Phil Plait at Bad Astronomy helps get an explanation to this amazing video from a while back.
The video is of a cumulonimbus cloud, a puffy white cauliflower-shaped cloud that forms when warm air rises rapidly. The camera view is between two buildings, and you can see the puffy cloud top just below the center of the frame. Keep your eye on the little wisp of cloud just above the cumulonimbus, right in the middle of the frame.
Assuming the video is real, I had a sneaky suspicion it had to do with the electric currents generated inside the cloud, the same currents that create lightning. Clouds can carry huge electric potentials — essentially, the ability to move charges around — and that stored energy can be suddenly released, creating lightning. When that happens, the electric field resets itselfs, and starts to store up energy again.
But I had no clue how that would make the cloud appear to dance like that!
Source: Bad Astronomy blog
Ask a real scientists how the world works. And gets an answer. Love it.