A mushroom cloud is a distinctive mushroom-shaped flammagenitus cloud of debris, smoke, and usually condensed water vapour resulting from a large explosion. The effect is most commonly associated with a nuclear explosion, but any sufficiently energetic detonation or deflagration will produce a similar effect. They can be caused by powerful conventional weapons, including large thermobaric weapons. Some volcanic eruptions and impact events can produce natural mushroom clouds.
Mushroom clouds result from the sudden formation of a large volume of lower-density gases at any altitude, causing a Rayleigh–Taylor instability. The buoyant mass of gas rises rapidly, resulting in turbulent vortices curling downward around its edges, forming a temporary vortex ring that draws up a central column, possibly with smoke, debris, condensed water vapor, or a combination of these, to form the "mushroom stem". The mass of gas plus entrained moist air eventually reaches an altitude where it is no longer of lower density than the surrounding air; at this point, it disperses, drifting back down, which results in fallout following a nuclear blast. The stabilization altitude depends strongly on the profiles of the temperature, dew point, and wind shear in the air at and above the starting altitude.
Early accounts and origins of term
Although the term appears to have been coined in the early 1950s, mushroom clouds generated by explosions were being described centuries before the Atomic Age. A contemporary aquatint by an unknown artist of the 1782 Franco-Spanish attack on Gibraltar shows one of the attacking force's floating batteries exploding with a mushroom cloud after the British defenders set it ablaze by firing heated shot.
In 1798, Gerhard Vieth published a detailed and illustrated account of a cloud in the neighborhood of Gotha that was "not unlike a mushroom in shape". The cloud had been observed by legation counselor Lichtenberg a few years earlier on a warm summer afternoon. It was interpreted as an irregular meteorological cloud and seemed to have caused a storm with rain and thunder from a new dark cloud that developed beneath it. Lichtenberg stated to have later observed somewhat similar clouds, but none as remarkable.
The 1917 Halifax Explosion produced a mushroom cloud. In 1930 Olaf Stapledon in his novel Last and First Men imagines the first demonstration of an atomic weapon "clouds of steam from the boiling sea.. a gigantic mushroom of steam and debris". The Times published a report on 1 October 1937 of a Japanese attack on Shanghai, China, that generated "a great mushroom of smoke". During World War II, the destruction of the Japanese battleship Yamato produced a mushroom cloud.Reynolds, Clark G (1982). The Carrier War. Time-Life Books. . p. 169.
The atomic bomb cloud over Nagasaki, Japan, was described in The Times of London of 13 August 1945 as a "huge mushroom of smoke and dust". On 9 September 1945, The New York Times published an eyewitness account of the Nagasaki bombing, written by William L. Laurence, the official newspaper correspondent of the Manhattan Project, who accompanied one of the three aircraft that made the bombing run. He wrote of the bomb producing a "pillar of purple fire" out of the top of which came "a giant mushroom that increased the height of the pillar to a total of 45,000 feet".Eyewitness Account of Atomic Bomb Over Nagasaki hiroshima-remembered.com. Retrieved on 2010-08-09.
In 1946, the Operation Crossroads nuclear bomb tests were described as having a "cauliflower" cloud, but a reporter present also spoke of "the mushroom, now the common symbol of the atomic age". Mushrooms have traditionally been associated both with life and death, food and poison, which made them a more powerful symbolic connection than, say, the "cauliflower" cloud.
Physics
Mushroom clouds are formed by many sorts of large explosions under Earth's gravity, but they are best known for their appearance after 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", 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.
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.
Nuclear detonations
Description At the moment of a nuclear explosion, a fireball is formed. The ascending, roughly spherical mass of hot, 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 cool into visible gases, forming a cloud; the 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 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.
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. A mass of air ascending from the troposphere to the stratosphere leads to the formation of acoustic gravity waves, 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 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 fission products and other weapon debris aerosols, and are usually dispersed by the wind, though weather patterns (especially rain) can produce nuclear fallout.Glasstone and Dolan 1977 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. 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. 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, 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. 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.Effects of Nuclear Explosions . Nuclearweaponarchive.org. Retrieved on 2010-02-08. A higher-yield detonation can carry the nitrogen oxides from the burst high enough in atmosphere to cause significant 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.Key Issues: Nuclear Weapons: History: Pre Cold War: Manhattan Project: Trinity: Eyewitness Philip Morrison . Nuclearfiles.org (1945-07-16). Retrieved on 2010-02-08.
The distribution of radiation in the mushroom cloud varies with the yield of the explosion, type of weapon, 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 beta burns, often presenting as discolored spots and lesions on the backs of exposed animals. The fallout from the Castle Bravo test had the appearance of white dust and was nicknamed Bikini snow; the tiny white flakes resembled snowflakes, 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.
Differences with detonation types