Funnel cloud
From Biocrawler, the free encyclopedia.
By definition, a funnel cloud is any rotating column of air extending from the base of a cloud (usually a cumulonimbus or towering cumulus cloud) but not reaching the ground. A funnel cloud is usually visible as a cone-shaped or needlelike protuberance from the main cloud base.
Funnel clouds frequently form in association with supercell thunderstorms, and these funnel clouds are a threat to become tornadoes if their circulation reaches the ground. Sometimes the visible funnel cloud may not reach all the way to the ground, but if a debris swirl is present on the ground underneath the funnel, a tornado exists. Conversely, some tornadoes may appear only as a debris swirl, with no obvious funnel extending below the rotating cloud base.
It is important to remember that even if a ground circulation is not visible from your vantage point, that does not mean that one does not exist.
Other funnel clouds, such as cold air funnels [1] (http://www.theweatherprediction.com/habyhints/329/), are associated with weaker convective clouds; these rarely become tornadoes-although it can happen.
