Condensation cloud

A transient condensation cloud, also called Wilson cloud, is observable at large explosions in humid air.

When a nuclear weapon or a large amount of a conventional explosive is detonated in sufficiently humid air, the "negative phase" of the shock wave causes a rarefaction (reduction in density) of the air surrounding the explosion, but not contained within it. This rarefaction results in a temporary cooling of that air, which causes a condensation of some of the water vapor contained in it. When the pressure and the temperature return to normal, the Wilson cloud dissipates.