Michael Davis
Mushroom clouds are the result of a massive release of heat, most commonly associated with nuclear explosions. However, any sufficiently energetic detonation or deflagration will produce a similar effect. They can be caused by powerful conventional weapons, including thermobaric weapons, as well as some volcanic eruptions and impact events. The iconic mushroom shape is formed when a bubble of hot gas rises and expands, creating a powerful updraft that picks up dust and debris, forming the stem of the mushroom cloud. As the cloud rises, it reaches a point in the atmosphere where the air is cold enough and dense enough to slow its ascent, causing it to flatten and spread out into the rounded cap of the mushroom.
| Characteristics | Values |
|---|---|
| Cause | Any massive release of heat |
| Formation | Explosion creates a very hot bubble of gas that rises and expands |
| Shape | Mushroom-shaped flammagenitus cloud |
| Composition | Debris, smoke, condensed water vapour |
| Colour | Initially red or reddish-brown, later obscured by white colour of water/ice clouds |
| Height | Reaches maximum height in about 10 minutes and is then stabilized |
| Duration | Persists in the atmosphere for about an hour |
| Fallout | Dry, ash-like flakes or invisible particles |
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What You'll Learn
A large explosion
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 continue to ascend into the stratosphere. The cloud will continue to grow laterally, forming the characteristic mushroom shape. The final height is reached in about 10 minutes, and the cloud may persist for about an hour before winds disperse it.
The mushroom cloud is not unique to nuclear explosions, although they are most commonly associated with them. Any sufficiently large release of heat or energetic detonation can produce a similar effect. For example, powerful conventional weapons, such as thermobaric weapons, can create mushroom clouds. Additionally, some natural phenomena, like volcanic eruptions and impact events, can also generate mushroom clouds.
The size and shape of the mushroom cloud can vary depending on the explosive yield and the height of detonation. The stem and cap of the cloud may merge into the classic mushroom profile for explosions stronger and/or closer to the ground. The radioactive fallout from the explosion may appear as dry, ash-like flakes or microscopic particles.
The phenomenon of mushroom clouds has been observed and recorded throughout history. For example, in 1798, Gerhard Vieth published an account of a mushroom-shaped cloud, and in 1937, a report described a Japanese attack on Shanghai that generated "a great mushroom of smoke". More recently, in the 21st century, online forums have discussed the science behind mushroom clouds, specifically those resulting from atomic bombs.
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Heat rises
The formation of a mushroom cloud is dependent on multiple factors, one of the most important being heat. The initial explosion creates a fireball that expands spherically in all directions. This fireball is a very hot bubble of gas or incandescent gases that rises due to convection. The extremely hot air of the explosion is warmer than the air above it, making it more buoyant and causing it to ascend rapidly. As the fireball rises, it cools and interacts with the surrounding atmospheric friction, changing from a spherical to a spheroidal shape.
The fireball continues to rise until it reaches a point in the atmosphere where the air is cold and dense enough to slow and eventually stop its ascent. This is typically around 6-8 miles above the Earth's surface, at the boundary between the troposphere and the stratosphere known as the tropopause. The fireball is no longer hot enough to break through this boundary, so it flattens out and expands laterally, forming the rounded cap of the mushroom cloud.
The formation of the mushroom cloud's stem is a result of the rising hot air creating a powerful updraft or vortex that draws in smoke, debris, and condensed water vapour. This central column of smoke and debris forms the visible stem of the mushroom cloud. The stem is most prominent when the explosion occurs near the ground, as the ground acts as a backstop for the blast to push against. The ground and nearby surroundings also contribute to the mushroom cloud's formation by reflecting and radiating heat and energy upward.
The height and shape of the mushroom cloud can vary depending on atmospheric conditions and the explosive yield of the bomb. If the cloud reaches the tropopause and still retains sufficient energy, it can ascend into the more stable air of the stratosphere. The cloud continues to grow laterally, and its distinctive shape may persist in the atmosphere for about an hour until winds and air currents disperse it.
In summary, the formation of a mushroom cloud is a complex process that involves the interaction of heat, atmospheric conditions, and the explosive force. The heat generated by the explosion creates a buoyant mass of hot air that rises, cools, and interacts with the surrounding atmosphere to form the characteristic mushroom shape.
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Smoke and debris
A mushroom cloud is a distinctive cloud of smoke, debris, and usually condensed water vapour resulting from a large explosion. The effect is most commonly associated with a nuclear explosion, but any sufficiently energetic detonation or deflagration will produce a similar effect. They can be caused by powerful conventional weapons, including thermobaric weapons such as the ATBIP and GBU-43/B MOAB. Some volcanic eruptions and impact events can produce natural mushroom clouds.
The smoke and debris are sucked into the updraft of the explosion. The initial fireball from a nuclear explosion expands in all directions. However, the expanding explosion quickly reaches equilibrium with the surrounding atmospheric pressure and stops growing. At that point, you have a superheated mass of rarefied air, which is far less dense than regular air, surrounded by a much colder mass of dense air. So the hot air rises very quickly, and as it moves up, air and gas are drawn in from the sides and bottom to fill the space the hot cloud once occupied. The cold air above the cloud is pushed aside, moved around the fireball, and then sucked back in underneath. This is what causes the "`rolling toroid` pattern" often observed.
The fireball soon reaches a point in the atmosphere where the air is cold enough and dense enough to slow its ascent, and the weight and density of the air flatten the fireball and its trailing smoke. The cloud continues to rise as it continues to flatten, forming the rounded cap of the mushroom. 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 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, mainly due to the water droplets (as in an ordinary cloud). The white colour can also be caused by the presence of nitrogen oxides, which are formed from initially ionized nitrogen, oxygen, and atmospheric moisture. In the high-temperature, high-radiation environment of the blast, ozone is also formed. The ozone gives the blast its characteristic corona discharge-like smell.
The smoke and debris in the cloud can cause harmful effects. Large amounts of newer, more radioactive particles deposited on the skin can cause beta burns, often presenting as discoloured spots and lesions on the backs of exposed animals. The fallout may appear as dry, ash-like flakes, or as particles too small to be visible; in the latter case, the particles are often deposited by rain.
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Atmospheric conditions
The shape of a mushroom cloud is influenced by local atmospheric conditions and wind patterns. The fireball from an atomic bomb explosion rises very quickly and experiences resistance from the air, which pushes it down sideways, leading to the flattening of the top of the cloud, which then appears like the cap of a mushroom. The displaced gas, which is at a lower temperature than the air in the centre of the column, trickles down the sides of the column, only to be sucked back in by the rising column to travel upwards again. This is why the edges of an explosion's fireball appear to be curling constantly.
The fireball from an H-bomb rises so high that it hits the tropopause, the boundary between the troposphere and the stratosphere. There is a strong temperature gradient at the tropopause, which prevents the two layers of the atmosphere from mixing. The hot bubble of the fireball initially expands and rises. By the time the bubble has risen from sea level to the tropopause, it is no longer hot enough to break through the boundary. So, the fireball flattens out and expands sideways into an exaggerated mushroom cap.
The formation of a mushroom cloud undergoes 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. The condensation of evaporated ground occurs most intensely during fireball temperatures between 3500 and 4100 Kelvin. In the rise and stabilization phase, which lasts from 20 seconds to 10 minutes, hot gases rise and early large fallout is deposited. In the late-time phase, which lasts until about 2 days later, the airborne particles are distributed by wind, deposited by gravity, and scavenged by precipitation.
The condensation of water droplets in the mushroom cloud depends on the amount of condensation nuclei. Too many condensation nuclei can inhibit condensation, as the particles compete for a relatively insufficient amount of water vapour. Chemical reactivity of the elements and their oxides, ion adsorption properties, and compound solubility influence particle distribution in the environment after deposition from the atmosphere.
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Rayleigh-Taylor instability
A mushroom cloud is caused by any massive release of heat, such as a thermonuclear explosion. The fireball rises through the atmosphere, creating a vacuum that is immediately filled with smoke and debris. As the fireball rises, it reaches a point where the air is cold and dense enough to slow its ascent, and the weight of the air flattens the fireball and its smoke, forming the rounded cap of the mushroom.
The Rayleigh-Taylor instability is a phenomenon that occurs at the interface of two fluids with different densities. It was studied by Lord Rayleigh, who observed that if a volume of heavier fluid is displaced downward, with an equal volume of lighter fluid displaced upwards, the disturbance will grow as the denser material moves down and the less dense material is further displaced upwards. This situation is equivalent to that of a less dense fluid accelerating into a denser fluid, which is precisely what happens in the case of a mushroom cloud.
The Rayleigh-Taylor instability can be observed in a simple experiment where a higher-density fluid like oil is suspended over a lower-density fluid like water under the influence of Earth's gravity. The total energy of the system would be lowered if the oil moves downward, reducing its potential energy. This disturbance leads to the upward movement of water, further reducing the potential energy of the system.
The evolution of the Rayleigh-Taylor instability follows four main stages. In the first stage, the perturbation amplitudes are small, and the equations of motion can be linearized, resulting in exponential instability growth. In the latter part of this stage, one can observe the beginnings of the formation of mushroom-shaped spikes and bubbles. In the second stage, the initial perturbations progress from a linear growth phase into a non-linear growth phase, with the spikes and bubbles tangling and rolling up into vortices. In the third stage, these vortices form the temporary 'vortex ring', which forms the stem of the mushroom cloud. Finally, in the fourth stage, the rising low-density air reaches an equilibrium altitude, where it is no longer at a lower density than the surrounding atmosphere. At this point, it stops rising and disperses downward, causing the mushroom shape.
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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.
A mushroom cloud can be caused by any sufficiently energetic detonation or deflagration. They are most commonly associated with nuclear explosions, but they can also be caused by powerful conventional weapons, volcanic eruptions, and impact events.
Mushroom clouds are composed of smoke, debris, condensed water vapour, or a combination of these elements. 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, the colour changes to white due to the water droplets.
Mushroom clouds rise upwards because hot air is less dense than cold air and therefore rises. The hot air from the explosion rises, creating a vacuum that is filled with smoke and debris, forming the central column of the mushroom cloud.
