Descent of radioactive materials from the atmosphere to the earth. Radioactivity in the atmosphere may arise from natural causes such as cosmic rays as well as from nuclear explosions and atomic reactor operations. The explosion of nuclear weapons leads to three types of fallout: local, tropospheric, and stratospheric. The first, intense but relatively short-lived, occurs as larger radioactive particles are deposited near the site of the explosion. Tropospheric fallout occurs when the finer particles enter the troposphere, and it spreads over a larger area in the month after the explosion. Stratospheric fallout, made of fine particles in the stratosphere, may continue years after the explosion, and the distribution is nearly worldwide. Many different radioisotopes are formed during a nuclear explosion, but only long-lived isotopes (e.g., cesium-137, strontium-90) are deposited as stratospheric fallout.
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