When I was in the Air Force we were taught that those were formed in bad thunderstorms and meant extremely unstable air. You usually see them around the same time as hailstorms and tornados. They relate to the phenomenom called 'micro bursts' which are sharp down drafts which can literally drop a plane a thousand feet in a few seconds. Really scary!
[This message has been edited by keldon02 (edited December 16, 2005).]
Bad weather on the ground, that is. K is right about them being an indication of dangerous downdrafts, though since it is the downdraft that forms them they tend to be most spectacular after the downdraft activity. If you see them forming in an area that you're flying towards, it's a bad sign. If they've finished forming and are beginning to dissapate, then it is merely creepy.
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