Mushroom cloud

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.

Condensation effects

Nuclear mushroom clouds are often accompanied by short-lived vapour clouds, known variously as "Wilson clouds", condensation clouds, or vapor rings. The "negative phase" following the positive overpressure behind a shock front causes a sudden rarefaction of the surrounding medium. This low pressure region causes an adiabatic drop in temperature, causing moisture in the air to condense in an outward moving shell surrounding the explosion. When the pressure and 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 visual similarity to a Wilson cloud chamber; the cloud chamber uses condensation from a rapid pressure drop to mark the tracks of electrically charged subatomic particles. Analysts of later nuclear bomb tests used the more general term "condensation cloud" in preference to "Wilson cloud".

The same kind of condensation is sometimes seen above the wings of jet aircraft at low altitude in high-humidity conditions. The top of a wing is a curved surface. The curvature (and increased air velocity) causes a reduction in air pressure, as given by Bernoulli's Law. This reduction in air pressure causes cooling, and when the air cools past its dew point, water vapour condenses out of the air, producing droplets of water, which become visible as a white cloud. In technical terms, the "Wilson cloud" is also an example of the Prandtl–Glauert singularity in aerodynamics.

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 300 mph. 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.

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. File:The Castle Bravo test.jpg|The mushroom cloud from the 15-megaton Castle Bravo hydrogen bomb test, showing multiple condensation rings, 1 March 1954 File:Castle romeo2.jpg|The mushroom cloud from the 11-megaton Castle Romeo hydrogen bomb test, showing a prominent condensation ring File:Castle Union.jpg|The mushroom cloud from the 6.9-megaton Castle Union hydrogen bomb test, showing multiple condensation rings File:Crossroads baker explosion.jpg|The water column from the 21-kiloton Crossroads Baker test, involving a nuclear underwater explosion, showing a prominent, spherical Wilson cloud File:Greenhouse George.jpg|The mushroom cloud from the 225-kiloton Greenhouse George test, showing a well-developed bell