Fig. 2. Early history of the Mike cloud. The figures indicate the number of days between detonation and the first ground observation of fission products. Fig. 3. Early history of the Bravo cloud. The figures indicate the number of days between detonation and the first ground observation of fission products. Fig. 4. Total radioactive fallout from the Mike cloud in the period from 2 to 35 days after detonation, in millicuries per 100 square miles. Hatching indicates the approximate Novemberposition of the Intertropical Convergence Zone, the belt of low pressure that tends to separate Northern and Southern Hemisphere air near the surface of the earth. 476 SCIENCE, VOL. 124