I recall accidentally making a
mushroom cloud on a small scale. Like an earlier poster, it was for a puppet show. Not having a
fog machine, I improvised by putting a little talcum powder in a small
bottle, and then feeding a short length of 1/2" dia tubing in and out of the
bottle. By blowing on it, I could make a small cloud of "
smoke". It actually was hard to not make a
mushroom cloud.
The air-cannon type of device is not ideal for forming a
mushroom cloud, because it tends to shoot out a fairly fast moving ring the size of the hole in the device. You would want something very slow moving and probably wider. So, aside from the space on the floor you would need, I don't think that is a good direction to go. I suspect that a cloth
drop, perhaps with battens to hold out the shape, or a netted
drop is the way to go. Just pile it onto the
stage floor, and lift as the cloud would lift up. Add some projected light
effect to the cloth to provide the general
effect of a swirling cloud. If it is a netted
drop, then you can back light. If
the light source is near the
drop, then any stray light going beyond the sides of the
drop may even add an
effect like of radiation streaming out from the cloud. If you don't have an effects light that will turn the
pattern, even a pair of simple linenbach projectors, cross-fading may be enough. Or a pair of ellipsoids with leaf gobos in them.
I also like the fan
effect that
gafftapegreenia mentioned. Though, you will probably need to start the fans before the actual explosion to get them up to speed soon enough.
-Joe