A mushroom cloud is a distinctive pyrocumulus mushroom-shaped cloud of condensed water vapor or debris resulting from a very large explosion. They are most commonly associated with nuclear explosions, but any sufficiently large blast will produce the same sort of effect. They can be caused by powerful conventional weapons like the GBU-43/B Massive Ordnance Air Blast bomb. Volcano eruptions and impact events can produce natural mushroom clouds.
Mushroom clouds form as a result of the sudden formation of a large mass of hot, low-density gases near the ground creating a Rayleigh–Taylor instability. The mass of gas rises rapidly, resulting in turbulent vortices curling downward around its edges, forming a vortex ring and drawing up a column of additional smoke and debris in the center to form its "stem". The mass of gas eventually reaches an altitude where it is no longer of lower density than the surrounding air and disperses, the debris drawn upward from the ground scattering and drifting back down (see fallout).
Although the term itself appears to have been coined at the start of the 1950s, mushroom clouds generated by explosions were being described before the atomic era. For instance, The Times published a report on 1 October 1937 of a Japanese attack on Shanghai in China which generated "a great mushroom of smoke". During the Second World War, descriptions of mushroom clouds were relatively common.
The atomic bomb cloud over Nagasaki, Japan was described in The Times of London of 13 August 1945 as a "huge mushroom of smoke and dust." On 9 September 1945, the New York Times published an eyewitness account of the Nagasaki bombing, written by William L. Laurence, the official newspaper correspondent of the Manhattan Project, who accompanied one of the three aircraft that made the bombing run. He wrote of the bomb producing a "pillar of purple fire", out of the top of which came "a giant mushroom that increased the height of the pillar to a total of 45,000 feet." (14 km)
Later in 1946, the Operation Crossroads nuclear bomb tests were described as having a "cauliflower" cloud, but a reporter present also spoke of "the mushroom, now the common symbol of the atomic age." Mushrooms have traditionally been associated both with life and death, food and poison, making them a more powerful symbolic connection than, say, the "cauliflower" cloud.
Mushroom clouds are formed by many sorts of large explosions under earth gravity, though they are best known for their appearance after nuclear detonations. In space the explosion would be somewhat spherical. Nuclear weapons are usually detonated above the ground (not upon impact, because most of the energy would be dispelled into the ground) in order to maximize the effect of their spherical expanding fireball and the blast wave. After immediate detonation, the fireball itself begins to rise into the air, acting on the same principle as a hot-air balloon.
One way to analyze the motion, once the hot gas has cleared the ground sufficiently, is as a 'spherical cap bubble' , as this gives agreement between the rate of rise and observed diameter.
While it rises, air is drawn into it and upwards (similar to the updraft of a chimney), producing strong air currents known as "afterwinds", while inside the head of the cloud the hot gases rotate in a toroidal shape. When the detonation itself is low enough, these afterwinds will draw in dirt and debris from the ground below to form the stem of the mushroom cloud.
After the mass of hot gases reaches the equilibrium level, the ascend stops and the cloud starts spreading to the characteristic mushroom shape.
Detonations produced high above the ground do not create mushroom clouds. The heads of the clouds themselves consist of highly radioactive particles, primarily the fission products, and usually are dispersed by the wind, though weather patterns (especially rain) can produce problematic nuclear fallout.
Detonations significantly below ground level or deep below the water (for instance, nuclear depth charges) also do not produce mushroom clouds, as the explosion causes the vaporization of a huge amount of earth and water in these instances. Detonations underwater but near the surface produce a pillar of water, which, in collapsing, forms a cauliflower-shape, which is mistaken for a mushroom cloud on many pictures (such as that seen in the well-known pictures of the Crossroads Baker test). Underground detonations of low depth produce a mushroom cloud and a base surge, two different distinct clouds. The amount of radiation vented into the atmosphere decreases with increasing detonation depth.
With surface and air bursts, the amount of debris lofted into the air decreases rapidly with increasing burst altitude. At burst altitude about 7 meters/kiloton a crater is not formed, with correspondingly lower amount of dust and debris produced. The fallout-free height, above which the radioactive particles consist only of the fine fireball condensation ones, is about 55 meters/kiloton; however even at these burst altitudes fallout may be formed by a number of mechanisms.
The distribution of radiation in the mushroom cloud varies with yield of the explosion, type of weapon, fusion/fission ratio, burst altitude, terrain type, and weather. Generally it can be said that lower-yield explosions have about 90% of radioactivity in the mushroom head and 10% in the stem. Megaton-range explosions tend to have most of the radioactivity in the lower third of the mushroom cloud.
At the moment of the explosion, the fireball is formed. The ascending, roughly spherical mass of hot, incandescent gases changes shape by friction against the atmosphere and cools its surface by radiating out energy, turning from a sphere to a violently swirling annular vortex. The vortex sucks air into its center, creating afterwinds and cooling itself; the speed of its swirling slows down as it cools, and may stop entirely at later phases. The fireball cools by expansion and radiating out the energy, the vaporized parts of the weapon and other materials condense into visible dust, forming the cloud; the white-hot vortex core becomes yellow, then red, then loses visible incandescence. With further cooling, the bulk of the cloud grows yet more as the atmospheric humidity condenses. As the cloud ascends and cools, its buoyancy lowers and its ascent slows.
If the fireball is comparable to the size of atmospheric density scale height, the movement of the cloud will be ballistic, overshooting large volume of denser air to greater altitudes. Significantly smaller fireballs produce clouds with buoyancy-governed ascent.
After reaching the tropopause, the region of strong static stability, the cloud tends to slow its ascent and spread out. If it still contains sufficient energy, part of it may continue rising up into stratosphere. Mass of air ascending from troposphere to stratosphere leads to formation of acoustic-gravity waves, virtually identical to those created by intense stratosphere-penetrating thunderstorms. Smaller scale explosions generate waves of higher frequency, classified as infrasound.
The explosion raises a large amount of moisture-laden air from lower altitudes. As the air rises, the temperature drops and the water vapor condenses as water droplets, and later freezes as ice crystals. The released latent heat of phase change heats the cloud, driving it to yet higher altitudes.
A mushroom cloud undergoes several phases of formation.
The shape of the cloud is influenced by the atmospheric conditions and the wind patterns. The distribution of the fallout is predominantly a downwind plume. However if the cloud reaches the tropopause, it may spread even against the wind direction, as the convection speed is higher than the ambient wind. The cloud shape in tropopause is roughly circular and spread out.
The initial color of some radioactive clouds can be colored red or reddish brown, due to presence of nitrogen dioxide and nitric acid, formed from nitrogen, oxygen, and atmospheric humidity in the high-temperature high-radiation environment of the blast; ozone is also formed. It is estimated that each megaton of yield produces about 5000 tons of nitrogen oxides. Yellow and orange hues are also described. The reddish hue is later obscured by the white color of water vapor, condensing in the fast-flowing air as the fireball cools, and the dark color of smoke and debris sucked into the updraft. The ozone gives the blast its characteristic corona discharge like smell.
The droplets of condensed water vapor gradually evaporate, leading to apparent disappearance of the cloud. The radioactive particles however remain suspended in the air, and the now invisible cloud continues depositing fallout along its trajectory.
The stem of the cloud is gray to brown for ground burst, as there is dust, dirt and soil sucked into the mushroom. Air bursts produce white and steamy stems. Dark mushrooms from ground bursts contain irradiated material from the ground in addition to only the bomb and its casing, and therefore produce more radioactive fallout with larger particles that deposit locally.
A double mushroom, with two levels, can be formed under certain conditions. For example, the Buster-Jangle Sugar shot formed the first head from the blast itself, followed by another one propelled by the heat from the freshly formed crater.
The fallout itself may appear as dry ash-like flakes, or as particles too small to be visible; in the latter case often deposited by rain. Higher amount of newer, more radioactive particles deposited on skin can cause beta burns, often presented as discolored spots and lesions on the backs of exposed animals. The fallout from the Castle Bravo test had the appearance of white dust and was nicknamed Bikini snow; the tiny white flakes resembled snowflakes, stuck to surfaces, and had salty taste. The fallout from the Operation Wigwam test consisted of 41.4% of irregular opaque particles, a bit over 25% of particles with transparent and opaque areas, about 20% were microscopic marine organisms, and 2% were microscopic radioactive threads of unknown origin.
The cloud contains three main classes of material: the remains of the weapon and its fission products, the material acquired from the ground (for burst altitudes below the fallout-free altitude, which depends on the weapon yield), and water vapor. The bulk of radiation contained in the cloud consists of the nuclear fission products; neutron activation isotopes from the weapon materials, air, and the ground debris are only a minor fraction. The neutron activation occurs during the neutron burst at the instant of the blast itself and the range of neutron reach is limited by the atmosphere absorption.
Most of the radiation is created by the fission products. Thermonuclear weapons are producing significant part of yield from nuclear fusion; fusion products are typically non-radioactive. The degree of radiation fallout production is therefore measured in kilotons of fission. Tsar Bomba, which produced 97% of the 50 Mt yield from fusion, was a relatively very clean weapon, as its fusion tamper was made of lead instead of uranium-238; otherwise the yield would be 100 Mt, 51 Mt of which would be of fission, and the fallout would amount to 25% of all nuclear weapon tests.
Initially, the fireball contains a highly ionized plasma consisting of atoms of the weapon, its fission products, and atmospheric gases. As the plasma cools, the atoms react, forming fine droplets and then solid particles of oxides. The particles coalesce to larger ones, and deposit on surface of other particles. Larger particles usually originate from material aspired into the cloud. Particles aspired while the cloud is still hot enough to melt them mix with the fission products throughout their volume. Larger particles get molten radioactive materials deposited on their surface. Particles aspired into the cloud later, when its temperature is low enough, do not become significantly contaminated. Particles formed only from the weapon itself are fine enough to stay airborne for long time and become widely dispersed and diluted to non-hazardous levels; higher-altitude blasts which do not aspire ground debris, or which aspire dust only after cooling enough and where the radioactive fraction of the particles is therefore small, cause much smaller degree of localized fallout than lower-altitude blasts with larger radioactive particles formed.
The concentration of condensation products is the same for the small particles and for the deposited surface layers of larger particles. About 100 kg of small particles par per kiloton of yield are formed. The volume, and therefore activity, of the small particles is almost three orders of magnitude lower than the volume of the deposited surface layers on larger particles.
For higher-altitude blasts, the primary processes for particle forming are condensation and subsequent coagulation. For lower-altitude and ground blasts, with involvement of soil particles, the primary process is deposition on the foreign particles.
A low-altitude detonation produces a cloud with dust loading of 100 tons per megaton of yield. A ground detonation produces clouds with about three times as much dust. About 200 tons per kiloton of soil, for a ground detonation, gets melted and comes in contact with radioactivity.
The fireball volume is the same for surface and atmospheric detonation. As in the first case the fireball is a hemisphere instead of a sphere, its radius is correspondingly larger.
The particle sizes range from submicrometer and micrometer sized (created by condensation of plasma in the fireball), through 10–500 micrometers (surface material agitated by the blast wave and raised by the afterwinds), to millimeter and above (ejecta from the crater). The size of particles together with the altitude they are carried to determines the length of their stay in the atmosphere, as larger particles are subject to dry precipitation. Smaller particles can be also scavenged by precipitation, whether of the moisture condensing in the cloud itself or by the mushroom meeting with a rain cloud; the fallout carried down by rain is known as rainout if scavenged during raincloud formation, washout if absorbed into already formed falling raindrops.
Particles from air bursts are smaller than 10–25 micrometers, usually in submicrometer range. They are composed mostly of iron oxides, with smaller proportion of aluminium oxide, and uranium and plutonium oxides. The particles larger than 1-2 micrometer are very spherical, corresponding to vaporized material condensing into droplets and then solidifying. The radioactivity is evenly distributed through the particle volume, making total activity of the particles linearly dependent on particle volume. About 80% of activity is present in more volatile elements, which condense only after the fireball cools to considerable degree. For example, strontium-90 will have less time to condense and coalesce into larger particles, resulting in greater degree of mixing in the volume of air and smaller particles. The particles produced immediately after the burst are small, with 90% of radiation activity present in particles below 300 nanometers. These coagulate with stratospheric aerosols. Coagulation in troposphere is more extensive, and at ground level most activity is present in particles between 300 nm and 1 micrometer. The coagulation offsets the fractionation processes at particle formation, evening out isotopic distribution.
For ground and low-altitude bursts, the cloud contains also vaporized, melted and fused soil particles. The distribution of activity through the particles depends on their formation; particles formed by vaporization-condensation have activity evenly distributed through volume as the air-burst ones, larger molten particles have the fission products diffused through the outer layers, and fused and non-melted particles that were not heated sufficiently but came in contact with the vaporized material or scavenged droplets before their solidification have a relatively thin layer of high activity material deposited on their surface. The composition of such particles depends on the character of the soil, usually a glass-like material formed from silicate minerals. The particle sizes do not depend on the yield but instead on the soil character, as they are based on individual grains of the soil or their clusters. Two types of particles are present; spherical, formed by complete vaporization-condensation or at least melting of the soil particles, with activity distributed evenly through the volume (or with a 10–30% volume of inactive core for larger particles between 0.5–2 mm), and irregular-shaped particles formed at the edges of the fireball by fusion of soil particles, with activity deposited in a thin surface layer. The amount of large irregular particles is insignificant. Particles formed from detonations above or in ocean will contain short-lived radioactive sodium isotopes, and salts from the sea water. Molten silica is a very good solvent for metal oxides and scavenges small particles easily; explosions above silica-containing soils will produce particles with isotopes mixed through their volume. In contrast, coral debris, based on calcium carbonate, tends to adsorb radioactive particles on its surface.
The elements undergo fractionation during particle formation, due to their different volatility. Refractory elements (Sr, Y, Zr, Nb, Ba, La, Ce, Pr, Nd, Pm) form oxides with high boiling points; these precipitate the fastest and at the time of particle solidification, at temperature of 1400 °C, are considered to be fully condensed. Volatile elements (Kr, Xe, I, Br) are not condensed at that temperature. Intermediate elements have their (or their oxides) boiling points close to the solidification temperature of the particles (Rb, Cs, Mo, Ru, Rh, Tc, Sb, Te). The elements in the fireball are present as oxides, unless the temperature is above the decomposition temperature of a given oxide. Less refractory products condense on surfaces of solidified particles. Isotopes with gaseous precursors solidify on the surface of the particles as they are produced by decay.
The largest and therefore the most radioactive particles are deposited by fallout in the first minutes to hours. Smaller particles are carried to higher altitudes and descend slower, reaching ground in less radioactive state as the shortest-time isotopes providing the most activity decay the fastest. The smallest particles can reach stratosphere and stay there for weeks, months, even years and reach the entire hemisphere by atmospheric currents. The high-danger, short-term, localized fallout is deposited primarily downwind from the blast site, in a cigar-shaped area, assuming a constant-strength, constant-direction wind; crosswinds, wind changes, and precipitation greatly alter the fallout pattern.
The condensation of water droplets in the mushroom cloud depends on the amount of condensation nuclei. Too large number of condensation nuclei actually inhibits condensation, as the particles compete for too low relative amount of water vapor.
Chemical reactivity of the elements and their oxides, adsorption properties of their ions, and solubility of their compounds influence their further distribution in the environment after deposition from the atmosphere. Bioaccumulation influences the propagation of fallout radioisotopes in the biosphere.
The primary radiation hazard of the fallout is gamma radiation from short-lived radioisotopes, which present the bulk of activity. Within 24 hours from the burst, the fallout gamma radiation level drops 60 times. Longer-life radioisotopes, typically caesium-137 and strontium-90, present a long-term hazard. Intense beta radiation from the fallout particles can cause beta burns shortly after the blast to people and animals coming in contact with the fallout. Ingested or inhaled particles cause internal dose of alpha and beta radiation, which may lead to long-term effects, including cancer.
The neutron irradiation of the atmosphere itself produces small amount of activation, mainly as a long-lived carbon-14 and short-lived argon-41. The elements most important for induced radioactivity for sea water are sodium-24, chlorine, magnesium, and bromine. For ground bursts, the elements of concern are aluminium-28, silicon-31, sodium-24, manganese-56, iron-59, and cobalt-60.
The bomb casing can be a significant sources of neutron-activated radioisotopes. The neutron flux in the bombs, especially the thermonuclear ones, is high enough for even high-threshold nuclear reactions. The induced isotopes include cobalt-60, 57 and 58, iron-59 and 55, manganese-54, zinc-65, yttrium-88, and likely also nickel-58 and 62, niobium-63, holmium-165, iridium-191, and short-lived manganese-56, sodium-24, silicon-31, and aluminium-28. Europium-152 and 154 can be also present, as well as two nuclear isomers of rhodium-102. During the Operation Hardtack, tungsten-185, 181 and 187 and rhenium-188 were produced from elements added as tracers to the bomb casings, to allow identification of fallout produced by specific explosions. Antimony-124, cadmium-109, and cadmium-113m are also mentioned as tracers.
The most significant radiation sources are the fission products from the primary fission stage, and in case of fission-fusion-fission weapons from the fission of the uranium tamper of the fusion stage. Many more neutrons per unit of energy are released in a thermonuclear explosion in comparison with purely fission one, influencing the fission products composition; e.g. the uranium-237 isotope is a unique marker of a thermonuclear explosion, as it is produced by a (n,2n) reaction from uranium-238, with the minimal neutron energy needed being about 5.9 MeV. Considerable amounts of neptunium-239 and uranium-237 are indicators of a fission-fusion-fission explosion; minor amounts of uranium-240 are also formed, and capture of large number of neutrons by individual nuclei leads to formation of negligible amounts of higher transuranium elements, e.g. einsteinium-255 and fermium-255.
The radioactivity of the particles decreases with time, with different isotopes being significant at different timespans. For soil activation products, during the first 15 minutes the aluminium-28 is the most important contributor, manganese-56 and sodium-24 follow until about 200 hours, iron-59 follows at 300 hours, and after 100–300 days, the significant contributor becomes cobalt-60.
Radioactive particles can be carried for considerable distances. Radiation from the Trinity test was washed out by a rainstorm in Illinois; this was deduced, and the origin traced, when Eastman Kodak found the x-ray films are being fogged by cardboard packaging produced in Midwest. Unanticipated winds carried lethal doses of Castle Bravo fallout over the Rongelap Atoll, forcing its evaluation; the crew of Daigo Fukuryu Maru, a Japanese fishing boat located outside of the predicted danger zone, was also affected. Strontium-90 found in a worldwide fallout later led to the Partial Test Ban Treaty.
The intense radiation in the first seconds after the blast may cause observable aura of fluorescence, the eerie blue-violet-purple glow of ionized oxygen and nitrogen at some distance from the fireball, surrounding the forming radioactive cloud. The light is best visible during the night or weak daylight. The brightness decreases rapidly, becoming barely visible in few tens of seconds.
Nuclear mushroom clouds are often also accompanied by short-lived vapor clouds, "Wilson clouds" also known as condensation clouds or vapor rings. These are created by the blast wave causing a sudden drop in the surrounding air temperatures, causing water vapor in the air to condense around the explosion cloud. When a nuclear weapon is detonated in humid air, the "negative phase" of the shock wave causes a rarefaction (reduction in density) of the air surrounding the explosion, but not contained within it. This rarefaction results in a temporary cooling of that air, which causes a condensation of some of the water vapor contained in it. When the pressure and the temperature return to normal, the Wilson cloud dissipates. Scientists observing the Operation Crossroads nuclear tests in 1946 at Bikini Atoll named that transitory cloud a "Wilson cloud" because of its superficial similarity to the appearance of the inside of a Wilson cloud chamber. (The cloud chamber effect is caused by electrically charged sub-atomic particles, not by a pressure wave.) Analysts of nuclear bomb tests later used the more general term "condensation cloud" for the "Wilson clouds".
The same kind of condensation is sometimes seen above the wings of low-altitude jet aircraft in a quite moist atmosphere. The top of a wing is the more-curved surface, and the curvature (and increased air velocity there) causes a reduction in the air pressure right there, as given by Bernoulli's Law. This reduction in air pressure causes a cooling, just as above, and the condensation of water vapor. Hence, the small, transient clouds appear. In technical terms, the "Wilson cloud" is also an example of the Prandtl–Glauert singularity in aerodynamics.
The shape of the shock wave, influenced by different speed in different altitudes, and the temperature and humidity of different atmospheric layers determines the appearance of the Wilson clouds. Condensation rings around or above the fireball are commonly observed. Rings around the fireball may become stable and form rings around the rising stem. Higher-yield explosions cause intense updraft, the air speed can reach 300 miles per hour; the entrainment of higher-humidity air together with the associated drop of pressure and temperature leads to formation of skirts and bells around the stem. If the water droplets become sufficiently large, the cloud structure they form may become heavy enough to descend; a rising stem with a descending bell around it can be formed. The layering of humidity in atmosphere that is responsible for the appearance of the condensation rings influences also the shape of the condensation artifacts along the mushroom stem, as the updraft causes laminar flow of air upwards. The same effect above the top of the cloud, where the expansion of rising cloud pushes a layer of humid air above the cloud from lower altitudes, and the low temperatures in high altitudes causes condensation of water vapors and freezing of the droplets, forming ice caps (or icecaps), similar in appearance and mechanism of formation to the scarf clouds.
The resulting structures can be fairly complex. The Castle Bravo cloud at various phases of its development had 4 condensation rings, 3 ice caps, 2 skirts and 3 bells.
Mushroom cloud with multiple condensation rings from the Castle Union 6.9 Mt hydrogen bomb test
A 21-kiloton underwater nuclear weapon test, showing a Wilson cloud
Greenhouse George, 225 kt, showing a well-developed bell
Of further note, the streaks of smoke seen to the left of the explosion at detonation are vertical smoke flares which are used to observe the shockwave.