Descent of radioactive materials from the atmosphere to the earth.
Radioactivity in the atmosphere may arise from natural causes such as 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 {{link=troposphere">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.