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What Happens When a Nuclear Bomb Goes Off?When the bomb goes off, it releases a lot of "gamma rays." The gamma rays can't go far; they're soaked up by the area right around the blast. The substances that absorb the blast become radioactive: the dirt, rocks, or asphalt on the ground, buildings and cars, even the air.
These gamma rays also hit the sucked-up tiny particles, and the particles become radioactive.
The fireball continues to rise, which causes the well-known mushroom appearance. As it rises, the fireball carries the radioactive particles high into the air. When the heat finally dissipates, the now-radioactive particles start to fall. The cloud of radioactive particles is the "fallout cloud." The particles are so tiny and lightweight that they fall very slowly.
The prevailing winds in the area carry the particles away from the center of the nuclear blast, so the fallout cloud affects a much larger area than the explosion does. Where particles land, they give off radioactivity until the radioactivity wears off.