Funnel cloud

Funnel clouds are directly related to tornadoes. 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. All tornadoes begin life as a funnel cloud, and many funnel clouds will not become a fully fledged tornado if they never make ground contact. 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. If a funnel cloud makes ground-level contact over water, it is generally termed a waterspout. However, it is important to note that the actual structure of a common waterspout is quite different than that of a tornado. Many waterspouts are not actually tornadic, despite their deadliness. (( disputed — see talk page) see Talk:Waterspout)

If no ground circulation is visible from your vantage point, that does not mean that one does not exist.

Cold-core funnel clouds

Cold-core or cold-air funnel clouds are usually short-lived and generally much weaker than the vortexes produced by supercells. Although cold-core funnels rarely make ground contact, they may touch down briefly and become weak tornadoes or waterspouts.

Unlike their cousins associated with the major supercell thunderstorms of the Midwest, cold-core funnels are generally associated with clear skies in the wake of cold fronts, where atmospheric instability and moisture is sufficient to support towering cumulus. The mixing of cooler air in the lower troposphere with air flowing in an opposing direction from the middle troposphere causes the rotation on a horizontal axis, which when deflected vertically by atmospheric conditions can form into a funnel cloud.

They are a common sight along the Pacific Coast of the USA, particularly in the spring or autumn.