Mushroom Cloud - Nuclear Mushroom Clouds

Nuclear Mushroom Clouds

Nuclear 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 are usually 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 altitudes of approximately 7 meters/kiloton, a crater is not formed, and correspondingly lower amounts of dust and debris are produced. The fallout-free height, above which the radioactive particles consist only of the fine fireball condensation, is approximately 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 due to atmospheric friction and cools its surface by energy radiation, turning from a sphere to a violently swirling annular vortex. A Rayleigh–Taylor instability is formed at the boundary between the hot fireball and the surrounding cooler air. This causes turbulence and a vortex which sucks air into its center, creating afterwinds and cooling itself. The speed of its swirling slows down as it cools, and may stop entirely during later phases. 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 as atmospheric moisture condenses. As the cloud ascends and cools, its buoyancy lessens, 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 contains sufficient energy, part of it may continue rising up into stratosphere. A 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 phase change releases latent heat which heats the cloud, driving it to yet higher altitudes.

A mushroom cloud undergoes several phases of formation.

• Early time, the first about 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.
• Rise and stabilization phase, 10 seconds to 10 minutes, when the hot gases rise up and early fallout is deposited.
• 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 cloud is influenced by the atmospheric conditions and wind patterns. Fallout distribution is predominantly a downwind plume. However if the cloud reaches the tropopause, it may spread against the wind direction, as the convection speed is higher than the ambient wind speed. The tropopause cloud shape 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 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 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 path.

The stem of the cloud is gray to brown in a 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 the bomb and its casing, and therefore produce more radioactive fallout with larger particles that deposit locally.

A higher-yield detonation can carry the nitrogen oxides 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 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.