A tornado is a violently rotating column of air in contact with and extending between a cloud (often a thunderstorm cloud) and the surface of the earth. Winds in most tornadoes blow at 100 mph or less, but in the most violent, and least frequent, wind speeds can exceed 250 mph.
Tornadoes, often nicknamed "twisters," typically track along the ground for a few miles or less and are less than 100 yards wide, though some monsters can remain in contact with the earth for well over fifty miles and exceed one mile in width.
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 contribute to the development of a thunderstorm, and a "trigger" (perhaps a cold front or other low level zone of converging winds) is needed to lift the moist air aloft.
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.
Atmospheric 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" - small, individual tornadoes rotating around a common center. Even others may be nearly invisible, with only swirling dust or debris at ground level as the only indication of the tornado's presence.