A funnel cloud is a long, thin cloud formation hanging far below the rest of a
cloud. Funnel clouds are the precursors to
tornados, although many funnel clouds never touch the ground. It is generally believed that funnel clouds, as well as
tornados, are caused by
wind shear, layers of wind moving in different directions adjacent to each other. Eddies form in these shear areas which may be entrained in the updraft of a
thunderstorm. When this occurs, the funnel is drawn into the storm and tightens, like an ice skater pulling in his/her arms. The
wind accelerates, and the rapidly rising air creates a long, thin cloud- thus the funnel cloud formation.
Funnel clouds may be quite small and insignificant, or may reach down thousands of feet from a cloud. Some are cone shaped, others form long, eerie ropes. Often, a funnel cloud is a sign of an imminent tornado, and for this reason they are often ominous and frightening. Watching a funnel cloud dance in the sky, trying to 'decide' if it will extend to the ground and ravage its surroundings, is an uneasy experience. It should be noted that a tornado can form before the actual funnel touches the ground. (in dry areas, such as California, the funnel may never even appear). Look for blowing dust under the funnel; if you see this, you have a tornado on your hands.
If you see a funnel cloud approaching, I definitely recommend that you take cover. Retreat to your storm cellar or basement, or if these are not available, an interior room with no windows. Use blankets or mattresses to protect yourself from flying debris. However, sometimes a funnel cloud is clearly offering no threat and can be observed. I am not officially 'suggesting' this becuase I don't want anyone to get killed or injured by a tornado. If there's any doubt, take cover. But observing a funnel cloud may be a once-in-a-lifetime experience.
The only clear funnel i have seen was extending from a small storm in the Sacramento Valley of northwestern California. The funnel was backlit by the sun and was hovering, its end only a few hundred feet from the ground, swirling in a white tube. Nearby, two police officers were watching the storm, dumbfounded. This being California, funnel clouds are rather rare. Luckily the funnel never touched the ground; or if it did, it was in a remote farm field far from people or structures.
Funnel clouds usually extend from thunderstorms, especially mesocyclones. However, they can sometimes extend from other types of clouds, and have been observed in most cloud types. Funnels extending from cold, small showers (most likely including the one I saw) are called 'cold air funnels'. These are relatively common but almost never touch down, and if they do, they only cause minimal damage.
Other cloud formations, such as virga or downbursts, may be mistaken as funnel clouds. The easiest way to determine if you are viewing a funnel cloud is to check for rotation in the cloud. Also, if the cloud is much longer than it is wide, and appears smooth, or if clouds are forming below it and being drawn into it, it is probably a funnel cloud.