Emma Wilson
Mushroom clouds are the result of a sudden, rapid, and concentrated release of heat in relatively cool surroundings. The phenomenon is usually associated with nuclear explosions, but it can also occur after a volcanic eruption, forest fire, impact event, or a powerful explosion. 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 be composed of smoke, debris, condensed water vapour, or a combination of these elements. The rings that form around the mushroom cloud are clouds of condensation, which are caused by the sudden drop in pressure and temperature, leading to the condensation of water vapour. These rings are often referred to as \Wilson clouds\ or condensation clouds.
| Characteristics | Values |
|---|---|
| Formation | The sudden formation of a large volume of lower-density gases at any altitude |
| Result | Rayleigh-Taylor instability |
| Gases | Buoyant mass of gas rises rapidly, resulting in turbulent vortices curling downward around its edges |
| Structure | Temporary vortex ring that draws up a central column, possibly with smoke, debris, condensed water vapour, or a combination of these |
| Altitude | The stabilization altitude depends on the profiles of temperature, dew point, and wind shear in the air at and above the starting altitude |
| Rings | Clouds of condensation |
| Explosion | Large explosions cause areas of high and low pressure in the air |
| Pressure | In areas of lowering pressure, the air can cool below its dew point and water vapour condenses into a cloud |
| Temperature | As the air returns to its normal temperature due to an increase in pressure, convection or warming from the explosion, the clouds vanish |
| Layering | Differences in humidity, pressure, and temperature |
| Wilson clouds | Transient condensation clouds that form surrounding large explosions in humid air |
| Shockwave | High-pressure front, and a trailing low-pressure zone |
| Drop in temperature | If the temperature drops below the dew point, moisture in the air condenses into water droplets |
| Nuclear explosion | A sudden, rapid and concentrated release of heat in relatively cool surroundings |
| Fireball | The giant fireball is extremely hot, which rises rapidly in the air, creating a vacuum that is then rapidly filled by the surrounding air |
| Fluids | When two fluids of different densities interact, the lighter fluid pushes on the denser fluid |
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What You'll Learn
The phenomenon of Wilson clouds
The name "Wilson cloud" is derived from the Wilson cloud chamber, a particle detector invented by Scottish physicist Charles Thomson Rees Wilson in the early 20th century. Wilson's chamber used condensation from a rapid pressure drop to track the paths of electrically charged subatomic particles, creating a zone of fog or a "Wilson Cloud". This phenomenon is visually similar to the clouds observed in nuclear explosions.
Wilson clouds are distinct from the larger and more persistent structures that form later, such as the "cauliflower cloud" and the base surge, which are composed of water, spray, and radioactive particles. The primary mushroom cloud that forms after a nuclear explosion is primarily composed of hot, compressed gas and bomb debris, rather than being a condensation phenomenon like the Wilson cloud.
The formation of Wilson clouds is influenced by the humidity and temperature of the ambient air. In humid air, the drop in temperature caused by the rarefaction phase can bring the air temperature below its dew point, leading to the condensation of moisture into a visible cloud. The layering of humidity in the atmosphere also contributes to the appearance of condensation rings or Wilson clouds, as opposed to a spherical cloud.
Wilson clouds are a striking reminder of the power and complexity of nuclear explosions. Their formation is a result of the interaction between the rapid release of energy, the subsequent shockwave, and the humidity and temperature conditions in the atmosphere. Understanding the Wilson cloud phenomenon provides valuable insights into the behaviour of matter under extreme conditions.
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Rayleigh-Taylor instability
Mushroom clouds are often associated with surface nuclear detonations and large volcanic eruptions. They are characterised by the occurrence of "dry lightning" or lightning without rain. These clouds are composed of debris, smoke, condensed water vapour, and sometimes, in the case of a nuclear explosion, radioactive isotopes.
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 include smoke, debris, condensed water vapour, or a combination of these, forming the "mushroom stem".
The Rayleigh-Taylor instability can be explained through an example: consider a higher-density fluid like oil suspended over a lower-density fluid like water, under the influence of Earth's gravity. Any system tends to achieve a state where its overall total energy is the lowest. In this case, the total energy of the oil-water system would be lowered if the oil moves downward, reducing its potential energy. This disturbance keeps growing in favour of reducing potential energy.
Taylor's insight was that a similar situation would arise when a less dense fluid is 'accelerated' into a more dense fluid, which is precisely what happens in the case of a mushroom cloud. The rapid upward movement of low-density gas leads to the formation of downward-directed turbulent vortices, forming a temporary 'vortex ring', which forms the stem of the mushroom cloud. Thereafter, the rising buoyant low-density air will reach an equilibrium altitude, where it is no longer of lower density than the surrounding atmosphere. At this point, it stops rising and disperses downwards, causing the mushroom shape.
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Shockwaves and condensation
Mushroom clouds are the result of a sudden, rapid, and concentrated release of heat in relatively cool surroundings. This can be caused by a nuclear explosion, a volcano, a forest fire, an impact event, or a powerful explosion, like those caused by vacuum bombs. 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 include smoke, debris, condensed water vapour, or a combination of these, forming the "mushroom stem".
The rings that sometimes appear around the mushroom cloud are clouds of condensation. Large explosions cause areas of high and low pressure in the air. In areas of low pressure, the air can cool below its dew point, and water vapour condenses into a cloud. As the air returns to its normal temperature due to an increase in pressure, convection, or warming from the explosion, the clouds vanish. The rings are layered due to differences in humidity, pressure, and temperature. The lack of humidity and high temperatures may be why they do not appear in tests in arid areas.
These condensation rings are also known as "Wilson clouds", named after the Wilson cloud chamber, a particle detector invented by Scottish physicist Charles Thomson Rees Wilson in the early 20th century. The chamber uses condensation from a rapid pressure drop to mark the tracks of electrically charged subatomic particles. The negative phase following the positive overpressure behind a shock front causes a sudden rarefaction of the surrounding medium. This low-pressure region causes an adiabatic drop in temperature, causing 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 shockwave from a large explosion causes an initial compression followed by a rapid expansion of the surrounding air. This rarefaction phase results in significant adiabatic cooling. If the ambient air is sufficiently humid, this cooling can drop the temperature below the dew point, causing water vapour to condense into a visible cloud.
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Explosions and pressure changes
Mushroom clouds are the result of a sudden, rapid, and concentrated release of heat in relatively cool surroundings. This phenomenon is known as Rayleigh-Taylor instability, which occurs when two fluids of different densities interact, causing the lighter fluid to push up against the denser fluid. This results in the formation of a buoyant mass of gas that 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, ultimately shaping the "mushroom stem".
The rings that are sometimes observed around mushroom clouds are clouds of condensation. Large explosions create areas of high and low pressure in the air. In regions of low pressure, the air can cool below its dew point, leading to water vapour condensing into a cloud. As the air returns to its normal temperature due to an increase in pressure, convection, or warming from the explosion, these clouds dissipate. These condensation rings are influenced by differences in humidity, pressure, and temperature, which also determine whether they remain as rings or transform into a spherical cloud.
The "rings" observed around mushroom clouds are also known as Wilson clouds, named after Scottish physicist Charles Thomson Rees Wilson, who pioneered the study of fog and rain. These clouds are formed by the sudden pressure changes caused by shock waves. The negative phase following the positive overpressure behind a shock front leads to a rapid rarefaction of the surrounding medium, resulting in 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 normalize, the Wilson cloud dissipates.
The formation of Wilson clouds can be understood through the principles of Wilson's invention, the Wilson cloud chamber. In this device, a sealed chamber containing air supersaturated with water or alcohol vapour undergoes rapid adiabatic expansion, causing significant cooling. This creates conditions conducive to vapour condensation, forming visible droplets that reveal the particle's track. Similarly, the shockwave from a large explosion causes initial compression followed by rapid rarefaction (expansion) of the surrounding air, leading to adiabatic cooling and condensation.
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Historical observations
Mushroom clouds are formed by large explosions under Earth's gravity. They are best known for their appearance after nuclear detonations. Nuclear weapons are usually detonated above the ground to maximize the effect of their spherically expanding fireball and blast wave. The fireball rises into the air, forming a Rayleigh-Taylor instability and producing strong air currents known as "afterwinds".
In 1917, the Halifax Explosion produced a mushroom cloud, and in 1937, a report by The Times described a Japanese attack on Shanghai that generated "a great mushroom of smoke." The Castle Union hydrogen bomb test also produced a mushroom cloud with multiple condensation rings.
The rings that sometimes appear around mushroom clouds are caused by condensation. Large explosions create areas of high and low pressure in the air, leading to a drop in temperature. When the temperature drops below the dew point, moisture in the air condenses into water droplets, forming clouds. These condensation clouds, also known as Wilson clouds, are short-lived and result from the rapid expansion and cooling of air behind the shock front of an explosion. They are composed of water droplets and, in the case of nuclear explosions, radioactive particles.
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Frequently asked questions
The rings around mushroom clouds are clouds of condensation. They are formed when a shockwave from a large explosion causes a sudden drop in pressure, which leads to a drop in temperature. If the temperature drops below the dew point, moisture in the air condenses into water droplets, forming a ring around the mushroom cloud.
The shockwave is caused by the sudden and rapid release of heat from an explosion, which interacts with the cooler surrounding air. This creates a high-pressure front and a trailing low-pressure zone, resulting in a drop in temperature and the formation of condensation rings.
No, the presence of the rings depends on the humidity and temperature of the air. The rings are more likely to form in humid air, and they may not appear in tests conducted in arid areas due to the lack of humidity and higher temperatures.
No, while mushroom clouds are often associated with nuclear explosions, they can also form following any event that results in a rapid release of heat, such as volcanic eruptions, forest fires, impact events, or powerful conventional explosions.
