Funnel cloud
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For the album of the same title by Hem, see Funnel Cloud (album).
A funnel cloud is a funnel-shaped cloud of condensed water droplets, associated with a rotating column of air and extending from the base of a cloud (usually a cumulonimbus or towering cumulus cloud) but not reaching the ground or a water surface. A funnel cloud is usually visible as a cone-shaped or needle like protuberance from the main cloud base. Funnel clouds frequently form in association with supercell thunderstorms.
If a funnel cloud touches the ground it becomes a tornado. Most tornadoes begin as funnel clouds, but many funnel clouds do not make ground contact and so do not become tornadoes. Also, a tornado does not necessarily need to have an associated condensation funnel—if strong cyclonic winds are occurring at the surface, then the feature is a tornado. Some tornadoes may appear only as a debris swirl, with no obvious funnel cloud extending below the rotating cloud base.
[edit] Cold-core funnel clouds
Cold-core or cold-air funnel clouds are usually short-lived and generally much weaker than the vortices produced by supercells. Although cold-core funnels rarely make ground contact, they may touch down briefly and become weak tornadoes or waterspouts.
Unlike the related phenomenon associated with severe thunderstorms, cold-core funnels are generally associated with partly cloudy skies in the wake of cold fronts, where atmospheric instability and moisture is sufficient to support towering cumulus clouds but not precipitation. The mixing of cooler air in the lower troposphere with air flowing in a different direction in the middle troposphere causes the rotation on a horizontal axis, which, when deflected vertically by atmospheric conditions, can become a funnel cloud.
They are a common sight along the Pacific Coast of the USA, particularly in the spring or autumn.[citation needed]
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