Laura Smith
Mushroom clouds are the result of a massive release of heat, often from a nuclear explosion. The reddish hue of the cloud is due to the presence of nitrogen dioxide and nitric acid, formed from initially ionized nitrogen, oxygen, and atmospheric moisture. As the fireball cools, vapors condense to form a cloud containing solid particles of weapon debris and water droplets. The cloud continues to rise and flatten, forming the rounded cap of the mushroom. Mushroom clouds can also be caused by powerful conventional weapons or natural events such as volcanic eruptions and forest fires.
| Characteristics | Values |
|---|---|
| Cause | Any massive release of heat |
| Explosion type | Nuclear, thermonuclear, or major chemical explosion |
| Cloud composition | Radioactive fission products, weapon residues, water droplets, particles of dirt, debris, smoke, dust |
| Initial colour | Red, reddish-brown, yellow, orange |
| Cloud shape | Caused by Rayleigh-Taylor instability |
| Maximum height | Reached in about 10 minutes |
| Persistence | Visible for about an hour |
| First observation | 1798, observed by Gerhard Vieth and legation counsellor Lichtenberg |
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What You'll Learn
Nitrogen dioxide and nitric acid
Nitrogen dioxide (NO2) is a nitrogen oxide that forms through the reaction of nitric acid with wood or metals, or through the oxidation of nitrogen oxide in the presence of light. It is a corrosive and toxic gas, with exposure leading to adverse effects such as wheezing and asthma exacerbations. In the context of mushroom clouds, nitrogen dioxide contributes to the reddish colour and, at high concentrations, can cause depletion of the ozone layer.
Nitric acid, on the other hand, is an inorganic and corrosive acid. It undergoes an oxidation-reduction reaction in the presence of light, producing nitrogen dioxide, water, and oxygen. Inhalation of nitric acid fumes leads to exposure to both nitric acid and nitrogen oxides, including nitrogen dioxide and nitric oxide. The presence of nitric acid in mushroom clouds, along with nitrogen dioxide, contributes to the reddish colour observed in the initial stages before the cloud cools and condensation occurs.
The reddish-brown colour of the mushroom cloud, indicative of the presence of nitrogen dioxide and nitric acid, eventually gives way to white as the fireball cools. This colour change is due to the condensation of water droplets, similar to those in ordinary clouds. The height reached by the radioactive cloud is dependent on the heat energy of the explosion and atmospheric conditions. If sufficient energy remains, a portion of the cloud may ascend into the stratosphere.
The formation of a mushroom cloud is not exclusive to nuclear explosions. Any sufficiently energetic detonation, powerful conventional weapons, or natural events like volcanic eruptions can produce this phenomenon. The iconic shape results from the Rayleigh-Taylor instability, where the buoyant mass of low-density gases rises rapidly, forming vortices that curl downward and create a temporary vortex ring—the "mushroom stem".
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Rayleigh-Taylor instability
The Rayleigh-Taylor instability is a phenomenon that occurs at the interface of two fluids with different densities. It is named after Lord Rayleigh, who studied the behaviour of fluids in a state of equilibrium, and G. I. Taylor, who realised that this setup was equivalent to the situation when less dense fluid is 'accelerated' into denser fluid. This is precisely what happens in the case of a mushroom cloud.
The Rayleigh-Taylor instability can be understood by considering a simple example. Imagine a higher-density fluid like oil suspended over a lower-density fluid like water, under the influence of Earth's gravity. The total energy of this system would be lowered if the oil moves downward, reducing its potential energy. This disturbance leads to the upward movement of water to compensate. As this disturbance grows, it further releases potential energy, with the denser material moving down and the less dense material moving up.
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 early portion of this stage, a sinusoidal initial perturbation retains its sinusoidal shape. However, as the first stage ends and non-linear effects begin to appear, the ubiquitous mushroom-shaped spikes and bubbles begin to form.
As the Rayleigh-Taylor instability develops, the initial perturbations progress from a linear growth phase into a non-linear growth phase. The spikes and bubbles of the instability tangle and roll up into vortices, 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 rising buoyant low-density air will eventually reach 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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Nuclear explosions
The fireball is formed from the sudden expansion of a large volume of lower-density gases, which rise rapidly and cause vortices that curl downward, forming a temporary vortex ring that draws up a central column of smoke, debris, and condensed water vapour. The cloud continues to rise and flatten, forming the rounded cap of the mushroom. The colour of the cloud is initially red or reddish-brown due to the presence of nitrogen dioxide and nitric acid, formed from ionized nitrogen, oxygen, and atmospheric moisture. The reddish hue is later obscured by the white colour of water/ice clouds as the fireball cools, and the dark colour of smoke and debris.
The fireball and cloud contain highly radioactive particles, primarily fission products, and other weapon debris aerosols. The blast wind at sea level may exceed 1,000 km/h (600 mph; 300 m/s), approaching the speed of sound in the air. The energy released from a nuclear explosion can be divided into four basic categories: blast, radiation, thermal radiation, and prompt electromagnetic radiation. The environment of the explosion will determine how much energy is distributed to the blast and how much to radiation. For example, denser media such as water absorbs more energy and creates more powerful shock waves while limiting the blast radius.
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Volcanic eruptions
Mushroom clouds are most commonly associated with nuclear explosions. However, they can also be caused by powerful conventional weapons and some natural events, such as volcanic eruptions and impact events. Volcanic eruptions can cause mushroom clouds, along with other types of clouds, such as ash clouds and pyroclastic clouds.
Pyroclastic clouds, also known as pyroclastic density currents or pyroclastic flows, are fast-moving currents of hot gas and volcanic matter that flow along the ground away from a volcano. They are produced as a result of certain explosive eruptions and can reach speeds of up to 700 km/h (430 mph). The word "pyroclast" comes from the Greek words for "fire" and "broken in pieces". A name for pyroclastic flows that glow red in the dark is "nuée ardente", which means "burning cloud". This name was used to describe the red clouds depicted by Edvard Munch in his painting 'The Scream', which were thought to be caused by the volcanic eruption of Krakatau in 1883.
Mushroom clouds are formed by the sudden generation of a large volume of lower-density gases at any altitude, resulting in a Rayleigh-Taylor instability. The buoyant mass of gas rises rapidly, forming turbulent vortices that curl downward around its edges and create a temporary vortex ring that draws up a central column of smoke, debris, and condensed water vapour. This central column becomes the "'stem" of the mushroom cloud, while the flattening of the rising mass of gas forms the rounded cap. The cloud continues to grow laterally, even as it is dispersed by winds, until it eventually merges with the surrounding natural clouds.
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Forest fires
The formation of mushroom clouds during forest fires occurs due to the rapid release of heat. The heat creates a powerful updraft, drawing dust and smoke upwards. As the column rises, it eventually reaches an atmospheric boundary layer, typically occurring at 30,000 to 50,000 feet. At this point, the rising column can no longer sustain its narrow structure and spreads out horizontally, forming the cap of the mushroom shape. This phenomenon is known as Rayleigh-Taylor instability, where lighter fluids push on denser fluids when interacting.
Pyrocumulus clouds play a significant role in the spread of smoke, ash, and pollutants during forest fires. They can reach extreme heights, dispersing these particles over a wide area. Additionally, these clouds can occasionally produce rain, which may extinguish the fire that created them. However, if there is a significant amount of moisture in the air, pyrocumulus clouds can transform into pyrocumulonimbus clouds, a type of thundercloud capable of generating lightning and causing further fires.
Lastly, forest fires have a profound impact on the fungi population. Pyrophilous fungi, or "fire fungi," often fruit from the charred remains of burned forests. These fungi play a crucial role in the growth of forests and their trees, providing various ecosystem services such as root symbiosis and wood decay. Fire ecology studies help us understand the complex relationships between fire, fungi, and forest regeneration, contributing to our knowledge of post-fire ecological dynamics.
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Frequently asked questions
The initial colour of some radioactive clouds is red or reddish-brown due to the presence of nitrogen dioxide and nitric acid, formed from initially ionized nitrogen, oxygen, and atmospheric moisture.
Mushroom clouds are caused by a massive release of heat, usually from a nuclear explosion. However, any sufficiently powerful source of heat, such as a volcano or forest fire, can produce one.
A mushroom cloud forms when an explosion creates a very hot bubble of gas. This hot air rises and expands, creating a powerful updraft that picks up dust and debris, forming the stem of the mushroom cloud. The cloud continues to rise and flatten, forming the rounded cap of the mushroom.
