John Miller
Mushroom clouds are distinctive mushroom-shaped clouds of debris, smoke, and condensed water vapour that form after a large explosion. Although the term mushroom cloud was coined in the 1950s, explosions of this kind have been described for centuries. The effect is most commonly associated with nuclear explosions, but any sufficiently energetic detonation or deflagration will produce a similar effect. So, are most explosions mushroom-shaped?
| Characteristics | Values |
|---|---|
| Cause | Any sufficiently energetic detonation or deflagration |
| Formation | A sudden, rapid, and concentrated release of heat in relatively cool surroundings |
| Appearance | A distinctive mushroom-shaped flammagenitus cloud of debris, smoke, and usually condensed water vapour |
| Explosion type | Nuclear, thermonuclear, chemical, volcanic, impact events, forest inferno |
| Radiation distribution | Varies with the yield of the explosion, type of weapon, fusion-fission ratio, burst altitude, terrain type, and weather |
| Fallout | Dry, ash-like flakes or invisible particles deposited by rain |
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What You'll Learn
Mushroom clouds are not exclusive to nuclear explosions
Mushroom clouds are often associated with nuclear explosions, but they are not exclusive to them. Any sufficiently energetic detonation or deflagration can produce a similar effect, including powerful conventional weapons such as thermobaric weapons.
Mushroom clouds were described centuries before the Atomic Age. For example, a contemporary account of the 1782 Franco-Spanish attack on Gibraltar depicted one of the attacking force's floating batteries exploding with a mushroom cloud after the British defenders set it ablaze. In 1798, Gerhard Vieth published an illustrated account of a cloud in Gotha that was "not unlike a mushroom in shape".
Mushroom clouds can also occur naturally, such as those produced by some volcanic eruptions and impact events. These natural mushroom clouds are the result of 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, forming turbulent vortices that curl downward, creating a temporary vortex ring that draws up a central column of smoke, debris, condensed water vapour, or a combination of these elements to form the "mushroom stem".
The distinctive mushroom shape of the cloud is due to the atmospheric friction experienced by the initial spherically expanding fireball. As the fireball rises, it cools and changes shape, forming a violently rotating spheroidal vortex. This causes turbulence and a vortex that sucks more air into the centre, creating external afterwinds and further cooling the fireball. The speed of rotation slows as the fireball cools and may stop entirely during later phases.
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The Rayleigh-Taylor instability
Mushroom clouds are the result of a large volume of lower-density gases being formed at a rapid rate, causing a Rayleigh-Taylor instability. This instability is observed when a parcel of heavier fluid is displaced downward, with an equal volume of lighter fluid moving upwards, resulting in a decrease in the potential energy of the system. This instability is not limited to gases and can also occur in fluids, including lava lamps.
Mathematically, the Rayleigh-Taylor instability can be described using the classical Navier-Stokes equations, which govern the behaviour of this hydro-dynamic instability. The equations consider factors such as velocity, pressure, and density to predict the evolution of the instability and the resulting turbulent mixing layers. The study of this instability provides an excellent springboard into the mathematical investigation of stability due to the simplicity of the base state.
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The distribution of radiation in the cloud
Mushroom clouds are not exclusive to nuclear explosions. They can occur when any detonation or deflagration is sufficiently energetic. This includes powerful conventional weapons, such as thermobaric weapons, as well as some volcanic eruptions and impact events.
The distribution of radiation in a mushroom cloud depends on several factors, including the yield of the explosion, the type of weapon, the fusion-fission ratio, burst altitude, terrain type, and weather. Lower-yield explosions tend to have around 90% of their radioactivity in the mushroom head, with the remaining 10% in the stem. Conversely, megaton-range explosions typically hold most of their radioactivity in the lower third of the mushroom cloud.
The fallout from these explosions may take the form of dry, ash-like flakes, or particles too small to see. These particles may be deposited by rain. The fallout from the Castle Bravo test resembled white dust and was nicknamed "Bikini snow".
The intense radiation in the seconds immediately following a blast can cause a visible aura of fluorescence, a blue-violet-purple glow of ionized oxygen and nitrogen surrounding the head of the mushroom cloud. This phenomenon is most easily observed at night or under weak daylight conditions. The light fades rapidly and becomes barely visible within tens of seconds.
Nuclear mushroom clouds are often accompanied by short-lived vapour clouds, known as "Wilson clouds", condensation clouds, or vapour rings. These are caused by a sudden rarefaction of the surrounding medium, leading to a drop in temperature and the condensation of moisture in an outward-moving shell.
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The formation of condensation rings
Condensation rings are a commonly observed feature of mushroom clouds. They are formed by the interaction of hot and cold air, which creates a distinctive mushroom shape. In the context of explosions, the formation of condensation rings is closely tied to the creation of mushroom clouds.
When an explosion occurs, it releases a large amount of heat energy, resulting in a rapid expansion of air. This expansion forms a bubble of hot gas, known as a fireball, which rises quickly through the cooler surrounding air. The fireball creates a powerful updraft, with air speeds reaching up to 300 miles per hour in higher-yield explosions.
As the hot air rises, it encounters the cooler, denser air above it. This interaction between the less-dense hot air and the more-dense cold air shapes the mushroom cloud. The cooler air pushes the rising hot air downward, forming the cap of the mushroom cloud. Simultaneously, the updraft created by the rising hot air picks up dust, debris, and moisture, forming the stem of the mushroom cloud.
The entrainment of higher-humidity air in the updraft, combined with the associated drop in pressure and temperature, leads to the formation of condensation rings. These rings, also known as skirts or bells, form around the stem of the mushroom cloud. The layering of humidity in the atmosphere influences the appearance of these condensation rings, contributing to the overall shape of the mushroom cloud.
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The explosion's yield and height
The mushroom shape of an explosion cloud is caused by a Rayleigh-Taylor instability, which occurs when a large volume of lower-density gases is suddenly formed at any altitude. The buoyant mass of gas rises rapidly, creating turbulent vortices that curl downward around its edges, forming a temporary vortex ring that draws up a central column. This central column, or "stem", may be comprised of smoke, debris, condensed water vapour, or a combination of these elements. The "head" of the mushroom shape is formed by the gases expanding laterally once they can no longer rise.
The formation of a mushroom cloud is not exclusive to nuclear explosions. Any sufficiently energetic detonation or deflagration will produce a similar effect. This includes powerful conventional weapons, such as thermobaric weapons, as well as some volcanic eruptions and impact events in nature.
The height of an explosion cloud is influenced by the energy of the explosion, which radiates outward in a sphere unless obstructed. When an explosion occurs on the ground, it is restricted in the downward direction and only expands in a half-sphere. The energy of the explosion, therefore, decreases with distance from the epicentre. The further away from an explosion, the less damage will be incurred.
The yield of an explosion refers to its explosive power and is typically given in terms of energy equivalents in kilotons or megatons of TNT. Yields of nuclear explosions can be very hard to calculate, even in controlled conditions, and the margins of error for less controlled conditions can be quite large. For fission devices, the most precise yield value is found from radiochemical fallout analysis, which measures the quantity of fission products generated.
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Frequently asked questions
No, mushroom clouds are not exclusive to nuclear explosions, but they are markedly different from regular explosions. Large explosions on the ground can also produce the iconic mushroom cloud shape.
A mushroom cloud forms when an explosion creates a very hot bubble of gas. The hot air is buoyant, so it quickly rises and expands. The rising cloud creates a powerful updraft that picks up dust and debris, forming the stem of the mushroom cloud.
A mushroom cloud is a cloud of debris, smoke, and usually condensed water vapour resulting from a large explosion. The effect is most commonly associated with a nuclear explosion.
Mushroom clouds emit radioactive particles that can cause beta burns, often presenting as discoloured spots and lesions on the backs of exposed animals.
Yes, some volcanic eruptions and impact events can produce natural mushroom clouds.
