How does a mushroom cloud work?

Answer 1

A mushroom cloud is a distinctive mushroom-shaped cloud of condensed water vapor or debris resulting from a very large explosion. They are most commonly associated with nuclear explosions, but any sufficiently large blast will produce the same sort of effect. They can be caused by powerful conventional weapons. Volcano eruptions and impact events can produce natural mushroom clouds.

Mushroom clouds form as a result of the sudden formation of a large mass of hot, low-density gases near the ground creating a Rayleigh-Taylor instability, an instability of an interface between two fluids of different densities, which occurs when the lighter fluid is pushing the heavier fluid. The mass of gas rises rapidly, resulting in turbulent vortices curling downward around its edges and drawing up a column of additional smoke and debris in the center to form its "stem". The mass of gas eventually reaches an altitude where it is no longer of lower density than the surrounding air and disperses, the debris drawn upward from the ground scattering and drifting back down.

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 dispelled into the ground) 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.

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. 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. Nuclear mushroom clouds are often also accompanied by short-lived vapor clouds, "Wilson condensation clouds," also known as vapor rings. These are created by the blast wave causing a sudden drop in the surrounding air temperatures, causing water vapor in the air to condense around the explosion cloud.

Detonations produced high above the ground do not create mushroom clouds. The heads of the clouds themselves consist of highly-radioactive particles and other fission products, and usually are dispersed by the wind, though weather patterns (especially rain) can produce problematic nuclear fallout.

Detonations below ground level or deep below the water (for instance, nuclear depth charges) also do not produce mushroom clouds, as the explosion causes the vaporization of a huge amount of earth and water in these instances. Detonations underwater but near the surface can produce mushroom clouds, however.