Emily Johnson
Mushroom clouds are the result of a large explosion, often associated with nuclear detonations. However, any event that results in a rapid release of heat, such as a volcanic eruption or a powerful conventional explosion, can produce a similar effect. The iconic mushroom shape is formed when a large volume of lower-density gases are suddenly released at high temperatures, causing a Rayleigh-Taylor instability. This buoyant mass of hot gases rises rapidly, cools, and flattens at the top, while the sides curl downward, forming a temporary vortex ring that draws up a central column of smoke, debris, and condensed water vapour. The eventual height and duration of the mushroom cloud depend on the heat energy of the explosion and the atmospheric conditions.
| Characteristics | Values |
|---|---|
| Cause | Any massive release of heat and energy, including explosions, volcanic eruptions, forest fires, impact events, and supernovae explosions |
| Shape | Mushroom-shaped cloud of smoke, debris, condensed water vapour, and solid particles |
| Formation | Explosion creates a hot bubble of gas (fireball) that rises rapidly, causing a vacuum that is filled with smoke and debris, forming the central column |
| Height | Depends on the heat energy of the explosion and atmospheric conditions; may reach the tropopause (6-8 miles high) and spread out or ascend into the stratosphere |
| Colour | Initially red or reddish-brown due to nitrogen compounds; changes to white due to water droplets, then darkens with smoke and debris |
| Stability | Reaches maximum height in about 10 minutes and is considered "stabilized"; continues to grow laterally |
| Duration | Visible for about an hour before being dispersed by winds and merging with natural clouds |
| Radioactivity | Lower-yield explosions have 90% radioactivity in the head and 10% in the stem; megaton-range explosions have most radioactivity in the lower third |
| Fallout | May appear as dry, ash-like flakes or microscopic particles, causing beta burns on exposed skin |
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What You'll Learn
Nuclear explosions
Mushroom clouds are most commonly associated with nuclear explosions. However, any sufficiently energetic detonation or deflagration will produce a similar effect. Mushroom clouds are the result of the sudden formation of a large volume of lower-density gases at any altitude, causing Rayleigh-Taylor instability.
As the fireball continues to rise, it experiences resistance from the air, which pushes it down sideways, leading to the flattening of the top of the cloud. The displaced gas, which is at a lower temperature than the air in the centre of the column, trickles down the sides, only to be sucked back in and travel upwards again. This is why the edges of an explosion's fireball appear to be curling constantly. The cloud continues to rise until it reaches the point where the surrounding air is no longer cool, which is high up in the atmosphere, where ozone heats up the surrounding gas by absorbing harmful solar radiation.
The eventual height reached by the radioactive cloud depends on the heat energy of the weapon and the atmospheric conditions. If the cloud reaches the tropopause, there is a tendency for it to spread out. However, if sufficient energy remains, a portion of the cloud will ascend into the more stable air of the stratosphere. The cloud attains its maximum height after about 10 minutes and is then said to be "stabilized". It continues to grow laterally, producing the characteristic mushroom shape.
The initial colour of some radioactive clouds can be reddish-brown due to the presence of nitrogen dioxide and nitric acid. As the fireball cools, the colour changes to white due to the water droplets. The distribution of radiation in the mushroom cloud varies with the yield of the explosion, the type of weapon, the fusion-fission ratio, and the burst altitude. Lower-yield explosions have about 90% of their radioactivity in the mushroom head and 10% in the stem, while megaton-range explosions tend to have most of their radioactivity in the lower third of the mushroom cloud.
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Rayleigh-Taylor instability
The process of Rayleigh-Taylor instability can be understood through the concept of potential energy. The system tends to achieve a state of lower total energy. In the case of two fluids with different densities, the potential energy of the system can be lowered if the higher-density fluid moves downwards, reducing its potential energy. This disturbance leads to the upward movement of the lower-density fluid to compensate. This disturbance continues to grow as the system seeks to reduce its potential energy further.
The Rayleigh-Taylor instability was studied by Lord Rayleigh, who examined the behaviour of fluids with different densities. G. I. Taylor made a significant contribution by realising that a similar situation occurs when a less dense fluid is 'accelerated' into a more dense fluid, which is precisely what happens during a mushroom cloud explosion.
The evolution of Rayleigh-Taylor instability follows four main stages. In the initial stage, 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. This non-linear stage is accelerated by natural convection in the context of a mushroom cloud explosion.
The Rayleigh-Taylor instability is not limited to mushroom cloud explosions but is also observed in various terrestrial and astrophysical phenomena. Examples include salt domes, weather inversions, the Crab Nebula, and the Sun's outer atmosphere. Additionally, Rayleigh-Taylor instability can be witnessed in everyday objects like lava lamps, where the active heating of the fluid layer at the bottom creates a similar dynamic.
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Atmospheric conditions
The height reached by the mushroom cloud depends on the heat energy released and the atmospheric conditions. If the cloud reaches the tropopause, a boundary between the troposphere and the stratosphere, it tends to spread out due to the stable air in the stratosphere. The cloud's maximum height is typically attained within about 10 minutes, after which it stabilises and continues to grow laterally, further developing the characteristic mushroom shape.
The fireball's interaction with the surrounding atmosphere leads to the formation of the mushroom cloud's distinctive cap. As the fireball rises, it cools down, and the vapours condense to form solid particles of weapon debris and water droplets derived from the air. The central part of the fireball remains the hottest, creating a rolling motion as it interacts with the outer, cooler portions. This results in turbulent vortices that curl downward, forming a temporary vortex ring. The upward movement of the hot gases and the downward curling of the edges create a constant rolling pattern.
The colour of the mushroom cloud can provide information about the explosion and the atmospheric conditions. Initially, the cloud may appear reddish-brown due to the presence of nitrogen dioxide and nitric acid, formed from ionised nitrogen, oxygen, and atmospheric moisture. As the fireball cools further, the colour transitions to white due to the formation of water droplets, similar to an ordinary cloud. The white colour can be obscured by the darker colours of smoke and debris drawn into the updraft.
The persistence of the mushroom cloud in the atmosphere is also influenced by atmospheric conditions. It can remain visible for about an hour or more, depending on weather conditions, before being dispersed by winds and merging with natural clouds. The duration of the cloud's visibility is impacted by factors such as wind speed and direction, humidity, and the presence of other atmospheric phenomena.
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Radioactivity
Mushroom clouds are most commonly associated with nuclear explosions. However, any sufficiently energetic detonation or deflagration will produce a similar effect. The formation of a mushroom cloud is caused by the sudden release of energy following an explosion, which heats up the surrounding air, causing it to expand.
The giant fireball created in the initial stages of the explosion can reach temperatures of up to millions of degrees Celsius. This hot air rises rapidly, creating a vacuum that is then filled by the surrounding air, which also expands and rises. As the fireball increases in size and cools, vapours condense to form a cloud containing solid particles of weapon debris and small drops of water derived from the air sucked into the rising fireball. 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 continues to rise, it experiences resistance from the air, which pushes it down sideways, leading to the flattening of the top of the cloud, resulting in the characteristic mushroom shape.
The eventual height reached by the radioactive 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. However, if there is sufficient energy remaining, a portion of the cloud will ascend into the more stable air of the stratosphere. The cloud attains its maximum height after about 10 minutes and is then said to be "stabilized". It continues to grow laterally, producing the mushroom shape.
The mushroom 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 distribution of radiation in the mushroom cloud varies with the yield of the explosion, type of weapon, fusion-fission ratio, burst altitude, terrain type, and weather. Lower-yield explosions tend to have about 90% of their radioactivity in the mushroom head and 10% in the stem, while megaton-range explosions have most of their radioactivity in the lower third of the mushroom cloud. The fallout may appear as dry, ash-like flakes or as particles too small to be visible, deposited by rain. Large amounts of newer, more radioactive particles deposited on the skin can cause beta burns, presenting as discoloured spots and lesions.
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Natural occurrences
Mushroom clouds are the result of a massive release of heat. While they are most commonly associated with nuclear explosions, any sufficiently energetic detonation or deflagration will produce a similar effect.
Mushroom clouds can also occur naturally, for example, as a result of volcanic eruptions. The 1782 Franco-Spanish attack on Gibraltar, which resulted in a mushroom cloud, was described in a contemporary account as follows:
> "One of the attacking force's floating batteries exploding with a mushroom cloud after the British defenders set it ablaze by firing heated shot."
In 1798, Gerhard Vieth published a detailed and illustrated account of a cloud in the neighborhood of Gotha that was "not unlike a mushroom in shape". The cloud had been observed by legation counselor Lichtenberg a few years earlier on a warm summer afternoon. It was interpreted as an irregular meteorological cloud and seemed to have caused a storm with rain and thunder from a new dark cloud that developed beneath it. Lichtenberg stated that he had later observed somewhat similar clouds, but none as remarkable.
Another example of a natural occurrence of a mushroom cloud is the 2020 Beirut explosion. This explosion was caused by the ignition of 2,750 tons of ammonium nitrate that had been improperly stored in a warehouse. The resulting blast created a shock wave that devastated the city, killing at least 200 people and injuring more than 6,500.
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Frequently asked questions
A mushroom cloud is a cloud of debris, smoke, condensed water vapour, and solid particles that forms in the sky following an extremely large explosion.
A mushroom cloud is caused by the rapid release of heat from an explosion. The hot air rises very quickly, creating a vacuum that is then filled with smoke and debris, forming the visible central column of what will become the mushroom cloud.
The initial fireball of a mushroom cloud explosion expands spherically. However, once it reaches equilibrium with the surrounding atmospheric pressure, it stops growing and moves upward due to convection. The cloud continues to rise and flatten, forming the rounded cap of the mushroom.
Mushroom clouds are most commonly associated with nuclear explosions. The nuclear explosion that occurred in Hiroshima, Japan, in 1945, and the atomic bombing of Nagasaki, Japan, on August 9, 1945, are notable examples. However, mushroom clouds can also be formed by powerful conventional weapons, volcanic eruptions, forest fires, and impact events.
