Fallout

Types of fallout

Fallout is usually divided into two major types, largely determined by the height of burst of the detonation. If detonated at a sufficient altitude that allows the fireball to avoid mixing significantly with ground debris, the radioactive byproducts in the fallout will generally stay aloft longer than weapons detonated at or near the surface. This additional time aloft allows the most acutely dangerous radioactive elements, with the shortest half-lives, to decay prior to their descending to the surface, reducing the overall radioactive intensity of the fallout that is deposited. This additional time also allows the radioactive cloud to diffuse over a larger area, resulting in less radioactive debris per area of land below. This more diffuse form of fallout is known as "global fallout," because its main effect is to raise background radiation exposure slightly over vast areas, and is contrasted with "local fallout," which produces a plume of concentrated radioactive byproducts downwind of the detonation within minutes or hours.