We all know the standard shape of a mushroom cloud
It looks something like this:
The question is why do they look like this?
Well, as can be seen in this post, the less in-depth answer is a simple difference in density between the rapidly expanding gases and the rest of the air.
However, this is a physics blog after all, and so I'm going to get into the Rayleigh-Taylor instability. In the most understandable terms (though to be quite honest after reading up on it I still don't really understand it), one fluid accelerates into another of a different density and convection magic and PHYSICS happen and things like supernovae and mushroom clouds happen.
I don't really get fluids. Gizmodo explains it better than me. But it's pretty cool. Woo.