A funnel cloud is a rotating condensation funnel that extends from the base of a convective cloud but is not known to be in contact with the ground. The American Meteorological Society defines it as a condensation cloud, typically funnel-shaped, associated with a rotating column of air that may or may not reach the surface.
How it forms
A funnel cloud develops when a small-scale vortex beneath a convective updraft drops pressure enough for water vapor in the inflow to condense. The vortex may originate from tilting of horizontal spin at cloud base, from stretching of pre-existing vertical vorticity above a boundary, or from a mesocyclone above the storm.
Whether the vortex reaches the surface depends on how strong the stretching is at low levels and how far the pressure drop extends downward. A funnel cloud that does not extend far below cloud base often marks a circulation that is weakening or that never fully organized.
How spotters interpret it
A spotter reports a funnel cloud when a persistent, clearly rotating condensation funnel is visible below cloud base and there is no evidence of ground contact. The moment a debris cloud, dust whirl, or clear touchdown is visible beneath the funnel, the report shifts to a tornado.
Not every dangling appendage under a storm is a funnel cloud. A scud tag or lowered inflow feature without visible rotation is not a funnel cloud. NWS spotter training emphasizes distinguishing rotation from motion; scud can move rapidly without rotating.
Where the label can mislead
Cold-air funnels sometimes form under high-based showers and cumulus in cold-core setups. They typically produce no ground effects and often go unwarned, but they are legitimate funnel clouds and warrant a report so that forecasters have data on the day's environment.
A funnel cloud over open country may still be a tornado. Without a surface signature such as debris or dust, it is hard to confirm ground contact at distance. The absence of confirmation does not mean the vortex is not reaching the ground; it means it has not been confirmed.