A tornado is a rotating column of air extending from a thunderstorm to the ground.
Tornadoes form most often from severe thunderstorms, especially supercells, when changes in wind speed and direction with height create rotating air. Rising air can tilt and tighten this rotation, producing a visible funnel when condensation or debris makes it apparent.
A tornado is classified as such when the rotating column connects the storm base with the surface. A funnel cloud that does not reach the ground is not technically a tornado.
Tornadoes occur on every continent except Antarctica, although the United States experiences more than any other country. A waterspout is a tornado over water, while a dust devil is usually a smaller, non-thunderstorm vortex.