[mannykannot]

I am sure you are right about the apparent cloud movement in this case, but shockwaves can result from sudden displacement. I read somewhere that Fermi made a good estimate of the Trinity bomb yield by dribbling a stream of shredded paper from his hand. After the shockwave passed him, he paced out the displacement of those that were in the air at the time.

http://www.dannen.com/decision/fermi.html