Sarah Brown
Mushroom clouds are formed by large explosions, most famously nuclear explosions. However, contrary to popular belief, mushroom clouds can also occur without a nuclear component. The shape of the cloud is influenced by local atmospheric conditions and wind patterns. When a fireball is formed by an explosion, it creates a very hot bubble of gas that rises and expands. As it rises, it cools and interacts with the air, creating a rolling motion. This forms a temporary vortex ring that draws up a central column of smoke, debris, condensed water vapour, or a combination of these, forming the mushroom stem.
| Characteristics | Values |
|---|---|
| Formation | Mushroom clouds are formed by large explosions under Earth's gravity. They are best known for their appearance after nuclear detonations. |
| Altitude | Detonations that occur significantly below ground level or deep below water do not produce mushroom clouds. |
| Explosion type | Any sufficiently energetic detonation or deflagration will produce a mushroom cloud. They can be caused by powerful conventional weapons, including large thermobaric weapons. |
| Explosion process | A mushroom cloud forms when an explosion creates a very hot bubble of gas. The hot air is buoyant, so it rises and expands, creating a powerful updraft that picks up dust, forming the stem of the mushroom cloud. |
| Phases of formation | Early time (first ~20 seconds), rise and stabilization phase (20 seconds to 10 minutes), late time (until ~2 days later). |
| Appearance | The cloud could appear white or dirty brown depending on whether the explosion occurs on the Earth's surface. |
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What You'll Learn
Explosions and altitude
The formation of a mushroom cloud is influenced by the altitude of the explosion. The shape of the cloud is not dependent on the explosion being nuclear; rather, it is the result of a Rayleigh-Taylor instability formed when a large volume of low-density gases is suddenly formed at any altitude. This buoyant mass of hot gas rises rapidly, creating vortices that curl downward, forming a temporary vortex ring that draws up a central column. This central column, or "stem", is formed from smoke, debris, condensed water vapour, or a combination of these elements.
The altitude of the explosion determines whether the stem of the mushroom cloud will form. A detonation high above the ground may produce a mushroom cloud without a stem. When the detonation altitude is low enough, afterwinds will draw in dirt and debris from the ground, forming the stem of the mushroom cloud. The fireball from an H-bomb rises so high that it hits the tropopause, the boundary between the troposphere and the stratosphere. The hot bubble of the fireball expands and rises, but by the time it reaches the tropopause, it is no longer hot enough to break through the boundary. This causes the fireball to flatten out and expand sideways into the characteristic mushroom shape.
The formation of a mushroom cloud can be divided into several phases. In the early phase, lasting about 20 seconds, the fireball forms, and fission products mix with material from the ground or ejected from the crater. This is followed by the rise and stabilization phase, lasting from 20 seconds to 10 minutes, during which hot gases rise, and early large fallout is deposited. The late phase occurs until about 2 days later, when airborne particles are distributed by wind, deposited by gravity, or scavenged by precipitation. The shape of the cloud during this phase is influenced by local atmospheric conditions and wind patterns.
The mushroom cloud is 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, including large thermobaric weapons. Some volcanic eruptions and impact events can also produce natural mushroom clouds.
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Rayleigh-Taylor instability
Mushroom clouds are formed by large explosions, usually associated with nuclear detonations. However, any sufficiently energetic explosion or deflagration will produce a similar effect, including powerful conventional weapons, large thermobaric weapons, and some volcanic eruptions.
The formation of a mushroom cloud can be explained in several phases. Firstly, the fireball forms, and fission products mix with the material from the ground or the crater. This is followed by the rise and stabilization phase, where hot gases rise, and early fallout is deposited. Finally, until about two days later, airborne particles are distributed by the wind, deposited by gravity, or scavenged by precipitation. The shape of the cloud is influenced by local atmospheric conditions and wind patterns.
The formation of a mushroom cloud is also associated with Rayleigh-Taylor instability. This phenomenon can be understood by considering the behaviour of fluids of different densities under the influence of Earth's gravity. In a system with a higher-density fluid suspended over a lower-density fluid, the system's total energy would be lowered if the higher-density fluid moves downward, reducing its potential energy. This disturbance leads to an upward movement of the lower-density fluid to compensate. This instability was studied by Lord Rayleigh and further developed by G. I. Taylor, who observed that a similar situation occurs when a less dense fluid is 'accelerated' into a more dense fluid—this is precisely what happens during a mushroom cloud formation.
The sudden explosion near the surface creates a large volume of low-density gases that start accelerating upwards rapidly against the higher-density gas above it. This rapid upward movement forms downward-directed turbulent vortices, creating a temporary 'vortex ring', which forms the stem of the mushroom cloud. As the low-density air continues to rise, it eventually reaches an equilibrium altitude where it is no longer at a lower density than the surrounding atmosphere. At this point, the upward movement stops, and the cloud begins to flatten and expand sideways, forming the characteristic mushroom shape.
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Formation of the stem
The formation of the stem of a mushroom cloud is a result of several factors and processes that occur during and after a large explosion. Firstly, it is important to understand that the stem is not always present in a mushroom cloud. Detonations that occur high above the ground may produce a mushroom cloud without a distinct stem.
The formation of the stem is closely linked to the altitude of the explosion. When the detonation occurs at a low altitude, the resulting afterwinds—strong air currents produced by the Rayleigh-Taylor instability—draw in dirt, dust, debris, and vapours from the ground below. This material forms the stem of the mushroom cloud. The Rayleigh-Taylor instability is caused by the interaction of cool air underneath the fireball with the hot gases of the fireball itself, creating a vortex that sucks in air and fuels the formation of the stem.
The fireball, a spherical mass of hot, incandescent gases formed during the explosion, plays a crucial role in the development of the stem. As the fireball rises, it cools, and the vapours condense to form a cloud containing solid particles of weapon debris and small water droplets derived from the air. This upward movement and condensation contribute to the formation of the stem as they create an upward draft, pulling in more dust and debris.
The size and yield of the explosion also influence the formation of the stem. Thermonuclear weapons, or hydrogen bombs, produce the characteristic flat, mushroom-shaped clouds due to the immense size of the explosion. The fireball rises to a higher altitude, encountering different atmospheric layers and temperature gradients, which affect the shape of the cloud and the formation of the stem.
In summary, the stem of a mushroom cloud is formed by the interaction of the explosion's by-product gases, the Rayleigh-Taylor instability, the upward movement of the fireball, condensation of vapours, and the drawing in of dirt, dust, and debris from the ground below. The specific conditions of the explosion, including its altitude and yield, determine the presence and characteristics of the stem.
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Rise and stabilisation
The rise and stabilisation phase of a mushroom cloud occurs approximately 20 seconds to 10 minutes after the initial explosion. During this phase, the hot gases rise, and the early large fallout is deposited. The fireball increases 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. This cloud may appear white and cloud-like or dirty brown, depending on whether the explosion occurred on the Earth's surface, as debris and dirt will be vaporised and sucked into the explosion in the latter case.
As the fireball rises, a Rayleigh-Taylor instability is formed, and air is drawn upwards and into the cloud, creating strong air currents known as "afterwinds". If the detonation altitude is low enough, these afterwinds will draw in dirt and debris from the ground to form the stem of the mushroom cloud. The ascent stops once the mass of hot gases reaches its equilibrium level, and the cloud begins to flatten into its characteristic mushroom shape, often assisted by surface growth from decaying turbulence.
The formation of the stem of the mushroom cloud can be explained by the principle of buoyancy. The hot air in the fireball is buoyant, so it rises and expands quickly. This rising cloud creates a powerful updraft that picks up dust and debris, forming the stem. The central part of the fireball is the hottest, creating a rolling motion as it interacts with the outer portions. This results in turbulent vortices curling downward around its edges, forming a temporary vortex ring that draws up a central column, possibly with smoke, debris, condensed water vapour, or a combination of these.
The fireball's shape changes from a sphere to a violently rotating spheroidal vortex due to atmospheric friction and energy radiation cooling the surface of the fireball. A Rayleigh-Taylor instability is formed as the cool air underneath initially pushes the bottom fireball gases into an inverted cup shape, causing turbulence and a vortex that sucks in more air, creating external afterwinds and further cooling the fireball.
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Distribution of particles
The distribution of particles in a mushroom cloud is influenced by various factors, including the local atmospheric conditions, wind patterns, and the altitude of the explosion. During the early stages of a mushroom cloud formation, the first 20 seconds are crucial for the initial distribution of particles. This is when the fireball forms, and the fission products mix with the material aspirated from the ground or ejected from the crater. The condensation of evaporated ground particles occurs within the first few seconds, with temperatures in the fireball ranging from 3500 to 4100 Kelvin.
As the mushroom cloud enters the rise and stabilization phase, which lasts from 20 seconds to 10 minutes, the hot gases rise, and early large fallout particles are deposited. The distribution of these particles is primarily influenced by wind patterns, creating a downwind plume. However, if the cloud reaches the tropopause, it may spread against the wind due to its higher convection speed compared to the ambient wind speed.
In the late-time phase, which lasts until about two days after the explosion, the remaining airborne particles continue to be distributed by wind and gravity. Additionally, these particles are scavenged by precipitation, which can lead to radioactive fallout in the case of nuclear explosions. The distribution of particles during this phase can cover vast distances, depending on the intensity of the explosion and the prevailing weather conditions.
It is important to note that the distribution of particles in a mushroom cloud is not limited to the immediate aftermath of the explosion. In the case of radioactive particles from nuclear detonations, the distribution can have long-term effects. These particles can contaminate the environment, leading to radioactive fallout over extended areas. The impact of the distributed particles can persist for extended periods, posing risks to human health, wildlife, and the ecosystem.
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Frequently asked questions
A mushroom cloud is a distinctive mushroom-shaped flammagenitus cloud of debris, smoke, and usually condensed water vapour resulting from a large explosion.
Mushroom clouds are formed by many sorts of large explosions under Earth's gravity. They are best known for their appearance after nuclear detonations.
A mushroom cloud undergoes several phases of formation. Early time, the first ~20 seconds, when the fireball forms and the fission products mix with the material aspired from the ground or ejected from the crater. The second phase is the rise and stabilization phase, lasting from 20 seconds to 10 minutes, when the hot gases rise up and early large fallout is deposited. The final phase occurs until about 2 days later, when the airborne particles are distributed by wind, deposited by gravity, and scavenged by precipitation.
Regular clouds are formed when warm air and water vapour rise and cool, and the vapour condenses into water droplets that are too small to fall as rain. Mushroom clouds, on the other hand, are the result of large explosions that create a very hot bubble of gas or a fireball. The vapours condense to form a cloud containing solid particles of weapon debris and small drops of water.
Yes, some volcanic eruptions and impact events can produce natural mushroom clouds.
