A tornado is a rapidly rotating column of air that extends from the base of a convective cloud to the ground, capable of producing damaging winds through a small area at the surface. The American Meteorological Society defines it as a rapidly rotating column of air extending from the surface to the base of a cumuliform cloud, often accompanied by near-surface circulating debris or spray.
How a tornado forms
Tornado formation requires two things to happen at once. Air near the ground has to acquire strong vertical rotation, and a persistent updraft has to stretch and concentrate that rotation into a narrow column that reaches the surface.
In supercells, the rotation source is usually the interaction of the storm's rear-flank downdraft with a horizontally rotating boundary layer near the ground. In non-supercell cases, pre-existing vertical vorticity along a surface boundary can be stretched by a growing updraft even without a mesocyclone aloft.
How it is identified in the field
A rotating condensation funnel that reaches the ground is a tornado. A rotating column that has clearly lofted dirt or debris at the surface is also a tornado, even if the funnel above it is only partial or absent. NWS spotter training makes both criteria explicit so that spotters do not withhold a report while waiting for a full condensation column.
Diameter typically ranges from tens of meters to over a mile in the strongest cases. Lifetimes span from under a minute to over an hour, and the AMS notes that the difference in wind speed across a typical tornado can exceed 40 m/s within 200 m of the surface.
What the label does not decide
Damage rating is assigned after a tornado through the Enhanced Fujita scale, based on the highest damage indicator surveyed. That rating tells you what the tornado did to a specific structure, not the maximum wind that occurred and not the ceiling of what any future tornado will do.
Tornado presence also does not require a mesocyclone or a hook echo. Landspouts and QLCS spin-ups fit the definition, and both can produce damaging winds without the classical supercell signatures on radar.
Reporting from the field
Spotters and chasers report a tornado when either condition holds: a rotating condensation funnel touching down, or a persistent debris cloud or dust whirl beneath a rotating cloud-base circulation. If you are not sure the ground contact is rotational, report it as a funnel cloud with your best description of what is happening at the surface.