The fallout radius of a nuclear detonation represents the area that may be contaminated by fallout. Fallout particles are held aloft by atmospheric winds, but begin to rain down shortly after the blast and represent a radiation hazard.
Wind patterns can significantly effect the fallout radius of a nuclear blast. Wind can blow deadly debris downwind and creating an elliptical fallout shadow.
The radioactive isotopes present in fallout decay relatively quickly. Fallout can be extremely dangerous in the hours following a nuclear detonation. A region that has been contaminated by fallout may be safe for humans in as little as three to five weeks.