Why a mushroom cloud?

[Saint_Cad]

Why do nuclear (or nuculer if you prefer) bomb explosions for a mushroom shape cloud?

[Polycarp]

Any large enough explosion will produce a mushroom cloud – but A-bomb and H-bomb explosions nearly always will owing to the size.

Essentially, the shock wave and blast effect from the explosion expands in a semicircle, the above-ground area around the explosion site, but much of the force is carried upwards. When it hits the tropopause, it begins to expand outward, producing the mushroom effect.

[Garfield226]

@Wikipedia

Mushroom clouds are formed by many sorts of large explosions, though they are best known for their appearance after nuclear detonations. Nuclear weapons are usually detonated above the ground (not upon impact) in order to maximize the effect of their spherical expanding fireball. After immediate detonation, the fireball itself begins to rise into the air, acting on the same principle as a hot-air balloon. While it rises, air is drawn into it and upwards (similar to the updraft of a chimney), producing strong air currents known as “afterwinds”, while inside the head of the cloud the hot gases rotate in a toroid shape. When the detonation itself is low enough, these afterwinds will draw in dirt and debris from the ground below to form the stem of the mushroom cloud.

http://en.wikipedia.org/wiki/Mushroom_cloud

[David_Simmons]

@Polycarp

Any large enough explosion will produce a mushroom cloud – but A-bomb and H-bomb explosions nearly always will owing to the size.

Essentially, the shock wave and blast effect from the explosion expands in a semicircle, the above-ground area around the explosion site, but much of the force is carried upwards. When it hits the tropopause, it begins to expand outward, producing the mushroom effect.

The explosion doesn’t even have to be large. On a calm day even a small explosion that takes place in the air produces a mushroom shaped cloud that lasts for a while.

[MikeS]

Even better than Wikipedia is Cecil’s column on the subject.