system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What creates the mushroom cloud when an atomic bomb blows up?
gpt
The air, dust, and ash caused by the explosion is *very* hot, which means it's way less dense than the cooler air around it. Like a hot air balloon, the hot mixture gets pushed up above that cooler surrounding air and rises. The atmosphere gets thinner as you go up, so even though it's cooler up there it's less dense. Eventually, the atmosphere will be less dense than the hot explosion mixture, which means the mixture can't rise anymore. It hits this "cap" of thin air and spreads out. Think of it like releasing a big bunch of balloons in a room, and when they hit the ceiling the ones on the bottom push the ones on top out of the way.

The ground is also still *really* hot from the explosion so as cool air rushes in to fill the space left by the hot air it gets heated by the ground and gets pushed up, too. As it does, it carries more ash and dust with it, creating the thin column of the "mushroom", squeezed inward by the cool air all around it.

That hot column of air keeps pushing up, forcing the hot gas mixture already up there out of the way. The mixture cools off, too, which makes it begin to sink back down a little. So hot column of stuff in the middle pushing up, which pushes the hot gasses out, and then down, to form the "cap" of the mushroom.

All explosions *kind* of do this, but they don't typically get hot enough to really show it. The layers of the atmosphere can be pretty distinct but they're still really deep, so it takes a big explosion to get the mixture of gasses hot enough to go all the way through the bottom layers and run into that really distinct line between two layers that makes the effect so pronounced.