The tremendous heat produced by any conventional or nuclear blast causes an up-draft of air which forms the familiar mushroom cloud.

When a nuclear blast occurs near the earth's surface, millions of vaporized dirt particles are also drawn into the cloud. As the heat diminishes, any radioactive materials that have been vaporized condense on the drawn-up dirt particles, which are also condensing.

Eventually these particles fall back to earth. This phenomenon is called radioactive fallout. This fallout material decays over a long period of time, and is the main source of the residual nuclear radiation.