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 like the Father of All Bombs. 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. 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 (see fallout).
The original descriptions of the first nuclear test explosion at Trinity Site contained many different associations with the cloud which formed after the blast, including a "multi-colored surging cloud," a "giant column," a "chimney-shaped column," a "dome-shaped" column, the "parasol," the "great funnel," and "geyser," the "convoluted brain," and even the "raspberry." In his official (then classified) description of the test, Enrico Fermi mentioned "a huge pillar of smoke with an expanded head like a gigantic mushroom".
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." (A similar phenomenon can be seen in the picture of the Nagasaki bomb cloud shown above.)
Later 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, making them a more powerful symbolic connection than, say, the "cauliflower" cloud. (Weart 1988)
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, lest most of the energy 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.
A quite successful way to analyze the motion, once the hot gas has cleared the ground sufficiently, is as a 'spherical cap bubble' , as this gives quite good 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.
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, as well as large "cloud chamber effect" (such as that seen in the well-known pictures of the Crossroads Baker test).
Nuclear mushroom clouds are often also accompanied by short-lived vapor 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.
Of further note, the streaks of smoke seen to the left of the explosion at detonation are vertical smoke flares which are used to observe the shockwave.
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