, but also turn with height in a clockwise or veering direction.
Tornadoes can appear as a traditional funner shape, or in a slender rope-like form. Some have a churning, smoky look to them, and other contain "multiple vorticles", which are small individual tornadoes rotating around a common center. Even others may be nearly invisible, with only swiriling dust or debris at ground levels as the only indication of the tornado's presence.