question: What causes significant explosions to produce the iconic mushroom shape cloud to form?
comment: The explosion creates a region of low density hot gas. This rises. Typically, given it is a big explosion, there's smoke/debris which are then pulled upwards with the smoke. The stem. Once the gas reaches an elevation where the air has the same density as itself, it stops ascending and spreads outwards, with the debris it contains. The cap.
passages: After the mass of hot gases reaches its equilibrium level, the ascent stops, and the cloud starts flattening the characteristic mushroom shape, usually aided by surface growth due to the decaying turbulence.
  
  Section::::Nuclear mushroom clouds.
  
  Nuclear detonations produced high above the ground might not create mushroom clouds with a stem. The heads of the clouds themselves consist of highly radioactive particles, primarily the fission products and other weapon debris aerosols, and are usually dispersed by the wind, though weather patterns (especially rain) can produce problematic nuclear fallout.
   A higher-yield detonation can carry the nitrogen oxides from the burst high enough in atmosphere to cause significant depletion of the ozone layer.
  
  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 generated by the heat from the hot, freshly formed crater.
   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, creating a bubble which then collapses in on itself; in the case of a less deep underground explosion, this produces a subsidence crater. Detonations underwater but near the surface produce a pillar of water, which, in collapsing, forms a cauliflower-like shape, which is easily mistaken for a mushroom cloud (such as in the well-known pictures of the "Crossroads Baker" test). Underground detonations at low depth produce a mushroom cloud and a base surge, two different distinct clouds. The amount of radiation vented into the atmosphere decreases rapidly with increasing detonation depth.
   Mushroom cloud
  
  A mushroom cloud is a distinctive pyrocumulus mushroom-shaped cloud of debris/smoke and usually condensed water vapor resulting from a large explosion. The effect is most commonly associated with a nuclear explosion, but any sufficiently energetic detonation or deflagration will produce the same effect. They can be caused by powerful conventional weapons, like thermobaric weapons, including the ATBIP and GBU-43/B Massive Ordnance Air Blast. Some volcanic eruptions and impact events can produce natural mushroom clouds.
   The intense radiation in the first seconds after the blast may cause an observable aura of fluorescence, the blue-violet-purple glow of ionized oxygen and nitrogen out to a significant distance from the fireball, surrounding the head of the forming mushroom cloud. This light is most easily visible at night or under conditions of weak daylight. The brightness of the glow decreases rapidly with elapsed time since the detonation, becoming only barely visible after a few tens of seconds.
  
  Section::::Nuclear mushroom clouds.:Condensation effects.
   BULLET::::- "Early time", the first ≈20 seconds, when the fireball forms and the fission products mix with the material aspired from the ground or ejected from the crater. The condensation of evaporated ground occurs in first few seconds, most intensely during fireball temperatures between 3500–4100 K.
  
  BULLET::::- "Rise and stabilization phase", 20 seconds to 10 minutes, when the hot gases rise up and early large fallout is deposited.
  
  BULLET::::- "Late time", until about 2 days later, when the airborne particles are being distributed by wind, deposited by gravity, and scavenged by precipitation.
   The shape of the shock wave is influenced by variation of the speed of sound with altitude, and the temperature and humidity of different atmospheric layers determines the appearance of the Wilson clouds. Condensation rings around or above the fireball are a commonly observed feature. Rings around the fireball may become stable, becoming rings around the rising stem. Higher-yield explosions cause intense updrafts, where air speeds can reach . The entrainment of higher-humidity air, combined with the associated drop in pressure and temperature, leads to the 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; in this way, a rising stem with a descending bell around it can be produced. Layering of humidity in the atmosphere, responsible for the appearance of the condensation rings as opposed to a spherical cloud, also influences the shape of the condensation artifacts along the stem of the mushroom cloud, as the updraft causes laminar flow. The same effect above the top of the cloud, where the expansion of the rising cloud pushes a layer of warm, humid, low-altitude air upwards into cold, high-altitude air, first causes the condensation of water vapour out of the air and then causes the resulting droplets to freeze, forming "ice caps" (or "icecaps"), similar in both appearance and mechanism of formation to scarf clouds.
   Mushroom clouds are formed by many sorts of large explosions under earth's gravity, but they are best known for their appearance after nuclear detonations. Without gravity, the explosive's by-product gases would remain spherical. Nuclear weapons are usually detonated above the ground (not upon impact, because some of the energy would be dissipated by the ground motions), to maximize the effect of their spherically expanding fireball and blast wave. Immediately after the detonation, the fireball begins to rise into the air, acting on the same principle as a hot-air balloon.
   A low-altitude detonation produces a cloud with a dust loading of 100 tons per megaton of yield. A ground detonation produces clouds with about three times as much dust. For a ground detonation, approximately 200 tons of soil per kiloton of yield is melted and comes in contact with radiation.
  
  The fireball volume is the same for a surface or an atmospheric detonation. In the first case, the fireball is a hemisphere instead of a sphere, with a correspondingly larger radius.
   The distribution of radiation in the mushroom cloud varies with the yield of the explosion, type of weapon, fusion/fission ratio, burst altitude, terrain type, and weather. In general, lower-yield explosions have about 90% of their radioactivity in the mushroom head and 10% in the stem. In contrast, megaton-range explosions tend to have most of their radioactivity in the lower third of the mushroom cloud.
   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 are formed per kiloton of yield. 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 particle forming processes 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.
   Although the term appears to have been coined at the start of the 1950s, mushroom clouds generated by explosions were being described centuries before the atomic era. A contemporary aquatint by an unknown artist of the 1782 Franco-Spanish attack on Gibraltar shows one of the attacking force's floating batteries exploding with a mushroom cloud, after the British defenders set it ablaze by firing heated shot. The 1917 Halifax Explosion produced one. "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 World War II, descriptions of mushroom clouds were relatively common.
   Section::::Earth impactor model.:Blast patterns.
  
  The explosion's effect on the trees near the hypocentre of the explosion was similar to the effects of the conventional Operation Blowdown. These effects are caused by the blast wave produced by large airburst explosions. The trees directly below the explosion are stripped as the blast wave moves vertically downward, while trees farther away are knocked over because the blast wave is travelling closer to horizontal when it reaches them.
   Initially, the fireball contains a highly ionized plasma consisting only of atoms of the weapon, its fission products, and atmospheric gases of adjacent air. 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 a 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 first pictures released of a nuclear explosion—the blast from the Trinity test—focused on the fireball itself; later pictures would focus primarily on the mushroom cloud that followed. After the United States began a regular program of nuclear testing in the late 1940s, continuing through the 1950s (and matched by the Soviet Union), the mushroom cloud has served as a symbol of the weapons themselves.
   With surface and near-surface air bursts, the amount of debris lofted into the air decreases rapidly with increasing burst altitude. At burst altitudes of approximately 7meters/kiloton, a crater is not formed, and correspondingly lower amounts of dust and debris are produced. The fallout-reducing height, above which the primary radioactive particles consist mainly of the fine fireball condensation, is approximately 55meters/kiloton. However, even at these burst altitudes, fallout may be formed by a number of mechanisms.
   Section::::Nuclear mushroom clouds.:Cloud composition.
   The explosion raises a large amount of moisture-laden air from lower altitudes. As the air rises, its temperature drops, and its water vapour first condenses as water droplets, and later freezes as ice crystals. The phase changes release latent heat, heating the cloud and driving it to yet higher altitudes.
  
  A mushroom cloud undergoes several phases of formation.
   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 initially ionized nitrogen, oxygen, and atmospheric moisture. 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 have also been described. This reddish hue is later obscured by the white colour of water/ice clouds, condensing out of the fast-flowing air as the fireball cools, and the dark colour of smoke and debris sucked into the updraft. The ozone gives the blast its characteristic corona discharge-like smell.
   For an explosion in the atmosphere, the fireball quickly expands to maximum size, and then begins to cool as it rises like a balloon through buoyancy in the surrounding air. As it does so it takes on the flow pattern of a vortex ring with incandescent material in the vortex core as seen in certain photographs. This effect is known as a mushroom cloud.
  
  Sand will fuse into glass if it is close enough to the nuclear fireball to be drawn into it, and is thus heated to the necessary temperatures to do so; this is known as trinitite.
   The resulting composite structures can become very complex. The Castle Bravo cloud had, at various phases of its development, 4 condensation rings, 3 ice caps, 2 skirts, and 3 bells.
  
  Section::::Bibliography.
  
  BULLET::::- Glasstone, Samuel, and Dolan, Philip J. "The Effects of Nuclear Weapons" 3rd edn. Washington, D.C.: United States Department of Defense and Energy Research and Development Administration, 1977. (esp. "Chronological development of an air-burst" and "Description of Air and Surface Bursts" in Chapter II)
  
  BULLET::::- Vigh, Jonathan. "Mechanisms by Which the Atmosphere Adjusts to an Extremely Large Explosive Event", 2001.
  
  Section::::External links.
   At the moment of the explosion, the fireball is formed. The ascending, roughly spherical mass of hot, incandescent gases changes shape due to atmospheric friction and cools its surface by energy radiation, turning from a sphere to a violently rotating spheroidal vortex. A Rayleigh–Taylor instability is formed as the underneath cool air initially pushes the bottom fireball gases into an inverted cup shape. This causes turbulence and a vortex that sucks more air into its center, creating external afterwinds and cooling itself. The speed of its rotating slows down as it cools, and may stop entirely during later phases. The vaporized parts of the weapon and ionized air cool into visible gases, forming the early cloud; the white-hot vortex core becomes yellow, then dark red, then loses visible incandescence. With further cooling, the bulk of the cloud fills in as atmospheric moisture condenses. As the cloud ascends and cools, its buoyancy lessens, and its ascent slows.
   The exhaust plume may also take on a corkscrew appearance as it is whipped around by upper level wind currents. It is typically seen within two to three minutes after a launch has occurred. Depending on weather conditions, it could remain in the sky for up to half an hour before dispersing.
   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.
  
  Section::::Physics.
   The crater itself is referred to as The Pitt, as is the event that created it. Within the fictional universe, it is occasionally also referred to as The Black Event (in contrast to the earlier White Event).
  