Emily Johnson
Mushroom clouds are formed by large explosions, but they are most commonly associated with the aftermath of nuclear detonations. The upward movement of the mushroom cloud is due to 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 and create a temporary vortex ring. This upward movement of gas, along with entrained moist air, eventually reaches an altitude where it is no longer less dense than the surrounding air, leading to dispersion and fallout. The shape and height of the mushroom cloud are influenced by factors such as temperature, dew point, wind shear, and atmospheric conditions.
| Characteristics | Values |
|---|---|
| Height reached by the cloud | Depends on the heat energy of the weapon and atmospheric conditions; if it reaches the tropopause, it will spread out |
| Color | Initially red or reddish-brown due to the presence of nitrous acid and oxides of nitrogen; changes to white due to water droplets |
| Formation | Sudden formation of a large volume of lower-density gases at any altitude, causing Rayleigh-Taylor instability; the buoyant mass of gas rises, forming a temporary vortex ring that draws up a central column of smoke, debris, condensed water vapor, or a combination |
| Explosion | Occurs in all directions, but the mushroom cloud forms due to gravity and the atmosphere; the explosion initially expands spherically but reaches equilibrium with atmospheric pressure and stops growing |
| Heat | A massive release of heat from the explosion interacts with the cooler surrounding air, making it less dense |
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What You'll Learn
Large explosions under Earth's gravity
Mushroom clouds are formed by large explosions under Earth's gravity. They are best known for appearing after nuclear detonations, but they can be created by any massive release of heat. Nuclear weapons are usually detonated above the ground to maximise the effect of their expanding fireball and blast wave.
When a bomb goes off, energy is released in all directions. This energy forms a sphere of hot air that rises rapidly through the atmosphere, creating a vacuum in its wake. The vacuum is immediately filled with smoke and debris, forming the central column of what will become the mushroom cloud. The hot air molecules move around rapidly, bouncing off each other at high velocities, creating space between themselves and forming a near-vacuum. This movement of hot air molecules is what causes the cloud to rise.
The fireball soon reaches a point in the atmosphere where the air is cold enough and dense enough to slow its ascent. The weight and density of the air flatten the fireball and its trailing smoke, causing it to take on the shape of a mushroom cap. The fireball continues to rise until it reaches an area of the atmosphere where the air is no longer cool. This is high up in the atmosphere, where ozone heats up the surrounding gas by absorbing harmful solar radiation.
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 will spread out. If it has sufficient energy remaining, a portion of it will ascend into the stratosphere, where it will stabilise after about 10 minutes. It will continue to grow laterally, producing the characteristic mushroom shape. The cloud may remain visible for about an hour or more before being dispersed by the wind.
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Rayleigh-Taylor instability
The Rayleigh-Taylor instability is a phenomenon observed in fluid mechanics. It occurs when a less dense fluid is accelerated into a denser fluid, which is what happens during a mushroom cloud explosion.
The explosion creates a large volume of low-density gases that rapidly accelerate upwards against the higher-density gas above it. This upward movement leads to the formation of turbulent vortices that curl downward, forming a temporary vortex ring that creates the ""stem" of the mushroom cloud. This is similar to the movement of fluids in a lava lamp, where the less dense fluid accelerates and moves upwards into the denser fluid.
The Rayleigh-Taylor instability can be explained by the principle that any system tends to achieve a state of minimum total energy. In the case of two fluids with different densities, the total energy of the system is lowered when the higher-density fluid moves down, reducing its potential energy. This disturbance will cause the lower-density fluid to move upwards, releasing more potential energy. This movement continues as the system seeks to minimize its potential energy.
The Rayleigh-Taylor instability is a key factor in the formation of mushroom clouds, along with the upward acceleration caused by the explosion. The size of the blast determines how quickly the Rayleigh-Taylor instability develops and how fast the mushroom cloud forms. The atmospheric conditions, such as wind, temperature, and altitude, also influence the growth of the instability and the final shape of the mushroom cloud.
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Fallout
The fallout from a mushroom cloud can have devastating consequences. The initial explosion of a nuclear bomb creates a fireball of fission products, which mixes with the material from the ground or the crater. This fireball rises rapidly, forming a vacuum that is filled with smoke and debris, which becomes the central column of the mushroom cloud. The fireball continues to rise until it reaches an altitude where the air is dense enough to slow its ascent. At this point, the top of the cloud flattens, forming the cap of the mushroom. The cloud continues to rise and spread out laterally, producing the characteristic mushroom shape. The cloud may be visible for about an hour before being dispersed by the wind, but the radioactive fallout can persist for much longer.
The extent of the fallout depends on various factors, including the height of the burst, the 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 in the cloud, a portion of it can ascend into the stratosphere. The height of the mushroom cloud is also influenced by the Rayleigh-Taylor instability, which is caused by the sudden formation of a large volume of lower-density gases.
The colour of the mushroom cloud can provide some indication of the radioactive fallout. Initially, the cloud is 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 reddish hue is obscured by the white colour of the water clouds and the dark colour of smoke and debris sucked into the updraft. A higher-yield detonation can carry these nitrogen oxides high enough into the atmosphere to cause depletion of the ozone layer.
The distribution of the fallout is predominantly a downwind plume, but if the cloud reaches the tropopause, it may spread against the wind due to its higher convection speed. The fallout can include radioactive particles, which can have severe health and environmental impacts. The prediction of fallout distribution is crucial for providing guidance on consequence management and protecting public health in the event of a nuclear crisis.
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Radioactive cloud height
The height of a radioactive mushroom cloud depends on several factors, including the heat energy of the weapon, atmospheric conditions, and the presence of a gravity well. During the initial phase of a mushroom cloud formation, which lasts about 20 seconds, a fireball forms as fission products mix with material from the ground. This is followed by a rise and stabilization phase, lasting from 20 seconds to 10 minutes, during which hot gases rise and early large fallout is deposited. The cloud attains its maximum height after about 10 minutes and is then considered stabilized, but it continues to grow laterally, forming the characteristic mushroom shape.
The Rayleigh-Taylor instability is a fluid mechanics phenomenon that occurs when a mushroom cloud forms. This instability causes air to be drawn upwards and into the cloud, creating strong air currents known as "afterwinds." The height of the burst also influences the amount of dirt and debris sucked into the cloud, with surface bursts producing darker mushroom clouds due to the inclusion of irradiated material from the ground.
The height of the radioactive cloud further determines the type of fallout. If the detonation occurs at a sufficient altitude, the fireball avoids mixing significantly with ground debris, and the radioactive byproducts stay aloft longer. This additional time allows the most dangerous radioactive elements with the shortest half-lives to decay before descending, reducing the overall radioactive intensity of the fallout. In contrast, detonations near the surface produce more radioactive fallout with larger particles that readily deposit locally.
The cloud's colour can also provide clues about its height. Initially, the presence of nitrogen dioxide and nitric acid, formed from ionized nitrogen, oxygen, and atmospheric moisture, gives the cloud a reddish hue. As the fireball cools, the white colour of water/ice clouds begins to obscure the reddish tint. If the cloud reaches the tropopause, about 6-8 miles above the Earth's surface, it tends to spread out, and a portion may ascend into the stratosphere if sufficient energy remains.
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Cooling and condensation
The formation of a mushroom cloud is a result of the cooling and condensation of a large volume of lower-density gases at any altitude, causing a Rayleigh-Taylor instability. The Rayleigh-Taylor instability occurs when there is a sudden release of energy, which heats up the surrounding air, causing it to expand and rise rapidly, creating a vacuum that is then filled by the surrounding air. This rising bubble of hot air forms the "'stem' of the mushroom cloud.
As the fireball cools and condensation occurs, the colour changes from reddish-brown to white, mainly due to the water droplets condensing out of the fast-flowing air. The water vapour condenses to form a cloud containing solid particles of weapon debris and small drops of water. The vapours condense into clouds, similar to the process that forms ordinary clouds. The condensation of evaporated ground occurs most intensely during fireball temperatures between 3500 and 4100 Kelvin.
The fireball continues to rise until it reaches an altitude where the surrounding air is no longer cool. At this point, the cloud stabilizes and stops rising. The stabilization altitude depends on the temperature, dew point, and wind shear in the air. The cloud may continue to grow laterally, producing the characteristic mushroom shape.
The cooling and condensation process also affects the shape of the mushroom cloud. The entrainment of higher-humidity air, combined with the drop in pressure and temperature, leads to the formation of skirts and bells around the stem. If the water droplets become large enough, they can descend, creating a rising stem with a descending bell around it. Additionally, the displaced gas, which is at a lower temperature, trickles down the sides of the column and is then sucked back in, contributing to the mushroom shape.
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Frequently asked questions
Mushroom clouds are the result of large explosions, often nuclear, that cause a Rayleigh-Taylor instability. 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.
The shape of a mushroom cloud is influenced by local atmospheric conditions and wind patterns. The cloud may spread out if it reaches the tropopause, but if there is sufficient energy remaining, a portion of the cloud will ascend into the stratosphere.
The initial colour of a mushroom cloud 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, and then to dark as smoke and debris are sucked into the updraft.
