Michael Davis
Mushroom clouds are distinctive mushroom-shaped clouds of dust, debris, smoke, and condensed water vapour that result from a large explosion. They are most commonly associated with nuclear explosions, but any sufficiently energetic detonation or deflagration will produce a similar effect. The formation of a mushroom cloud is caused by the sudden formation of a large volume of lower-density gases at any altitude, resulting in a Rayleigh-Taylor instability. The buoyant mass of gas rises rapidly, forming turbulent vortices that curl downward around its edges, creating a temporary vortex ring that draws up a central column of smoke, debris, condensed water vapour, or a combination of these elements 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, but as the fireball cools and condensation occurs, the colour changes to white, mainly due to the formation of water droplets.
| Characteristics | Values |
|---|---|
| Cause | Any massive release of heat |
| Formation | A large volume of lower-density gases is formed at any altitude, causing a Rayleigh-Taylor instability |
| Shape | The low-density fluid in the middle rises the fastest, forming the mushroom stem. The cloud continues to rise and flatten, forming the rounded cap of the mushroom |
| Height | The height reached depends on the heat energy of the weapon and the atmospheric conditions |
| Color | Initially red or reddish-brown due to the presence of nitrous acid and oxides of nitrogen. Later, the color changes to white due to water droplets |
| Fallout | Dry, ash-like flakes or invisible particles. Larger amounts of newer, more radioactive particles deposited on the skin can cause beta burns |
| Duration | Can persist in the atmosphere for about an hour until dispersed by winds |
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What You'll Learn
A massive release of heat
A mushroom cloud is caused by a massive release of heat. This occurs when there is a large explosion, often associated with a nuclear detonation, but it can also be caused by powerful conventional weapons or natural events such as volcanic eruptions.
The explosion creates an outburst of energy that forms a sphere of hot air, which rises through the atmosphere, creating a vacuum that is filled with smoke and debris. This is the central column of what will become the mushroom cloud. The fireball continues to rise and expand until it reaches a point in the atmosphere where the air is cold and dense enough to slow and eventually stop its ascent.
The fireball's ascent is stopped when it reaches its equilibrium level, and the cloud begins to flatten and spread out, forming the rounded cap of the mushroom. This is due to the Rayleigh-Taylor instability, where the cool air underneath pushes the bottom of the fireball into an inverted cup shape, causing turbulence and a vortex that sucks in more air, further cooling the fireball. The cloud continues to rise and grow laterally, producing the characteristic mushroom shape.
The colour of the mushroom cloud can provide information about the explosion. Initially, the cloud is red or reddish-brown due to the presence of nitrous acid and oxides of nitrogen. As the fireball cools, the colour changes to white due to the formation of water droplets, similar to an ordinary cloud. The distribution of radioactivity in the mushroom cloud depends on the explosion's yield, with lower-yield explosions having most of their radioactivity in the mushroom head, while megaton-range explosions have more in the lower third of the cloud.
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Rayleigh-Taylor instability
In the context of a mushroom cloud, Rayleigh-Taylor instability is instigated by the sudden explosion near the surface, which leads to the formation of a large volume of low-density gases that start accelerating upwards rapidly against the higher-density gas above it. This rapid upward movement results in the formation of downward-directed turbulent vortices, creating a temporary "vortex ring" that forms the stem of the mushroom cloud. The rising buoyant low-density air will eventually reach an equilibrium altitude, where it stops rising and disperses downwards, contributing to the mushroom shape.
The evolution of Rayleigh-Taylor instability follows four main stages. Initially, the perturbation amplitudes are small, and the equations of motion can be linearized, resulting in exponential instability growth. As the instability progresses, it enters a non-linear growth phase, where the spikes and bubbles of the instability tangle and roll up into vortices. During this stage, the ubiquitous mushroom-shaped spikes and bubbles begin to form.
The size of the blast determines the rate at which Rayleigh-Taylor instability develops. A smaller blast produces a weaker instability and slower progression, resulting in a longer stem for the mushroom cloud. Conversely, a larger blast yields increased convection and a stronger instability, leading to a quicker formation of the mushroom shape with a shorter stem.
It is important to note that not all fire-clouds evolve into mushroom clouds due to Rayleigh-Taylor instability. The blast must be sufficiently large to drive the instability into the non-linear stage, where its "fingers" constitute a large flow of material that is sheared relative to the higher-density surroundings. Additionally, natural convection plays a role in accelerating the instability and influencing its development.
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Atmospheric conditions
The formation of a mushroom cloud is dependent on the atmospheric conditions. Firstly, a mushroom cloud is formed from any massive release of heat, which can be from a nuclear explosion or other sufficiently energetic detonations or deflagrations. The heat rises, and the blast of heat and energy from an explosive fireball quickly moves through the atmosphere, creating a vacuum that is immediately filled with smoke and debris. This forms the central column of what will become the mushroom cloud.
The fireball continues to rise until it reaches a point in the atmosphere where the air is cold and dense enough to slow and eventually stop its ascent. The weight and density of the air flatten the fireball and its trailing smoke, causing it to spread out. The cloud continues to rise as it flattens, forming the rounded cap of the mushroom shape.
The height reached by the cloud depends on the heat energy of the explosion and the atmospheric conditions. If the cloud reaches the tropopause, around 6-8 miles above the Earth's surface, it may spread out further. However, if there is sufficient energy remaining, a portion of the cloud may continue to ascend into the more stable air of the stratosphere.
The cloud attains its maximum height after about 10 minutes and is then considered stabilized. It continues to expand laterally, contributing to the mushroom shape. The cloud may persist in the atmosphere for about an hour or more before being dispersed by winds and merging with natural clouds.
The colour of the mushroom cloud can vary depending on the atmospheric conditions and the composition of the explosion. Initially, the cloud may appear reddish-brown due to the presence of nitrous acid and oxides of nitrogen. As the fireball cools, condensation occurs, and the colour changes to white due to the formation of water droplets. The reddish hue may be obscured by the white colour of the water clouds and the dark colour of smoke and debris.
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Explosion height
The height of a mushroom cloud depends on the heat energy of the explosion and the atmospheric conditions. The cloud reaches its maximum height after about 10 minutes and is then considered "stabilized".
The height of the explosion also determines the shape of the mushroom cloud. If detonated high enough, the cap and stem of the cloud do not merge. The bombs that caused the clouds observed by the sources were detonated at a height of nearly 2,000 feet (610 meters) above the ground, and the cap and stem did not meet.
In contrast, for explosions that occur closer to the ground, the stem and cap merge into the classic mushroom profile. This is because, as the fireball rises, it creates a vacuum that is immediately filled with smoke and debris, forming the visible central column of what will become the mushroom cloud.
The fireball continues to rise and flatten, forming the rounded cap of the mushroom. The cloud continues to rise and grow laterally, forming the characteristic mushroom shape.
The height of the explosion also determines the amount of dirt and debris drawn into the cloud. A strong updraft with inflowing winds, called "afterwinds", are produced depending on the height of the burst. These afterwinds can cause varying amounts of dirt and debris to be sucked up from the earth's surface into the cloud.
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Explosion energy
A mushroom cloud is the result of a large explosion that releases heat and energy. The explosion creates a fireball that rises through the atmosphere, forming a vacuum that is filled with smoke and debris. This smoke and debris form the visible central column of what will become the mushroom cloud. The fireball continues to rise and flatten, forming the rounded cap of the mushroom. The height reached by the cloud depends on the heat energy of the explosion and the atmospheric conditions. If the cloud reaches the tropopause, it tends to spread out, but if it has sufficient energy, it will ascend into the stratosphere.
The formation of a mushroom cloud is not limited to nuclear explosions. Any sufficiently energetic detonation or deflagration will produce a similar effect. Powerful conventional weapons, such as thermobaric weapons, can create mushroom clouds. Additionally, some natural events, such as volcanic eruptions and impact events, can also produce mushroom clouds.
The energy released during an explosion plays a crucial role in the formation of the mushroom cloud. The initial outburst of energy forms a sphere of hot air, but the shape quickly evolves due to the buoyancy of the gases. The middle column of the sphere experiences more buoyancy than the edges, causing the sphere to rise and form a temporary vortex ring. This vortex ring draws up smoke, debris, and condensed water vapour to form the "mushroom stem".
The colour of the mushroom cloud can provide information about the explosion. Initially, the cloud may be red or reddish-brown due to the presence of nitrous acid and oxides of nitrogen. As the fireball cools, condensation occurs, and the colour changes to white due to the formation of water droplets. The distribution of radioactivity within the mushroom cloud varies, with lower-yield explosions having most of their radioactivity in the mushroom head, while megaton-range explosions tend to have more radioactivity in the lower third of the cloud.
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Frequently asked questions
A mushroom cloud is a distinctive mushroom-shaped cloud of debris, smoke, and usually condensed water vapour resulting from a large explosion.
A mushroom cloud can be created by any massive release of heat. The explosion creates a vacuum that is immediately filled with smoke and debris, forming the visible central column of what will become the mushroom cloud.
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 the water droplets.
The eventual height reached by the radioactive cloud depends upon 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.
