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How does radioactive fallout form after a surface nuclear explosion?

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Atomic bomb — atomic bomb The first atomic bomb test, near Alamogordo, New Mexico, July 16, 1945.

When a nuclear explosion occurs on or near the Earth's surface, the intense heat and blast force vaporize soil, water, and other materials in the immediate vicinity. These materials are sucked upward into the rising fireball and cloud, where they mix with the radioactive fission products created by the weapon.12

As this cloud cools, the radioactive materials condense onto the particles of vaporized earth and debris. Because these particles are relatively large and heavy, they begin to settle back to the ground relatively quickly, often within the first 24 hours. This process creates what is known as local fallout, which can contaminate large areas and pose an immediate, extreme biological hazard.1

In addition to these larger particles, finer radioactive dust may be carried by prevailing winds to settle over more extensive regions as delayed fallout. The total radioactivity of these fission products is initially very high but decreases rapidly over time due to radioactive decay.1