Understanding Mushroom Clouds: A Guide To Their Formation

Mushroom clouds are the result of a massive release of heat, causing a Rayleigh-Taylor instability. This can be caused by a thermonuclear explosion, a volcano, or a large-scale explosion such as the 2020 Beirut blast. As the fireball rises, it cools and flattens, forming the rounded cap of the mushroom shape. The buoyant mass of gas rises rapidly, creating turbulent vortices that curl downward, forming a temporary vortex ring that draws up a central column of smoke, debris, condensed water vapour, or a combination of these, to form the mushroom stem. The shape of the cloud is influenced by local atmospheric conditions and wind patterns.

Formation: a large volume of lower-density gases forms at high altitude, causing Rayleigh-Taylor instability

Mushroom clouds are often associated with surface nuclear detonations and large volcanic eruptions. They are formed by the sudden generation of a large volume of low-density gases at high altitudes, resulting in Rayleigh-Taylor instability.

The Rayleigh-Taylor instability occurs when a lower-density fluid is accelerated into a higher-density fluid. In the context of a mushroom cloud, the explosion creates a large volume of low-density gases that rapidly accelerate upwards against the higher-density air above it. This upward acceleration results in turbulent vortices curling downward around its edges, forming a temporary vortex ring. This vortex ring forms the stem of the mushroom cloud.

The buoyant low-density gases continue to rise until they reach an equilibrium altitude, where their density is no longer lower than that of the surrounding atmosphere. At this point, the gases stop rising and begin to disperse downward. This downward dispersion contributes to the formation of the mushroom-shaped cloud.

The specific shape of the mushroom cloud is influenced by various factors, including the height of the explosion and local atmospheric conditions. For example, when a bomb explodes closer to the ground, the stem and cap of the cloud may merge, forming the classic mushroom profile. Additionally, the afterwinds produced by the explosion can cause dirt and debris to be sucked up into the cloud, further shaping its structure.

The Rayleigh-Taylor instability is a well-studied phenomenon that provides insight into the formation of mushroom clouds. By understanding this instability and the physics behind it, scientists can predict the behaviour of radioactive particles and provide guidance on consequence management to protect public health in the event of a nuclear crisis.

Phases: early time, rise and stabilization, late time

Early time

The first phase of a mushroom cloud's formation, "early time", occurs during the first 20 seconds after detonation. During this phase, a 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 during these first few seconds, most intensely during fireball temperatures between 3500 and 4100 Kelvin. 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. The intense radiation in the first seconds after the blast may cause an observable aura of fluorescence—a blue-violet-purple glow of ionized oxygen and nitrogen surrounding the head of the forming mushroom cloud. This light is most visible at night or under weak daylight.

Rise and stabilization

The "rise and stabilization" phase occurs between 20 seconds and 10 minutes after detonation. During this phase, the hot gases rise, and early large fallout is deposited. 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 stabilization altitude depends on the profiles of temperature, dew point, and wind shear in the air at and above the starting altitude.

Late time

The "late time" phase occurs until about two days after detonation, when the airborne particles are distributed by wind, deposited by gravity, and scavenged by precipitation. The shape of the cloud is influenced by the local atmospheric conditions and wind patterns. The fallout distribution is predominantly a downwind plume. However, if the cloud reaches the tropopause, it may spread against the wind, as its convection speed is higher than the ambient wind speed. The cloud may continue to be visible for about an hour or more before being dispersed by the wind and merging with natural clouds in the sky.

Fallout: radioactive particles are deposited in the first few hours, causing danger in a downwind plume

Mushroom clouds are the result of the sudden formation of a large volume of lower-density gases at any altitude. This causes a Rayleigh-Taylor instability, leading to the formation of a temporary vortex ring that draws up a central column of smoke, debris, condensed water vapour, or a combination of these, to form the "mushroom stem".

The fallout following a nuclear blast is a critical consequence of mushroom clouds. The largest and most radioactive particles are deposited in the first few hours after the blast, causing danger in a downwind plume. This is due to the fact that the higher-altitude, smaller particles descend more slowly, reaching the ground in a less radioactive state as the isotopes with the shortest half-lives decay the fastest. The short-term, localized fallout is deposited primarily downwind from the blast site in a cigar-shaped area, assuming a constant wind. This downwind plume can carry radioactive particles over considerable distances, leading to dangerous levels of radiation exposure.

The shape and extent of the fallout distribution are influenced by local atmospheric conditions, wind patterns, crosswinds, changes in wind direction, and precipitation. The colour of the mushroom cloud is initially 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 formation of water droplets.

The danger associated with the downwind plume of radioactive particles is significant. In the event that one witnesses a mushroom cloud, it is advised to stick out one's arm and hold one's thumb over the cloud. If the cloud is bigger than one's thumb, it indicates that one is in the radiation zone and should evacuate.

Mushroom shape: the fireball cools and flattens, forming the rounded cap, with the stem created by a vacuum sucking up material

The mushroom cloud is the result of a thermonuclear explosion, or any massive release of heat, such as a volcano. The explosion creates a large volume of lower-density gases at a rapid rate of ascent, causing a Rayleigh-Taylor instability. As the fireball increases in size, it cools and flattens, and vapours condense to form a cloud. This cloud contains solid particles of weapon debris and small drops of water derived from the air sucked into the rising fireball. This process continues, forming the rounded cap of the mushroom shape.

The buoyant mass of gas rises rapidly, resulting in turbulent vortices curling downward around its edges, forming a temporary vortex ring. This vortex ring draws up a central column, possibly with smoke, debris, condensed water vapour, or a combination of these, to form the "mushroom stem". The mass of gas and entrained moist air eventually reach an altitude where it is no longer of lower density than the surrounding air, and it disperses, drifting back down, resulting in fallout.

The stabilization altitude depends on the temperature, dew point, and wind shear in the air at and above the starting altitude. The stem of the mushroom cloud is formed by a jet of material being sucked into the vacuum that is pushing up. This vacuum is created by the rising fireball, which causes a rush of cooler air to fill the space below the bubble, distorting the sphere into a torus or doughnut shape. This phenomenon is known as Rayleigh-Taylor instability. The mushroom cloud undergoes several phases of formation, from the initial fireball and mixing of fission products, to the rise and stabilization phase, and finally, the late-time phase when particles are distributed by wind and deposited by gravity.

Causes: mushroom clouds can be caused by any massive release of heat, not just nuclear explosions

Mushroom clouds are often associated with nuclear explosions, but they can also be caused by any massive release of heat. This includes powerful conventional weapons, such as thermobaric weapons, and natural events like volcanic eruptions and impact events.

Mushroom clouds are formed by the sudden release of a large volume of lower-density gases at any altitude, resulting in a Rayleigh-Taylor instability. The buoyant mass of gas rises rapidly, creating turbulent vortices that curl downward around its edges. This forms a temporary vortex ring, which draws up a central column of smoke, debris, condensed water vapour, or a combination of these elements to create the iconic "mushroom stem". The mass of gas rises until it reaches an altitude where it is no longer of lower density than the surrounding air, at which point it disperses and begins to fall back down, resulting in fallout.

The shape of the mushroom cloud is influenced by the height of the explosion and the local atmospheric conditions and wind patterns. When a bomb is detonated high above the ground, the cap and stem of the cloud may not meet, resulting in a detached mushroom shape. However, when the explosion occurs closer to the ground, the stem and cap merge into the classic mushroom profile.

The colour of the mushroom cloud also provides information about its composition. Initially, the cloud is 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 formation of water droplets, similar to an ordinary cloud.

It is important to note that mushroom clouds cannot form in the absence of an atmosphere or gravity. The explosion's by-product gases would remain spherical without the presence of an atmosphere or gravity to distort the initial sphere and create the characteristic mushroom shape.

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