Emma Wilson
Mushroom clouds are the result of a large explosion, often associated with nuclear detonations. However, any sufficiently powerful explosion or natural event, such as a volcanic eruption, can produce a similar mushroom-shaped cloud. The formation of this distinctive shape occurs due to several factors, including the initial spherical expansion of the blast, the interaction with the cooler atmosphere, the principles of buoyancy, and the subsequent vacuum formation, which together result in the iconic mushroom cloud.
| Characteristics | Values |
|---|---|
| Cause | Any massive release of heat, such as a nuclear explosion, conventional bomb, volcanic eruption, or impact event |
| Formation | Several phases, including early time (first ~20 seconds), rise and stabilization phase (20 seconds to 10 minutes), and late time (until about 2 days later) |
| Shape | Result of Rayleigh-Taylor instability, where lower-density gases form at high altitudes, causing a buoyant mass of gas to rise rapidly and form a vortex ring that draws up a central column of smoke, debris, and condensed water vapor |
| Color | Initially red or reddish-brown due to nitrous acid and oxides of nitrogen, then changes to white due to water droplets |
| Duration | Can persist in the atmosphere for about an hour until winds and air currents disperse it |
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What You'll Learn
Any large explosion can create a mushroom cloud
Mushroom clouds are most commonly associated with nuclear explosions. However, any sufficiently energetic detonation or deflagration will produce a similar effect. For instance, the 1917 Halifax Explosion produced a mushroom cloud, as did the detonation of a 2,750-ton stockpile of ammonium nitrate in Beirut in 2020.
When a bomb goes off, energy is released in all directions, initially forming a spherical fireball of hot air. As this fireball rises, it creates a vacuum, which is filled with smoke and debris, forming 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 dense enough to slow its ascent. The weight and density of the air then flatten the fireball and its trailing smoke, forming the rounded cap of the mushroom.
The formation of a mushroom cloud can be divided into 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. From 20 seconds to 10 minutes, the hot gases rise, and early fallout is deposited. Until about two days later, the airborne particles are distributed by the wind, deposited by gravity, and scavenged by precipitation. The shape of the cloud is influenced by the local atmospheric conditions and wind patterns.
For a mushroom cloud to form, an atmosphere is required, as it won't occur in a vacuum. The explosion must be sufficiently powerful to generate a large volume of lower-density gases at any altitude, causing a Rayleigh-Taylor instability. This instability results in the upward movement of air, forming strong air currents known as "afterwinds", which can draw dirt and debris into the cloud, forming the stem of the mushroom.
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The Rayleigh-Taylor instability phenomenon
The Rayleigh-Taylor instability occurs when a less dense fluid is accelerated into a more dense fluid. In the context of a mushroom cloud, the explosion results in the sudden formation of a large volume of low-density gases that start accelerating upwards rapidly against the higher-density gas above it. This upward movement leads to the formation of downward-directed turbulent vortices, creating a temporary vortex ring that forms the stem of the mushroom cloud. This phenomenon is also observed in supernova explosions, where expanding core gas is accelerated into denser shell gas.
The formation of a mushroom cloud can be divided into several phases. Initially, a fireball forms, and the fission products mix with the material aspirated from the ground or ejected from the crater. This is followed by the rise and stabilization phase, where hot gases rise, and early large fallout is deposited. Finally, the late-time phase occurs, where airborne particles are distributed by wind, deposited by gravity, or scavenged by precipitation. The shape of the cloud is influenced by local atmospheric conditions and wind patterns.
The Rayleigh-Taylor instability is named after Lord Rayleigh and G. I. Taylor, who studied the behaviour of fluids of different densities under the influence of gravity. In their experiments, they observed that when a higher-density fluid, such as oil, is suspended over a lower-density fluid, such as water, the system tends to reduce its total energy by displacing the denser fluid downward, leading to the upward movement of the less dense fluid. This disturbance continues to grow as the system seeks to reduce its potential energy.
The understanding of the Rayleigh-Taylor instability has practical applications beyond the formation of mushroom clouds. It is relevant in various fields, including plasma fusion reactors, inertial confinement fusion, and the study of supernova explosions. Additionally, the instability provides a springboard into the mathematical study of stability and the investigation of quantum phenomena in Bose-Einstein condensates.
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The height of the explosion matters
The height of the explosion determines the amount of afterwinds generated and their strength. If the explosion occurs closer to the ground, stronger afterwinds are produced, pulling in larger amounts of dirt and debris from the ground. This results in the formation of a more prominent stem in the mushroom cloud. The stem and cap of the cloud may even merge into the classic mushroom profile, as observed by researcher Katie Lundquist.
The height of burst also influences the colour of the mushroom cloud. Initially, the cloud is 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 due to the formation of water droplets, similar to an ordinary cloud.
Additionally, the height of the explosion impacts the overall height of the mushroom cloud. The fireball continues to rise until it reaches equilibrium with the surrounding air, which occurs at higher altitudes in the atmosphere for nuclear explosions, typically in the ozone layer. Therefore, a higher explosion will result in a taller mushroom cloud.
Furthermore, the height of the explosion can determine whether a mushroom cloud forms at all. If the explosion occurs significantly below ground level or deep underwater, there is no mushroom cloud. Instead, a vast amount of earth or water vaporizes, creating a bubble that collapses in on itself, forming a subsidence crater.
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Natural occurrences of mushroom clouds
Mushroom clouds are most commonly associated with nuclear explosions, but they can occur naturally and have been observed long before the Atomic Age. Natural mushroom clouds can be produced by any massive release of heat, such as volcanic eruptions, forest fires, and impact events.
Volcanoes, for example, can generate mushroom clouds of smoke and dust due to the massive release of heat and energy. Similarly, forest fires can produce mushroom-shaped clouds of water vapour, known as pyrocumulus. These clouds are formed by the updraft of hot air and gases rising through the atmosphere, creating a vacuum that is filled with smoke and debris, ultimately resulting in the characteristic mushroom shape.
Historical accounts and artworks provide evidence that mushroom clouds were observed and described centuries before the Atomic Age. For instance, in 1782, an unknown artist depicted a floating battery exploding with a mushroom cloud during the Franco-Spanish attack on Gibraltar. In 1798, Gerhard Vieth published an illustrated account of a cloud in the neighbourhood of Gotha that resembled a mushroom in shape.
Even in modern times, natural occurrences of mushroom clouds have been witnessed. For example, the 2020 Beirut explosion resulted in a mushroom cloud, demonstrating that powerful conventional weapons or accidents can also create such cloud formations without the involvement of nuclear detonations.
It is important to note that the formation of mushroom clouds relies on the presence of an atmosphere, as it provides the necessary fluid material for the cloud to take shape. In the absence of an atmosphere, such as on the moon, a mushroom cloud would not occur, as there would be no distortion of the initial sphere into a torus shape and no difference in air densities to facilitate the growth of the cloud.
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The colour of the cloud changes
The colour of the mushroom cloud changes. The initial colour of some radioactive clouds is red, reddish-brown, or reddish, due to the presence of nitrous acid, nitrogen dioxide, nitric acid, and oxides of nitrogen, formed from initially ionized nitrogen, oxygen, and atmospheric moisture. Yellow, orange, violet, blue, and green hues have also been described. The reddish hue is later obscured by the white colour of water/ice clouds, condensing out of the fast-flowing air as the fireball cools, and the dark colour of smoke and debris sucked into the updraft. The white colour is caused by the water droplets, like in an ordinary cloud.
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 cloud may continue to be visible for about an hour or more before being dispersed by the winds into the surrounding atmosphere, where it merges with natural clouds in the sky.
The colour changes are due to the cooling of the fireball, which causes the temperature at its centre to fall, changing the colour of the fireball to red and yellow, which appear through the cloud produced around it. The expansion of the fireball also produces a drop in the pressure and temperature of the surrounding air, turning vapour into water droplets and the fireball into fleecy, white clouds.
The colour changes of the mushroom cloud can also be influenced by the height of the burst and local atmospheric conditions and wind patterns. For example, when the detonation altitude is low enough, the afterwinds will draw in dirt and debris from the ground below to form the stem of the mushroom cloud. The higher the cloud rises, the more it will spread out. If the cloud reaches the tropopause, about 6-8 miles above the Earth's surface, it may spread against the wind.
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Frequently asked questions
Mushroom clouds are the result of a Rayleigh-Taylor instability, which occurs when a large volume of lower-density gases is formed at any altitude. The buoyant mass of gas rises rapidly, forming a temporary vortex ring that draws up a central column of smoke, debris, condensed water vapour, or a combination of these, to form the "stem" of the mushroom.
No. Any sufficiently energetic detonation or deflagration will produce a similar effect. They can be caused by powerful conventional weapons, volcanic eruptions, or impact events.
Mushroom clouds contain solid particles of weapon debris, small drops of water derived from the air, and dirt and debris from the ground below.
The cloud is often described as having a "stem" and a "cap". The stem is brown and the cap is white. The stem and cap do not always meet, but when they do, the classic mushroom profile is formed.
Depending on weather conditions, a mushroom cloud can persist in the atmosphere for about an hour until winds and air currents disperse it.
