A mushroom cloud is a distinctive mushroom-shaped flammagenitus cloud of debris, smoke, and usually condensed water vapor resulting from a large explosion. The effect is most commonly associated with a nuclear explosion, but any sufficiently energetic detonation or deflagration will produce the same effect. They can be caused by powerful conventional weapons, like thermobaric weapons such as the ATBIP and GBU-43/B MOAB. 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 (see fallout). 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 era.
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.
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".
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.