This debris combines with the condensed vapor to form the familiar mushroom-shaped cloud. Detonation of the nuclear bomb creates a blast wave that travels out in all directions at an initial speed greater than the speed of sound. When the wave strikes the earth's surface, another wave is formed by reflection. At some distance from ground zero, the primary and reflected waves combine to form a reinforced blast wave. Pressure at the wave front, called overpressure, is many times that of normal atmospheric pressure and is what causes most of the physical damage.
Overpressure decreases as distance from the blast increases.
The greatest danger from residual radiation is fallout or the return to earth of radioactive particles of the cloud. In an airburst, most of the particles are carried high into the air where they are scattered by the winds and returned to the earth slowly. Fallout from low-altitude airburst presents a greater hazard because the heavy particles of debris picked up from the surface settle rapidly and are highly radioactive, but the hazard is not so great as that from surface and subsurface bursts. Surface burst Produces the worst fallout. The fireball touches the ground.