A Tornado is a rotating column of air "in contact with the ground". The same phenomenon, "without ground-contact", is a Funnel Cloud.
All Tornadoes should be reported as soon it is safe to do so.
The most intense of all atmospheric phenomena, tornadoes come in many shapes and sizes, but are typically in the form of a visible condensation funnel, whose narrow end touches the earth and is often encircled by a cloud of debris and dust. Most tornadoes have wind speeds between 40 mph and 110 mph, are approximately 250 feet across, and travel a few miles before dissipating. The most extreme can attain wind speeds of more than 300 mph, stretch more than a mile across, and stay on the ground for dozens of miles.
Various types of tornadoes include the landspout, multiple vortex tornado, and waterspout. Waterspouts have similar characteristics to tornadoes, characterized by a spiraling funnel-shaped wind current that forms over a body of water, connecting to a large cumulus or cumulonimbus cloud. Waterspouts are generally classified as non-supercellular tornadoes that develop over bodies of water. These spiraling columns of air frequently develop in tropical areas close to the equator, and are less common at high latitudes.
Also see Enhanced Fujita Scale