Meteorological conditions greatly influence fallout , particularly local fallout . Atmospheric winds are able to bring fallout over large areas . For example , as a result of a '' Castle Bravo '' surface burst of a 15 Mt thermonuclear device at Bikini Atoll on March 1 , 1954 , a roughly cigar-shaped area of the Pacific extending over 500 km downwind and varying in width to a maximum of 100 km was severely contaminated . There are three very different versions of the fallout pattern from this test , because the fallout was only measured on a small number of widely spaced Pacific Atolls . The two alternative versions both ascribe the high radiation levels at north Rongelap to a downwind hotspot caused by the large amount of radioactivity carried on fallout particles of about 50 – 100 micrometres size
Castle Bravo
Castle Bravo
Nuclear fallout