The Fallout Pattern
The details of the actual fallout pattern depend on wind speed and direction and on the terrain. The fallout will contain about 60 percent of the total radioactivity. The largest particles will fall within a short distance of ground zero. Smaller particles will require many hours to return to earth and may be carried hundreds of miles. This means that a surface burst can produce serious contamination far from the point of detonation.
From the 15-megaton thermonuclear device tested at Bikini Atoll on March 1, 1954 - the BRAVO shot of Operation CASTLE - the fallout caused substantial contamination over an area of more than 7,000 square miles. The contaminated region was roughly cigar-shaped and extended more than 20 miles upwind and over 350 miles downwind.
Fallout can also enter into the stratosphere. In this stable region, radioactive particles can remain from 1 to 3 years before returning to the surface.