Why does an atom bomb form a mushroom cloud?

Dear Cecil: Why do nuclear explosions form a mushroom-shaped cloud? If you would tell me why frantic and furious fusion and fission have a fondness for the fungus form, I would certainly appreciate it. Paul Smith, Tampa, Florida

Cecil replies:

Shame on you, Paul. You know I cringe at F-words. You don’t need an atom bomb to make a mushroom cloud, just convection. Mushroom clouds typically occur when an explosion produces a massive fireball. Since the fireball is very hot and thus less dense than the surrounding air, it rises rapidly, forming the cap of the mushroom cloud. In its wake the fireball leaves a column of heated air. This acts as a chimney, drawing in smoke and hot gases from ground fires. These form the stalk of the mushroom. Since the center is the hottest part of the mushroom cloud, it rises faster than the outer edges, giving the impression that the cap is curling down around the stalk. Thus the familiar fungal form.

Hydrogen bomb explosions are so huge the cloud may reach the tropopause, the boundary in the atmosphere where a fairly sharp rise in temperature starts. The cloud generally can’t break through this and the top flattens out, producing an especially pronounced mushroom shape. (The tropopause also forms a ceiling for thunderheads, producing their anvil shape.)

Mushroom clouds aren’t necessarily big. One of the Teeming Millions tells me he once set off a carbide noisemaker-type cannon with the igniter mechanism removed. Out of the hole where the igniter was supposed to go there issued a 10-inch mushroom cloud with a stem of fire and a cap of black smoke. And, we must suppose, a fabulously fierce FOOMP.

Cecil Adams

Send questions to Cecil via cecil@straightdope.com.