Emily Johnson
Mushroom clouds are clouds of smoke and debris that form after a massive release of heat, such as a nuclear explosion. They can also form after a conventional explosion or a volcanic eruption. Mushroom clouds are the result of the sudden formation of a large volume of lower-density gases, causing a Rayleigh-Taylor instability. The buoyant mass of gas rises rapidly, resulting in turbulent vortices that curl downward, forming a temporary vortex ring that draws up a central column of smoke, debris, and condensed water vapour to form the mushroom stem. The fireball increases in size and cools, and the vapours condense to form a cloud containing solid particles of weapon debris and small drops of water.
| Characteristics | Values |
|---|---|
| Formation | Mushroom clouds are formed by the sudden release of a large volume of lower-density gases at any altitude, causing Rayleigh-Taylor instability. |
| Phases | Early time (first 20 seconds), rise and stabilization phase (20 seconds to 10 minutes), and late time (until about 2 days later). |
| Appearance | The iconic shape of a mushroom cloud is due to the interaction of hot, less dense air in the middle with denser cold air on the outside. |
| Cause | Any massive release of heat, including nuclear explosions, volcanic eruptions, and conventional bomb explosions. |
| Height | The height at which a bomb explodes contributes to the shape and features of the resulting mushroom cloud. |
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What You'll Learn
Formation
Mushroom clouds are the result of a massive release of heat, such as from a nuclear explosion, a volcano, or a conventional bomb. They can even be created by smaller explosions, such as those using just a few kilograms of TNT. The formation of mushroom clouds can be understood through the following phases:
Early Time Phase
During the first 20 seconds after the explosion, a fireball forms, and fission products mix with the material aspirated from the ground or ejected from a crater. The condensation of evaporated ground occurs in the first few seconds, with temperatures between 3500 and 4100 Kelvin. This intense heat causes a sudden release of energy, creating a hot bubble of gas or a large volume of lower-density gases, which interacts with the cooler surrounding air, making it less dense. This interaction between gases of different densities is known as Rayleigh-Taylor instability.
Rise and Stabilization Phase
From 20 seconds to 10 minutes after the explosion, the hot gases, including vapors from the evaporated ground, rise and form a cloud. The buoyant mass of gas rises rapidly, forming turbulent vortices that curl downward, creating a temporary vortex ring. This upward movement of gases forms the "'stem' of the mushroom cloud. The upward rush of gases also creates a vacuum that gets filled with smoke and debris, forming the central column of what will become the mushroom cloud. The fireball continues to rise until it reaches an altitude where the air is dense and cold enough to slow its ascent.
Late Time Phase
Until about two days after the explosion, the particles in the cloud are distributed by the wind, deposited by gravity, or scavenged by precipitation. The cloud continues to rise and flatten, forming the rounded cap of the mushroom. The shape of the cloud during this phase is influenced by local atmospheric conditions and wind patterns. Eventually, the cloud reaches an altitude where it is no longer of lower density than the surrounding air, causing it to disperse and drift back down, leading to fallout.
It's important to note that the formation of a mushroom cloud is not specific to nuclear explosions. While nuclear explosions typically create the most massive mushroom clouds, the underlying physics applies to all fluids and gases interacting at different densities.
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Phases
Mushroom clouds are the result of a massive release of heat, often from a nuclear explosion, but also from other sources such as conventional bombs, volcanic eruptions, or industrial explosions. The formation of a mushroom cloud occurs in several distinct phases.
The first phase, known as the "early time" phase, occurs in the first 20 seconds after the explosion. During this phase, a fireball forms, and fission products mix with material aspirated from the ground or ejected from a crater. The condensation of evaporated ground occurs during this initial phase, most intensely when fireball temperatures are between 3500 and 4100 Kelvin. This phase is characterised by an intense release of energy, creating a hot bubble of gas, or fireball, that interacts with the cooler surrounding air, reducing its density.
The second phase is the "rise and stabilization" phase, which lasts from 20 seconds to about 10 minutes after the explosion. During this phase, the hot gases and vapours rise, forming a cloud containing solid particles of weapon debris and small drops of water derived from the air. The buoyant mass of gas rises rapidly, resulting in turbulent vortices curling downward around its edges. This forms a temporary vortex ring that draws up a central column of smoke, debris, condensed water vapour, or a combination of these, forming the stem of the mushroom cloud. The upward movement of the gases is influenced by the rebound of the shock wave off the surface of the earth, contributing to the asymmetry of the mushroom shape.
The third phase is the "late time" phase, which lasts until about two days after the explosion. During this phase, the mushroom cloud has reached its maximum height, and the shape of the cloud is influenced by local atmospheric conditions and wind patterns. The cloud continues to rise and flatten, forming the rounded cap of the mushroom. The cloud can persist in the atmosphere for about an hour until winds and air currents disperse it, causing the fallout of airborne particles.
It is important to note that the phases of mushroom cloud formation can vary depending on the type of explosion and the environmental conditions. The height of the explosion, the explosive yield, and the surrounding temperature, dew point, and wind shear can all influence the formation and characteristics of the mushroom cloud.
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Nuclear explosions
Mushroom clouds are most commonly associated with nuclear explosions. However, they can also be caused by powerful conventional weapons, including thermobaric weapons, as well as some volcanic eruptions and impact events. In fact, mushroom clouds generated by explosions were being described centuries before the Atomic Age.
A mushroom cloud can be created by any massive release of heat. Heat rises, and the blast of heat and energy from an explosive fireball quickly ascends through the atmosphere, creating a vacuum in its wake. This vacuum is immediately filled with smoke and debris, forming the visible central column of what will become the mushroom cloud. The fireball soon reaches a point in the atmosphere where the air is cold enough and dense enough to slow its ascent, and the weight and density of the air flatten the fireball and its trailing smoke. The cloud continues to rise as it continues to flatten, forming the rounded cap of the mushroom.
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 centre, 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 formation of a mushroom cloud occurs in several phases. In the first 20 seconds, the fireball forms and the fission products mix with the material aspirated from the ground or ejected from the crater. The condensation of evaporated ground occurs most intensely during fireball temperatures between 3,500 and 4,100 Kelvin. From 20 seconds to 10 minutes, the hot gases rise and early large fallout is deposited. Until about two days later, airborne particles are distributed by the wind, deposited by gravity, and scavenged by precipitation. The shape of the cloud is influenced by 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.
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Non-nuclear explosions
Mushroom clouds are most commonly associated with nuclear explosions, but they can also form from any sufficiently energetic detonation or deflagration. Powerful conventional weapons, such as thermobaric weapons, can produce mushroom clouds. Natural mushroom clouds can also occur as a result of some volcanic eruptions and impact events.
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, forming turbulent vortices that curl downward around its edges. This creates a temporary vortex ring that draws up a central column, possibly with smoke, debris, condensed water vapour, or a combination of these, to form the "mushroom stem".
The colour of the cloud is initially red or reddish-brown due to the presence of nitrous acid and oxides of nitrogen. As the fireball cools and condensation occurs, the colour changes to white, mainly due to water droplets. The cloud consists chiefly of very small particles of radioactive fission products, weapon residues, water droplets, and larger particles of dirt and debris carried up by the afterwinds.
The eventual height reached by the cloud depends on the heat energy of the weapon and the atmospheric conditions. If the cloud reaches the tropopause, about 6-8 miles above the Earth's surface, it tends to spread out. If there is sufficient energy remaining, a portion of the cloud will ascend into the more stable air of the stratosphere.
Detonations that occur significantly below ground level or deep below water do not produce mushroom clouds. This is because the explosion vaporises a large amount of earth or water, creating a bubble that collapses in on itself. A detonation high above the ground may also produce a mushroom cloud without a stem.
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Fallout
Mushroom clouds are a result of a thermonuclear explosion, but they can also be created by any massive release of heat, such as from a volcano or conventional bomb. They are formed by the sudden formation of a large volume of lower-density gases at any altitude, causing 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. This central column may be comprised of smoke, debris, condensed water vapour, or a combination of these, forming the "mushroom stem".
The fallout from a mushroom cloud can be extremely dangerous. Radioactive particles can be carried for considerable distances, and radiation from nuclear tests can be spread by rainstorms, wind, gravity, and precipitation. This fallout can lead to the evacuation of affected areas, as in the case of the Rongelap Atoll, which was evacuated due to lethal doses of radiation from the Castle Bravo test.
The shape of the mushroom cloud is influenced by the local atmospheric conditions and wind patterns. The cloud can persist in the atmosphere for about an hour until winds and air currents disperse it. The height at which a bomb is detonated also influences the shape of the cloud. When bombs are detonated at a high altitude, the cap and stem of the mushroom cloud do not meet. However, when bombs are detonated closer to the ground, the stem and cap merge into the classic mushroom profile.
The formation of the mushroom cloud can be divided into several phases. The first phase occurs in the early moments after the explosion, when the fireball forms and fission products mix with material from the ground or the crater. This is followed by the rise and stabilization phase, during which hot gases rise and early large fallout is deposited. The final phase occurs until about two days after the explosion, when airborne particles are distributed by wind, deposited by gravity, or scavenged by precipitation.
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Frequently asked questions
Mushroom clouds form after a massive release of heat, such as a nuclear explosion.
Mushroom clouds are clouds of smoke and debris that move through the air following an explosion.
When a bomb goes off, energy is released in all directions. This creates a sphere of hot air that rises through the atmosphere, forming a vacuum that is filled with smoke and debris. This forms the "'stem' of the mushroom, while the cloud continues to rise and flatten, forming the rounded "'cap'".
No, mushroom clouds can form after any event that creates heat very rapidly, such as the eruption of a conventional bomb or a volcano.
The first phase is the "early time", which lasts about 20 seconds. During this phase, a fireball forms and fission products mix with material from the ground or ejected from a crater. The second phase is the "rise and stabilization phase", which lasts from 20 seconds to 10 minutes. During this phase, hot gases rise and early large fallout is deposited. The final phase is the "late time", which lasts until about 2 days after the explosion. During this phase, airborne particles are distributed by wind, deposited by gravity, or scavenged by precipitation.
