Why does a bomb explosion take the shape of a mushroom cloud?

The rise of hot gases results in the appearance of the “foot”.

Due to differences in density and temperature between the air located in the explosion zone and that higher up. Often associated with nuclear bombs, pyrocumulus clouds – those famous mushroom-shaped clouds – can actually be created by any explosion that generates enough heat, such as a volcanic eruption.

“During an explosion, the surrounding gases suddenly heat up and become less dense than the cold air further away.” explains Jean-Marie Brom, CNRS research director in nuclear physics at the University of Strasbourg. However, the least dense fluids tend to rise above the densest: “Burning gases and light elements, such as burning dust, therefore rise, generating a column of smoke continues the physicist. This is called the mushroom base. ”

Bouncing descent

The more powerful the explosion, the greater the temperature generated, and therefore the higher this column rises. But it's not over: “As they rise, the hot gases cool and become more and more dense, points out Jean-Marie Brom. When they reach the same density as the surrounding air, they can no longer rise. Pushed by the hotter gases coming from below, they therefore begin to spread out to the sides and move back down. ”

There, the gases are heated again by the “foot”, still boiling. They rise once again, before cooling, then falling again… and so on. “Due to this repetitive movement, this kind of ball characteristic of the mushroom cap appears” concludes the physicist.