Michael Davis
Mushroom clouds are clouds of smoke and debris that form after a large explosion. They are most commonly associated with nuclear explosions, but any sufficiently energetic detonation or deflagration will produce a similar effect. The formation of a mushroom cloud is the result of the interaction between hot, less dense air and the denser, cold surrounding air. This interaction is known as Rayleigh-Taylor instability.
| Characteristics | Values |
|---|---|
| Cause | Any massive release of heat and energy, including explosions and volcanic eruptions |
| Explosion type | Nuclear, conventional, or thermobaric weapons |
| Shape | Mushroom-shaped flammagenitus cloud of debris, smoke, and condensed water vapour |
| Colour | Red, reddish-brown, yellow, orange, or white |
| Height | Up to 6-8 miles above the Earth's surface, depending on heat energy and atmospheric conditions |
| Duration | Visible for about an hour until dispersed by winds |
| Radioactivity | Lower-yield explosions have 90% of radioactivity in the mushroom head; megaton-range explosions have most in the lower third |
| Fallout | Dry, ash-like flakes or small, invisible particles; can cause beta burns on exposed skin |
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What You'll Learn
Nuclear explosions
Mushroom clouds are distinctive mushroom-shaped clouds of debris, smoke, and usually condensed water vapour that result from a large explosion. Although the term mushroom cloud is most commonly associated with nuclear explosions, any sufficiently energetic detonation or deflagration will produce a similar effect. For example, powerful conventional weapons, such as thermobaric weapons, and some volcanic eruptions and impact events can produce natural mushroom clouds.
Mushroom clouds result from the sudden formation of a large volume of lower-density gases at any altitude, causing a Rayleigh-Taylor instability. 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. This central column may contain smoke, debris, condensed water vapour, or a combination of these, forming the "mushroom stem". The initial colour of some radioactive clouds can be red or reddish-brown due to the presence of nitrogen dioxide and nitric acid, formed from initially ionized nitrogen, oxygen, and atmospheric moisture. As the fireball cools, the colour changes to white due to the water droplets. The distribution of radiation in the mushroom cloud varies with the yield of the explosion, type of weapon, fusion-fission ratio, and burst altitude. Lower-yield explosions have about 90% of their radioactivity in the mushroom head and 10% in the stem, while megaton-range explosions tend to have most of their radioactivity in the lower third of the mushroom cloud.
Nuclear mushroom clouds are often accompanied by short-lived vapour clouds, known as "Wilson clouds", condensation clouds, or vapour rings. The "negative phase" following the positive overpressure behind a shock front causes a sudden rarefaction of the surrounding medium, leading to an adiabatic drop in temperature. This causes moisture in the air to condense in an outward-moving shell surrounding the explosion. When the pressure and temperature return to normal, the Wilson cloud dissipates. The same effect can also occur above the top of the cloud, where the expansion of the rising cloud pushes a layer of warm, humid, low-altitude air upwards into cold, high-altitude air, causing the condensation of water vapour and the formation of ice caps.
The height reached by the radioactive cloud depends on the heat energy of the weapon and atmospheric conditions. If the cloud reaches the tropopause, about 6-8 miles above the Earth's surface, it tends to spread out. However, if sufficient energy remains, a portion of the cloud will ascend into the more stable air of the stratosphere. The cloud attains its maximum height after about 10 minutes and is then considered "stabilized". It continues to grow laterally, producing the characteristic mushroom shape. The cloud may remain visible for about an hour or more before being dispersed by the winds and merging with natural clouds in the sky.
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Conventional weapons
Mushroom clouds are clouds of smoke and debris that form after an explosion. Although commonly associated with nuclear explosions, mushroom clouds can also be caused by powerful conventional weapons. These include thermobaric weapons such as the ATBIP and GBU-43/B MOAB.
The formation of a mushroom cloud is the result of the interaction of two fluids or gases with different densities. In the case of an explosion, the less dense hot air rises through the more dense cold air, creating a vacuum in its wake. This causes the denser cold air to be sucked in, forming a vortex. The speed of rotation slows as the fireball cools, and the vapours condense to form a cloud containing solid particles of weapon debris, water droplets, and dirt and debris from the ground. The cloud continues to grow laterally, producing the characteristic mushroom shape.
The height reached by the cloud depends on the heat energy of the weapon and atmospheric conditions. If the cloud reaches the tropopause, it tends to spread out. If there is sufficient energy remaining, a portion of the cloud will ascend into the stratosphere. The cloud attains its maximum height after about 10 minutes and is then considered "stabilized". It may remain visible for about an hour before being dispersed by the wind.
Mushroom clouds can also occur naturally, such as in the case of volcanic eruptions. Historical accounts of mushroom clouds include the 1917 Halifax Explosion and an attack on Shanghai, China, reported in The Times on 1 October 1937.
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Volcanic eruptions
Mushroom clouds are often associated with nuclear explosions. They are caused by the rapid heat release creating a vacuum that pulls up debris and smoke into a tall column, spreading out at the top. The top of this column spreads out as it cools, making the classic mushroom shape. It is a mix of smoke, dust, and water vapour that can reach miles into the sky.
While mushroom clouds are commonly associated with nuclear explosions, they can also be caused by other powerful man-made explosions and natural events. Volcanic eruptions, for instance, can create mushroom clouds. The hot ash, gases, and lava shooting up into the sky during a volcanic eruption can form mushroom-shaped plumes that resemble those produced by nuclear explosions. The shape and size of the mushroom cloud depend on the volcano's contents and the nature of the eruption.
The 1980 eruption of Mount St. Helens is a well-known example of a volcanic eruption that generated a huge mushroom cloud visible for miles. This cloud was composed of ash, rock, and steam. Other volcanic eruptions can also produce different types of mushroom clouds, ranging from tall and skinny to short and wide shapes.
Additionally, volcanic eruptions can have both immediate and long-term effects on the environment and human activities. The release of ash, gas, and lava during an eruption can pose hazards to nearby populations, including respiratory issues, infrastructure damage, and disruption of transportation. The ash and aerosols injected into the atmosphere can also affect aviation, leading to flight cancellations and diversions due to reduced visibility and potential damage to aircraft engines.
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Rayleigh-Taylor instability
Mushroom clouds are the result of a large volume of lower-density gases being formed at any altitude, causing a Rayleigh-Taylor instability. This instability occurs due to the sudden formation of a large volume of lower-density gases, which creates a buoyant mass of gas that rises rapidly. The ascent of the gas results in turbulent vortices that curl downward around its edges, forming a temporary vortex ring that draws up a central column. This central column may include smoke, debris, condensed water vapour, or a combination of these elements, forming the "mushroom stem" of the cloud.
The Rayleigh-Taylor instability is a well-known phenomenon in fluid dynamics that occurs at the interface of two fluids with different densities. It was first studied by Lord Rayleigh, who examined the behaviour of a parcel of heavier fluid displaced downward with an equal volume of lighter fluid displaced upwards. This disturbance leads to a release of potential energy, causing the denser material to move downwards and the less dense material to be further displaced upwards.
The Rayleigh-Taylor instability is characterised by the development of "plumes" and the formation of vortices as the instability progresses from a linear growth phase to a non-linear growth phase. This instability is distinct from the Plateau-Rayleigh instability, also known as the Rayleigh instability, which occurs in a liquid jet due to surface tension. The Rayleigh-Taylor instability, on the other hand, occurs in accelerated fluid layers and can lead to turbulent mixing layers.
The mathematical study of the Rayleigh-Taylor instability begins with the classical potential flow theory for moving contact surfaces and extends to various fluid systems, including inhomogeneous, viscous, compressible, and isobaric flows. The stability analysis considers different geometries, boundary conditions, and physical conditions to understand the behaviour of layered materials during the instability.
The Rayleigh-Taylor instability is not just a theoretical concept but has important practical applications as well. It plays a significant role in a wide range of engineering, geophysical, and astrophysical flows, often serving as a triggering event for large-scale turbulent mixing. The understanding and prediction of this instability are crucial in various fields, with efforts being made over the past 140 years to model and study the evolution of the Rayleigh-Taylor instability and its induced mixing layers.
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Radioactivity
Mushroom clouds are clouds of smoke and debris that form after a large explosion. They are most commonly associated with nuclear explosions, but any sufficiently energetic detonation or deflagration will produce a similar effect. For example, powerful conventional weapons, such as thermobaric weapons, and some volcanic eruptions can also create mushroom clouds.
The formation of a mushroom cloud occurs when a large volume of lower-density gases is formed at any altitude, causing a Rayleigh-Taylor instability. This instability is caused by the interaction between two materials (fluids or gases) of different densities. The buoyant mass of gas rises rapidly, resulting in turbulent vortices that curl downward around its edges, forming a temporary vortex ring that draws up a central column. This central column may contain smoke, debris, condensed water vapour, or a combination of these, forming the "mushroom stem".
The initial fireball of an explosion expands in all directions, but the upward movement is exacerbated in ground or near-ground detonations as heat and energy are reflected upwards from the ground. The upward flow of air after the explosion interacts with the smoke from the explosion to form the "mushroom cap". The cloud will continue to rise until it reaches equilibrium with the surrounding air, which is normally in the ozone layer.
The eventual height of the radioactive cloud depends on the heat energy of the weapon and atmospheric conditions. If the cloud reaches the tropopause, it tends to spread out. However, if there is sufficient energy remaining, a portion of the cloud will ascend into the stratosphere, where it is more stable. The cloud will continue to grow laterally, forming the characteristic mushroom shape. The cloud may remain visible for about an hour or more before being dispersed by the wind and merging with natural clouds.
The heads of mushroom clouds consist of highly radioactive particles, primarily fission products and other weapon debris aerosols. These particles remain suspended in the air even after the cloud disappears as the water droplets evaporate. The radioactive fallout is deposited along the path of the invisible cloud and can cause beta burns, presenting as discoloured spots and lesions on the backs of exposed animals. The distribution of radioactivity in the mushroom cloud varies depending on the explosion's yield, type of weapon, fusion-fission ratio, burst altitude, terrain type, and weather. Generally, lower-yield explosions have about 90% of their radioactivity in the mushroom head and 10% in the stem, while megaton-range explosions tend to have most of their radioactivity in the lower third of the cloud.
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Frequently asked questions
A mushroom cloud is a cloud of smoke and debris that moves through the air following an explosion.
Mushroom clouds are caused by the interaction of two fluids or gases of different densities. This interaction is called Rayleigh-Taylor instability.
Mushroom clouds are most commonly associated with nuclear explosions. However, mushroom clouds can be caused by any sufficiently energetic detonation or deflagration, including powerful conventional weapons such as thermobaric weapons, volcanic eruptions, and impact events.
An explosion creates a very hot bubble of gas, which interacts with the cooler surrounding air, making it less dense. This less dense hot air rises from the initial fireball and creates a vacuum in its wake. This causes the denser cold air to get sucked in as the fireball continues to rise. The rising hot air meets resistance from the denser cold air, which slows its ascent. The cloud continues to rise as it flattens, forming the rounded cap of the mushroom.
