A condensation funnel is a funnel-shaped cloud made of condensed water droplets that forms within a rotating column of low pressure hanging below a convective cloud base. NWS Norman defines it as a funnel-shaped cloud associated with rotation and consisting of condensed water droplets, distinct from a cloud of dust, smoke, or debris.
Why it condenses
Inside a tornado-scale vortex, the pressure drops sharply toward the center. Air spiraling into that low pressure expands and cools, and if it cools past its dew point the water vapor inside condenses into visible droplets. The funnel outlines the region where the temperature has fallen below the local condensation level.
The exact depth of the funnel depends on how far the pressure has dropped and on how moist the air is at the cloud base. In dry environments, such as the High Plains in summer, a vortex can be strong at the ground with only a shallow, wispy condensation funnel or none at all.
How spotters read it
A condensation funnel that reaches the ground indicates a tornado. A funnel that does not reach the ground can be a funnel cloud, or it can be a tornado whose contact at the surface is marked by a debris cloud rather than a fully condensed column. Spotters look for rotation at cloud base and for any dust, dirt, or debris being lofted below the funnel.
The shape of the funnel gives some information about intensity, though not a strict one. A wide, laminar cone or a rope contorted into loops both indicate strong rotation. A fat, ragged column with rapid width changes suggests a mature, still-organizing tornado.
What it does not decide
Because the funnel is only the condensed part of the pressure drop, its absence does not rule out a dangerous vortex at the surface. Photos of thin ropes lofting significant damage indicators are common, as are surveys of tornadoes visible only as debris.
In practical terms, the funnel is a helpful visual but not the criterion for warning or for public action. The criterion is whether a rotating column of air is in contact with the ground.