The base surge is a cloud that rolls outward from the bottom of the column produced by a subsurface explosion. After the water evaporates, an invisible base surge of small radioactive particles may persist. For subsurface land bursts, the surge is made up of small solid particles, but it still behaves like a fluid. A soil earth medium favors base surge formation in an underground burst. Meteorological conditions will greatly influence fallout, particularly local fallout.
Atmospheric winds are able to distribute fallout over large areas.
Snow and rain, especially if they come from considerable heights, will accelerate local fallout. Under special meteorological conditions, such as a local rain shower that originates above the radioactive cloud, limited areas of heavy contamination may be formed.