Emily Johnson
A mushroom cloud is a distinctive mushroom-shaped cloud of debris, smoke, condensed water vapour, and highly radioactive particles resulting from a large explosion. The term mushroom cloud was coined in the early 1950s, but mushroom clouds generated by explosions were described centuries earlier. They are most commonly associated with nuclear explosions, but any sufficiently energetic detonation or deflagration will produce a similar effect. These include powerful conventional weapons, volcanic eruptions, and impact events.
| Characteristics | Values |
|---|---|
| Formation | A mushroom cloud is formed when a large volume of lower-density gases is formed at any altitude, causing a Rayleigh-Taylor instability. |
| Shape | The upward movement of the central column of hot gas is faster than the rest of the cloud, causing it to expand sideways. The cloud continues to rise and flatten, forming the rounded cap of the mushroom. |
| Colour | The initial colour can be red, reddish-brown, yellow or orange due to the presence of nitrogen dioxide and nitric acid. This is later obscured by white water/ice clouds and the dark colour of smoke and debris. |
| Altitude | The cloud reaches its maximum height in about 10 minutes and is then stabilized. |
| Persistence | Depending on weather conditions, the cloud can persist for about an hour until winds and air currents disperse it. |
| Causes | Mushroom clouds are typically associated with nuclear explosions, but they can also be caused by any massive release of heat, such as large forest fires, volcanic eruptions, or even sound. |
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What You'll Learn
Formation
Mushroom clouds are formed by a massive release of heat, which can be caused by nuclear explosions, large forest fires, or volcanic eruptions. During the early moments of a nuclear explosion, a fireball is formed, and fission products mix with the material aspirated from the ground or ejected from the crater. This fireball, a spherical mass of hot, incandescent gases, changes shape due to atmospheric friction, and its surface is cooled by energy radiation. The fireball increases in size and cools, and the vapors condense to form a cloud containing solid particles of weapon debris and small water droplets derived from the air. This process is known as condensation, which occurs most intensely during fireball temperatures between 3500 and 4100 Kelvin.
As the fireball rises, it encounters colder, denser air that slows and flattens it, forming the rounded cap of the mushroom. This creates a vacuum that is filled with smoke and debris, forming the visible central column of what becomes the mushroom cloud. The hot central gas column rises rapidly, pushing upwards and expanding sideways as it encounters the gas at the top of the mushroom, which has slowed due to stationary air above it. This forms a vortex ring, with turbulent vortices curling downward around its edges, drawing up a central column of smoke, debris, condensed water vapour, or a combination of these, to form the "mushroom stem".
The cloud continues to rise and flatten, attaining its maximum height after about 10 minutes, at which point it is considered stabilized. However, it continues to grow laterally, producing the characteristic mushroom shape. The ascent stops once the mass of hot gases reaches its equilibrium level, and the cloud begins to flatten further, often assisted by surface growth from decaying turbulence. The shape of the cloud is influenced by local atmospheric conditions and wind patterns.
The mushroom cloud may remain visible for about an hour or more, depending on weather conditions, before being dispersed by winds and merging with natural clouds in the sky. The fallout distribution is predominantly a downwind plume, but if the cloud reaches the tropopause, it may spread against the wind due to its higher convection speed compared to the ambient wind speed. The stabilization altitude depends on temperature profiles, dew point, and wind shear in the air at and above the starting altitude.
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Rayleigh-Taylor instability
In the context of a mushroom cloud, a sudden explosion near the surface creates a large volume of low-density gases that accelerate upwards against the higher-density gas above it. This upward movement of less dense fluid is compensated by the downward movement of the denser fluid, leading to the formation of downward-directed turbulent vortices. These vortices form a temporary "vortex ring", which creates the stem of the mushroom cloud.
The growth of the Rayleigh-Taylor instability can be divided into four main stages. Initially, perturbation amplitudes are small, and the equations of motion can be linearized, resulting in exponential instability growth. As non-linear effects begin to appear, the characteristic mushroom-shaped spikes and bubbles start to form. These spikes and bubbles are fluid structures of heavy fluid growing into light fluid and vice versa. The growth becomes non-linear as these structures tangle and roll up into vortices, leading to the formation of the mushroom shape.
The size of the blast determines the development of the mushroom cloud. A smaller blast produces a weaker instability and less convection, resulting in a longer stem. Conversely, a larger blast creates stronger instability with increased convection, leading to a shorter stem. The blast sets the initial conditions for the instability in the linear stage, and sufficient energy is required to drive the instability into the nonlinear stage before mixing takes over. Additionally, local atmospheric conditions, such as wind, temperature, and altitude, have a more significant impact on the nonlinear stage.
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Phases
A mushroom cloud undergoes several distinct phases of formation. In the first phase, lasting about 20 seconds, a fireball forms, and fission products mix with the material aspirated from the ground or ejected from the crater. The condensation of evaporated ground occurs during this phase, most intensely when fireball temperatures are between 3500 and 4100 Kelvin. This phase is marked by the formation of a vacuum that is immediately filled with smoke and debris, creating the visible central column that will become the mushroom cloud.
In the second phase, called the "rise and stabilization phase," the hot gases rise, and early large fallout is deposited. This phase lasts from 20 seconds to 10 minutes, with the cloud attaining its maximum height and assuming a stabilized form. The cloud continues to grow laterally, producing the characteristic mushroom shape.
The third phase, called "late time," occurs until about two days after the initial blast. During this phase, airborne particles are distributed by the wind, deposited by gravity, and scavenged by precipitation. The cloud may remain visible for about an hour or more, depending on weather conditions, before being fully dispersed by winds and merging with natural clouds in the sky.
The formation of a mushroom cloud is influenced by local atmospheric conditions and wind patterns. The distinctive shape results from the interaction of the hot central gas column rising rapidly against the stagnant colder air, creating swirling currents that form the mushroom cap. This Rayleigh-Taylor instability results in a buoyant mass of gas that rises rapidly, forming a temporary vortex ring that draws up a central column of smoke, debris, condensed water vapour, or a combination of these elements.
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Colour
The colour of a mushroom cloud is initially red or reddish-brown due to the presence of nitrous acid, nitrogen dioxide, nitric acid, and oxides of nitrogen, formed from initially ionized nitrogen, oxygen, and atmospheric moisture. As the fireball cools and condensation occurs, the colour changes to white, mainly due to the water droplets (as in an ordinary cloud). The white colour of water/ice clouds condensing out of the fast-flowing air obscures the reddish hue. The cloud may also appear grey, due to the presence of dirt and debris sucked up from the Earth's surface.
The first images of the atomic bomb that were widely circulated in the United States were black-and-white photographs, leading to the widespread perception that the mushroom cloud was white or grey. However, eyewitness accounts of the bombing of Hiroshima described the cloud as having "swirling, lovely colours like a rainbow". Some survivors' drawings portray the cloud as orange or violet, while others depict it as white or grey. One eyewitness, Hiroyuki Miyakawa, recalled seeing "flames of orange, green, yellow, and purple".
The colour of the cloud can also be influenced by the height of the burst. A strong updraft with inflowing winds, known as "afterwinds", can cause varying amounts of dirt and debris to be drawn up into the cloud. In an air burst with moderate or small amounts of dirt and debris, only a small proportion becomes contaminated with radioactivity. However, in a burst near the ground, larger amounts of dirt and debris are drawn into the cloud, which can affect its colour.
The height reached by the cloud depends on the heat energy of the weapon and atmospheric conditions. If the cloud reaches the tropopause, about 6-8 miles above the Earth's surface, it tends to spread out. However, if there is sufficient energy remaining, a portion of the cloud may ascend into the stratosphere, where it stabilizes and continues to grow laterally, forming the characteristic mushroom shape.
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Nuclear explosions
Mushroom clouds are the result of a large explosion, most commonly associated with nuclear explosions. However, any sufficiently energetic detonation or deflagration will produce a similar effect. They can be caused by powerful conventional weapons, volcanic eruptions, and impact events.
The formation of a mushroom cloud is due to the sudden generation of a large volume of lower-density gases at any altitude, resulting in Rayleigh-Taylor instability. The Rayleigh-Taylor instability forms an inverted cup shape as the cool air underneath pushes up on the bottom of the fireball gases. This instability creates turbulence and a vortex that sucks in more air, forming a central column of smoke, debris, condensed water vapour, or a combination thereof. The cloud continues to rise and flatten, forming the rounded cap of the mushroom.
Nuclear weapons are typically detonated above ground to maximize the effect of their expanding fireball and blast wave. The initial fireball expands in all directions, forming a sphere of hot air that rises due to buoyancy. As the fireball increases in size, it cools, and the vapours condense to form a cloud containing weapon debris and water droplets from the air. This cloud of hot gases continues to rise until it reaches an equilibrium level, at which point it stops ascending and begins to flatten, forming the characteristic mushroom shape.
The height reached by the mushroom cloud depends on the heat energy of the explosion and atmospheric conditions. If the cloud reaches the tropopause, it tends to spread out. However, if there is sufficient energy remaining, a portion of the cloud will ascend into the stratosphere. The cloud attains its maximum height in about 10 minutes and is then considered stabilized. It continues to expand laterally, contributing to the mushroom shape. The cloud may remain visible for about an hour or more before being dispersed by the wind and merging with natural clouds.
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Frequently asked questions
Mushroom clouds are caused by a Rayleigh-Taylor instability formed when there is a very hot central feature, causing a strong updraft shaft where heated air rises. This rising against the stagnant colder air around the circular rising centre "stem" causes swirling currents in the form of a mushroom cap.
Mushroom clouds can be caused by any massive release of heat, such as a nuclear explosion, large forest fires, or volcanoes.
Mushroom clouds are composed of vapours, solid particles of weapon debris, small drops of water, smoke, and debris. The initial colour of some radioactive clouds can be reddish-brown due to the presence of nitrogen dioxide and nitric acid.
