Editing Mushroom cloud (section)

=== Description ===
At the moment of a nuclear explosion, a fireball is formed. The ascending, roughly spherical mass of hot, [[Incandescence|incandescent]] gases changes shape due to atmospheric friction, and the surface of the fireball is cooled by energy radiation, turning from a sphere to a violently rotating spheroidal vortex. A Rayleigh–Taylor instability is formed as the cool air underneath initially pushes the bottom fireball gases into an inverted cup shape. This causes turbulence and a vortex that sucks more air into the center, creating external afterwinds and further cooling the fireball. The speed of rotation slows as the fireball cools and may stop entirely during later phases. The vaporized parts of the weapon and [[Ionized-air glow|ionized air]] cool into visible gases, forming a cloud; the [[black-body radiation|white-hot]] vortex core becomes yellow, then dark red, then loses visible incandescence. With further cooling, the bulk of the cloud fills in as atmospheric moisture condenses. As the cloud ascends and cools, its [[buoyancy]] lessens, and its ascent slows. If the size of the fireball is comparable to the atmospheric density [[scale height]], the whole cloud rise will be [[wikt:ballistic|ballistic]], overshooting a large volume of overdense air to greater altitudes than the final stabilization altitude. Significantly smaller fireballs produce clouds with buoyancy-governed ascent.

[[Image:Bomba atomica.gif|right|thumb|The evolution of a nuclear mushroom cloud; 19 kt at 120 m • kt <sup>−{{frac|1|3}}</sup>. [[Operation Tumbler-Snapper|Tumbler-Snapper Dog]]. The sandy [[Nevada desert]] soil is "popcorned" by the intense ''flash'' of light emitted by the [[Critical mass|prompt supercriticality]] event; this "popcorning effect" results in more soil being lofted into the stem of the mushroom cloud than would otherwise be the case if the device had been placed above a more typical surface or soil]]After reaching the [[tropopause]] (the bottom of the region of strong static stability) the cloud tends to slow and spread out. If it contains sufficient energy, the central part may continue rising up into the [[stratosphere]] as an analog of a standard [[thunderstorm]].<ref>{{cite web |title=The Mushroom Cloud |url=http://www.atomicarchive.com/Effects/effects9.shtml |url-status=dead |archive-url=https://web.archive.org/web/20130830043914/http://www.atomicarchive.com/Effects/effects9.shtml |archive-date=2013-08-30 |access-date=January 14, 2018 |work=Atomic Archive}}</ref> A mass of air ascending from the [[troposphere]] to the stratosphere leads to the formation of acoustic [[gravity wave]]s, virtually identical to those created by intense stratosphere-penetrating thunderstorms. Smaller-scale explosions penetrating the tropopause generate waves of higher frequency, classified as [[infrasound]]. The explosion raises a large amount of moisture-laden air from lower altitudes. As the air rises, its temperature drops and its water vapour first condenses as water droplets and later freezes as ice crystals. The [[Phase transition|phase change]] releases [[latent heat]], heating the cloud and driving it to yet higher altitudes. The heads of the clouds consist of highly [[radioactive]] particles, primarily the [[Nuclear fission product|fission products]] and other weapon debris aerosols, and are usually dispersed by the wind, though weather patterns (especially rain) can produce [[nuclear fallout]].<ref name="gd">Glasstone and Dolan 1977</ref> The droplets of condensed water gradually evaporate, leading to the cloud's apparent disappearance. The radioactive particles, however, remain suspended in the air, and the invisible cloud continues depositing fallout along its path.

A mushroom cloud undergoes several phases of formation.<ref>{{cite book |author1=National Research Council |url=https://books.google.com/books?id=_JFf_bepeeEC&pg=PA53 |title=Effects of Nuclear Earth-Penetrator and Other Weapons |author2=Division on Engineering and Physical Sciences |author3=Committee on the Effects of Nuclear Earth-Penetrator and Other Weapons |publisher=National Academies Press |year=2005 |isbn=978-0-309-09673-7 |page=53}}</ref>
* ''Early time'', the first ~20 seconds, when the fireball forms and the fission products mix with the material aspired from the ground or ejected from the crater. The condensation of evaporated ground occurs in first few seconds, most intensely during fireball temperatures between 3500 and 4100 K.<ref name="google1" />
* ''Rise and stabilization phase'', 20 seconds to 10 minutes, when the hot gases rise up and early large fallout is deposited.
* ''Late time'', until about 2 days later, when the airborne particles are being distributed by wind, [[Deposition (aerosol physics)|deposited by gravity]], and scavenged by precipitation.
The shape of the cloud is influenced by the local atmospheric conditions and wind patterns. The fallout distribution is predominantly a downwind [[plume (hydrodynamics)|plume]]. However, if the cloud reaches the tropopause, it may spread against the wind, because its convection speed is higher than the ambient wind speed. At the tropopause, the cloud shape is roughly circular and spread out. The initial color of some radioactive clouds can be colored red or reddish-brown, due to presence of [[nitrogen dioxide]] and [[nitric acid]], formed from initially ionized [[nitrogen]], [[oxygen]], and atmospheric moisture. In the high-temperature, high-radiation environment of the blast, [[ozone]] is also formed. It is estimated that each megaton of yield produces about 5,000 tons of nitrogen oxides.<ref>[http://nuclearweaponarchive.org/Nwfaq/Nfaq5.html Effects of Nuclear Explosions] {{webarchive|url=https://web.archive.org/web/20140428174041/http://nuclearweaponarchive.org/Nwfaq/Nfaq5.html|date=2014-04-28}}. Nuclearweaponarchive.org. Retrieved on 2010-02-08.</ref> A higher-yield detonation can carry the nitrogen oxides from the burst high enough in atmosphere to cause significant [[ozone depletion|depletion]] of the [[ozone layer]]. Yellow and orange hues have also been described. This reddish hue is later obscured by the white colour of water/ice clouds, condensing out of the fast-flowing air as the fireball cools, and the dark colour of smoke and debris sucked into the updraft. The ozone gives the blast its characteristic [[corona discharge]]-like smell.<ref>[http://www.nuclearfiles.org/menu/key-issues/nuclear-weapons/history/pre-cold-war/manhattan-project/trinity/eyewitness-philip-morrison_1945-07-16.htm Key Issues: Nuclear Weapons: History: Pre Cold War: Manhattan Project: Trinity: Eyewitness Philip Morrison] {{webarchive|url=https://web.archive.org/web/20140721200833/http://www.nuclearfiles.org/menu/key-issues/nuclear-weapons/history/pre-cold-war/manhattan-project/trinity/eyewitness-philip-morrison_1945-07-16.htm|date=2014-07-21}}. Nuclearfiles.org (1945-07-16). Retrieved on 2010-02-08.</ref>

[[Image:Nukecloud.png|thumb|Mushroom cloud size as a function of [[nuclear weapon yield|yield]].<ref>[https://www.jstor.org/stable/pdf/443658.pdf The Mushroom Cloud, by Virginia L. Snitow]</ref>]]
The distribution of radiation in the mushroom cloud varies with the [[Nuclear weapon yield|yield]] of the explosion, type of weapon, [[Nuclear fusion|fusion]]–fission ratio, burst altitude, terrain type, and weather. In general, lower-yield explosions have about 90% of their radioactivity in the mushroom head and 10% in the stem. In contrast, megaton-range explosions tend to have most of their radioactivity in the lower third of the mushroom cloud. The fallout may appear as dry, ash-like flakes, or as particles too small to be visible; in the latter case, the particles are often deposited by rain. Large amounts of newer, more radioactive particles deposited on skin can cause [[Radiation burn|beta burns]], often presenting as discolored spots and [[lesion]]s on the backs of exposed animals.<ref>{{cite book |author1=Thomas Carlyle Jones |url=https://books.google.com/books?id=8fXzJrDfFgUC&pg=PA690 |title=Veterinary Pathology |author2=Ronald Duncan Hunt |author3=Norval W. King |publisher=Wiley-Blackwell |year=1997 |isbn=978-0-683-04481-2 |page=690}}</ref> The fallout from the [[Castle Bravo]] test had the appearance of white dust and was nicknamed ''Bikini snow''; the tiny white flakes resembled [[snowflake]]s, stuck to surfaces, and had a salty taste. In [[Operation Wigwam]], 41.4% of the fallout consisted of irregular opaque particles, slightly over 25% of particles with transparent and opaque areas, approximately 20% of microscopic marine organisms, and 2% of microscopic radioactive threads of unknown origin.<ref name="undercloud" />