Editing Mushroom cloud (section)

==Physics==
[[Image:Mushroom cloud.svg|thumb|left|Inside a rising mushroom cloud: denser air rapidly forces itself into the bottom center of the [[vortex ring|toroidal]] fireball, which turbulently mixes into the familiar cloud appearance.]]
Mushroom clouds are formed by many sorts of large explosions under Earth's gravity, but they are best known for their appearance after [[Nuclear explosion|nuclear detonations]]. Without gravity, or without a thick atmosphere, the explosive's by-product gases would remain spherical. Nuclear weapons are usually detonated above the ground (not upon impact, because some of the energy would be dissipated by the ground motions), to maximize the effect of their spherically expanding fireball and [[blast wave]]. Immediately after the detonation, the fireball 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",<ref>{{cite book|author=Batchelor, G. K.|title=An Introduction to Fluid Dynamics|publisher=Cambridge University Press|chapter=6.11, Large Gas Bubbles in Liquid|page=470|chapter-url=https://books.google.com/books?id=im8CSUpZbh0C&pg=RA1-PA470|isbn=978-0-521-66396-0|year=2000|url-status=live|archive-url=https://web.archive.org/web/20160428062144/https://books.google.com/books?id=im8CSUpZbh0C&pg=RA1-PA470|archive-date=2016-04-28}}</ref> as this gives agreement between the rate of rise and observed diameter.

[[Image:Castle Bravo Blast.jpg|thumb|15-megaton [[Castle Bravo]] explosion at Bikini Atoll, 1 March 1954, showing multiple condensation rings and several ice caps]]

As it rises, a [[Rayleigh–Taylor instability]] is formed, and air is drawn upwards and into the cloud (similar to the [[Vertical draft|updraft]] of a [[chimney]]), producing strong air currents known as "afterwinds", while, inside the head of the cloud, the hot gases rotate in a [[vortex ring|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.

Once the mass of hot gases reaches its equilibrium level, the ascent stops, and the cloud begins to flatten into the characteristic mushroom shape, often assisted by surface growth from decaying turbulence.