The Trinity Testing Area Fallout, also called radioactive precipitation or nuclear precipitation, is the designation of the radiation hazard that is formed by descending dust which is radioactive in the minutes to days after a nuclear explosion in the atmosphere or on the earth's surface dust descends around the explosion point or (by spreading in the air and on the wind) far from there. Also the radioactive substance itself is referred to as a fallout.
The fall-out consists of three types of radioactive material:
Core explosions high in the atmosphere give little fall-out and explosions underground give no fall-out. The biggest fallout occurs at an explosion at the ground level, which wakes up a lot of dust that becomes radioactive by irradiation.
Fallout also occurs in accidents at nuclear reactors where clouds escape radioactive material, such as after Chernobyl disaster. It is characterized by a larger proportion of isotopes with a long half-life, because these isotopes are present in the reactor to a large extent. Fall-out caused by a nuclear weapon has a larger share of short-lived isotopes formed immediately after nuclear explosion. Footnote
wiki
