Physics
Mushroom clouds are formed by many sorts of large explosions under earth gravity, though they are best known for their appearance after nuclear detonations. In space the explosion would be somewhat spherical. Nuclear weapons are usually detonated above the ground (not upon impact, because most of the energy would be dissipated by the ground) in order to maximize the effect of their spherical expanding fireball and the blast wave. Immediately after detonation, the fireball itself begins to rise into the air, acting on the same principle as a hot-air balloon.
One way to analyze the motion, once the hot gas has cleared the ground sufficiently, is as a 'spherical cap bubble', as this gives agreement between the rate of rise and observed diameter.
As it rises, a Rayleigh–Taylor instability is formed and air is drawn upwards and into the cloud (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 toroidal shape. When the detonation altitude is low enough, these afterwinds will draw in dirt and debris from the ground below to form the stem of the mushroom cloud.
After the mass of hot gases reaches the equilibrium level, the ascent stops and the cloud starts forming the characteristic mushroom shape. It can sometimes occur during demolitions.
Read more about this topic: Mushroom Cloud
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