A funnel cloud is a rotating column of air extending from a thunderstorm but not reaching the ground.
It forms when water vapour condenses in the low-pressure, rotating air beneath a convective storm, making part of the circulation visible as a funnel-shaped cloud. The visible condensation funnel may not show the full rotating air column, and strong winds can exist below it even when the funnel does not touch the surface.
If the rotating circulation reaches the ground, it is generally classified as a tornado; if it reaches a water surface, it is called a waterspout. A funnel cloud is therefore not simply another name for a tornado. Some funnels are narrow and short-lived, while others are associated with powerful supercell thunderstorms. Meteorologists use radar, eyewitness reports, damage surveys, and visual observations together because cloud appearance alone cannot reliably reveal the strength of the circulation.