Laura Smith
Mushroom clouds are the result of a massive release of heat, such as a thermonuclear explosion. They can be formed by any explosion of sufficient heat and magnitude, including non-nuclear explosions. The formation of a mushroom cloud occurs in several phases, from the initial fireball and mixing of fission products with ground material, to the rise and stabilization phase, and finally, the late-time phase when particles are distributed by wind and deposited by gravity. The iconic mushroom shape is a result of the upward flow of hot air, creating a vacuum that is filled with smoke and debris, forming the stem of the mushroom.
| Characteristics | Values |
|---|---|
| Formation | The sudden formation of a large volume of lower-density gases at any altitude, causing a Rayleigh-Taylor instability |
| Shape | The upward flow of air after the explosion hits the smoke from the explosion |
| Contributing factors | Explosion, heat, fireball, vacuum |
| Examples | Nuclear explosions, Beirut explosion, volcanic eruptions |
| Phases | Early time (first 20 seconds), rise and stabilization phase (20 seconds to 10 minutes), late time (until about 2 days later) |
| Fallout | Radioactive particles, radiation |
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What You'll Learn
Formation
Mushroom clouds are formed by a massive release of heat, which can be from a nuclear explosion, volcanic eruption, or conventional explosions involving a few kilograms of TNT. They are not unique to nuclear blasts, though nuclear explosions usually result in larger mushroom clouds.
When a bomb detonates, energy is released in all directions, initially forming a spherical fireball. As the fireball increases in size, it cools, and the vapors condense to form a cloud containing solid particles of weapon debris and small drops of water from the air. This is the first phase of mushroom cloud formation, which occurs in the first 20 seconds.
In the next phase, the hot gases rise, and early large fallout is deposited. This is the "Rise and stabilization phase," which lasts from 20 seconds to 10 minutes. The buoyant mass of gas rises rapidly, resulting in turbulent vortices curling downward around its edges, forming a temporary vortex ring that draws up a central column of smoke, debris, and condensed water vapor. This central column forms the "'stem' of the mushroom cloud.
The mass of gas continues to rise until it reaches an altitude where it is no longer of lower density than the surrounding air. At this point, it disperses and drifts back down, resulting in fallout. The stabilization altitude depends on the temperature, dew point, and wind shear in the air at and above the starting altitude. The shape of the cloud is influenced by local atmospheric conditions and wind patterns.
The final phase occurs until about two days after the explosion, when airborne particles are distributed by the wind, deposited by gravity, or scavenged by precipitation.
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Phases of formation
Mushroom clouds can be formed by any massive release of heat, such as a volcanic eruption, a nuclear explosion, or a conventional explosion. They are not unique to nuclear explosions, but these explosions usually result in larger mushroom clouds.
Early Time
The first phase of formation occurs in the first 20 seconds, when a fireball forms and the fission products mix with the material aspirated from the ground or ejected from the crater. The condensation of evaporated ground occurs in the first few seconds, most intensely during fireball temperatures between 3500 and 4100 Kelvin. The colour of the cloud is initially red or reddish-brown due to the presence of nitrous acid and oxides of nitrogen.
Rise and Stabilization
In the next phase, from 20 seconds to 10 minutes, hot gases rise, and early large fallout is deposited. The fireball continues to increase in size and cools, and the 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 cloud continues to rise and flatten, forming the rounded cap of the mushroom.
Late Time
The final phase occurs until about two days later, when the airborne particles are distributed by the wind, deposited by gravity, and scavenged by precipitation. The fallout distribution is predominantly a downwind plume. However, if the cloud reaches the tropopause, it may spread against the wind because its convection speed is higher than the wind speed.
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Mushroom clouds and nuclear explosions
Mushroom clouds are the result of a large explosion, often nuclear, that releases a massive amount of heat. The explosion creates a vacuum as the blast wave of hot air rises, which is then filled with smoke and debris, forming a central column. This column of smoke and debris is the "stem" of the mushroom cloud. The fireball of hot, incandescent gases changes shape due to atmospheric friction, and the surface is cooled by energy radiation, turning from a sphere to a violently rotating vortex. This vortex sucks more air into its centre, creating external afterwinds and further cooling the fireball.
The upward flow of air after the explosion is what forms the mushroom cap. The cloud 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 will spread out. However, if there is sufficient energy remaining, a portion of the cloud will enter the stratosphere. The cloud will continue to grow laterally, forming the characteristic mushroom shape.
Mushroom clouds are most commonly associated with nuclear explosions, but 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 were described centuries before the Atomic Age, with an example from 1782 depicting the Franco-Spanish attack on Gibraltar showing one of the attacking force's floating batteries exploding with a mushroom cloud.
Nuclear weapons are usually detonated above the ground to maximize the effect of their expanding fireball and blast wave. The fireball formed by a nuclear explosion is composed of incandescent gases, which change shape due to atmospheric friction and cool over time. 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 due to the formation of water droplets. The heads of the clouds consist of highly radioactive particles, which are usually dispersed by the wind. However, weather patterns, especially rain, can produce nuclear fallout.
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Mushroom clouds and radioactive particles
Mushroom clouds are the result of a massive release of heat, which can be from a nuclear explosion, volcanic eruption, or even a warehouse fire. When a bomb detonates, energy is released in all directions, forming a sphere of hot air. However, due to buoyancy, the middle column of this sphere rises faster, causing the sphere to distort into a torus or doughnut shape. This creates a vacuum, which is quickly filled with smoke, debris, and condensed water vapour, forming the "stem" of the mushroom cloud. The cloud continues to rise and flatten, eventually reaching an altitude where it is no longer less dense than the surrounding air, at which point it disperses.
The formation of a mushroom cloud can be divided into several phases. In the early moments, a fireball forms, and fission products mix with material aspirated from the ground or ejected from a crater. This is followed by the "rise and stabilization phase", lasting from 20 seconds to 10 minutes, during which hot gases rise, and early fallout occurs. The final phase occurs until about two days later, when airborne particles are distributed by wind, deposited by gravity, or scavenged by precipitation.
The iconic shape of a mushroom cloud is not unique to nuclear explosions, but they are typically associated with them due to their distinctive appearance. The height at which a bomb explodes and its explosive yield determine the features of the resulting mushroom cloud. When the bomb explodes closer to the ground, the stem and cap of the cloud merge into the classic mushroom profile.
Mushroom clouds are not only visually striking but also carry radioactive particles that can have significant consequences. Radioactive fallout from nuclear explosions, such as the Castle Bravo test, can be carried by winds over long distances, affecting areas outside the predicted danger zones and leading to evacuations. The radiation can cause observable fluorescence, creating an aura of blue-violet-purple light surrounding the head of the mushroom cloud due to the ionization of oxygen and nitrogen.
The study of mushroom clouds and the behaviour of radioactive particles during and after a nuclear explosion is crucial for understanding fallout patterns and providing guidance on consequence management to protect public health in the event of a nuclear crisis.
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Mushroom clouds in nature
Mushroom clouds are most commonly associated with nuclear explosions. However, they can also form from any sufficiently energetic detonation or deflagration, including powerful conventional weapons. Some volcanic eruptions and impact events can produce natural mushroom clouds.
Mushroom clouds are the result of a Rayleigh-Taylor instability, which occurs when a large volume of lower-density gases forms at any altitude. The buoyant mass of gas rises rapidly, creating turbulent vortices that curl downward around its edges. This forms a temporary vortex ring that draws up a central column of smoke, debris, condensed water vapour, or a combination of these elements, forming the "mushroom stem".
The formation of a mushroom cloud can be divided into several phases. In the early phase, a fireball forms, and fission products mix with material aspirated from the ground or ejected from a crater. The condensation of evaporated ground occurs in the first few seconds, most intensely during fireball temperatures between 3500 and 4100 Kelvin. The rise and stabilization phase lasts from 20 seconds to 10 minutes, during which hot gases rise, and early large fallout is deposited. In the late phase, which can last up to two days, airborne particles are distributed by wind, deposited by gravity, or scavenged by precipitation.
The shape of a mushroom cloud is influenced by local atmospheric conditions and wind patterns. While the cloud typically spreads in the direction of the wind, it may spread against the wind if it reaches the tropopause due to its higher convection speed compared to the ambient wind speed. The cloud's colour changes from reddish-brown to white as it cools, mainly due to the formation of water droplets.
Mushroom clouds have been observed in nature for centuries, predating the Atomic Age. For example, 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.
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Frequently asked questions
A mushroom cloud is the result of a large volume of lower-density gases forming at any altitude, causing a Rayleigh-Taylor instability. The mass of gas rises rapidly, forming a vortex ring that draws up a central column of smoke, debris, and condensed water vapour.
Mushroom clouds are most commonly associated with nuclear explosions, but they can be caused by any massive release of heat. For example, the 2020 Beirut explosion formed a mushroom cloud.
Mushroom clouds form above explosions because heat rises. The upward flow of hot air after the explosion meets the smoke from the blast, forming the "cap" of the mushroom.
Mushroom clouds have a "cap" and a "stem". The cap is formed by the rising hot air and smoke, while the stem is a vertical column of dirt and debris sucked up from beneath the explosion.
