mushroom cloud

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The mushroom cloud of the atomic bomb dropped on Nagasaki, Japan on August 9, 1945.

A mushroom cloud is a distinctive pyrocumulus 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 GBU-43/B Massive Ordnance Air Blast bomb. 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).

Origin of the term

Although the term itself appears to have been coined at the start of the 1950s, mushroom clouds generated by explosions were being described before the atomic era. For instance, The Times published a report on 1 October 1937 of a Japanese attack on Shanghai in China which generated "a great mushroom of smoke". During the Second World War, descriptions of mushroom clouds were relatively common.

Ascending cloud from Redoubt Volcano from an eruption in 1990. The mushroom-shaped plume rose from avalanches of hot debris (pyroclastic flows) that cascaded down the north flank of the volcano.

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." (14 000 m)

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)

Physics

Inside a mushroom cloud: cooler air is drawn into the rising toroidal fireball, which itself cools into the familiar cloud appearance.

Mushroom cloud with prominent condensation ring from the Castle Romeo hydrogen bomb test.

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, because most of the energy would 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.

One way to analyze the motion, once the hot gas has cleared the ground sufficiently, is as a 'spherical cap bubble' ^[1], as this gives 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. ^[2]

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 produce a pillar of water, which, in colapsing, forms a cauliflower-shape, which is mistaken for a mushroom cloud on many pictures (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, "Wilson clouds" also known as or condensation clouds or 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.

References

Wikimedia Commons has media related to: Mushroom cloud

• Batchelor, G. K. An Introduction to Fluid Dynamics, Cambridge, UK: Cambridge University Press, 1967.
• Glasstone, Samuel, and Dolan, Philip J. The Effects of Nuclear Weapons, 3rd edn. Washington, DC: United States Department of Defense and Energy Research and Development Administration, 1977. (esp. "Chronological development of an air-burst" and "Description of Air and Surface Bursts" in Chapter II)
• Vigh, Jonathan. Mechanisms by Which the Atmosphere Adjusts to an Extremely Large Explosive Event, 2001. (See: http://euler.atmos.colostate.edu/~vigh/other_works/at735/vigh_adjustment_mechanisms.pdf .)
• Weart, Spencer. Nuclear Fear: A History of Images. Cambridge, MA: Harvard University Press, 1987.

External links

• Carey Sublette's Nuclear Weapon Archive has many photographs of mushroom clouds
• DOE Nevada Site Office has many photographs of nuclear tests conducted at the Nevada Test Site and elsewhere