The irregular variations in mass distribution around the bomb core create the mottled blob-like appearance. The various light and dark patches are caused by the varying vapor density of the material splashing against the back of the shock front. However, this acceleration happens in a short period, so the material is trapped behind the shock front, even though it eventually travels faster than the shock front. These vapors are accelerated to very high velocities, several tens of kilometers per second, faster than the shock front. In the first few microseconds after detonation, the bomb casing and shot cab are destroyed and vaporized.
At the point in the explosion captured in the photo above, the shock front has passed the original radiative fireball and has about twice its size. The shock wave contains so much energy that the compression heating it causes in the air causes it to glow. After a brief period this shock front reaches and then passes the initial radiative fireball. This moves outward at supersonic speeds, creating a hydrodynamic shock wave at its outer edge. Inside the radiative fireball the bomb itself is rapidly expanding due to the heat generated by the nuclear reactions. This is known as a "radiatively driven" fireball. These x-rays cannot travel very far in the lower atmosphere before reacting with molecules in the air, so the result is a fireball that rapidly forms within about 10 metres (33 ft) in diameter and does not expand. In the initial microseconds after the explosion, a fireball is formed around the bomb by the massive numbers of thermal x-rays released by the explosion process. The cause of a surface mottling is more complex. Because of the lack of guy wires, no "rope trick" effects were observed in surface-detonation tests, free-flying weapons tests, or underground tests. Malik observed that when the rope was painted black, spike formation was enhanced, and if it were painted with reflective paint or wrapped in aluminium foil, no spikes were observed – thus confirming the hypothesis that it is heating and vaporization of the rope, induced by exposure to high-intensity visible light radiation, which causes the effect. The "rope tricks" that protrude from the bottom of the fireball are caused by the heating, rapid vaporization and then expansion of guy wires (or specialized rope trick test cables) that extend from the shot cab, the housing at the top of the tower that contains the explosive device, to the ground. Anything solid in the area absorbs the light and rapidly heats. The surface of the fireball, with a temperature over 20,000 kelvins, emits huge amounts of visible light radiation, more than 100 times the intensity at the Sun's surface. The adjacent photograph shows two unusual phenomena: bright spikes projecting from the bottom of the fireball, and the peculiar mottling of the expanding fireball surface.
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