A funnel cloud gets its name from its shape — it is a funnel-shaped protuberance from the base of a thunderstorm.
It is composed of water droplets and is often associated with a supercell storm. The funnel cloud often has rotation, and when it does, it’s a harbinger of possible severe weather.
A supercell thunderstorm is a large storm, sometimes 20 miles or more across, that almost always produces dangerous weather.
Supercell storms produce one or more of the following weather conditions: strong wind gusts, large hail, dangerous lightning and tornadoes. The severity of these storms is primarily a result of the structure of the environment in which the storms form. Severe weather requires warm, moist air near the ground and a change in wind speed and direction, or wind shear, with height above the surface.
Funnel clouds are not dangerous unless they reach the ground. We are interested in reported funnel clouds since it is possible that a funnel cloud can become a tornado. If the rotating funnel cloud stretches down and touches the ground, it is called a tornado.
Many tornadoes are at one time funnel clouds, but not all funnel clouds become tornadoes. When a trained weather spotter observes a funnel cloud, he reports it to the National Weather Service, who may then warn the public.
Less threatening is a cold-air funnel. These are generally observed in partly cloudy skies after the passage of a cold front. While they look threatening, they don’t pose a hazard.