contour of 0.03 milliroentgen per hour around the hypocenter. The highest intensity within this area was about 0.1 milliroentgen per hour. 6.2.2 Radioactivity Downwind of the Cities. As the radioactive cloud was borne downwind from the center of each city, rainshowers within the hour after the detonation caused some of the fission products and unfissioned residue of the bomb to be carried to earth in a manner similar to fallout. This "rainout" produced a small pattern of radioactivity to the west of Hiroshima, near the village of Takasu; and a somewhat larger area to the east of Nagasaki, in the vicinity of the Nishiyama Reservoir. Other areas of fallout were documented farther downwind of the Nishiyama rainout. Figures 18 and 19 show the areas and intensities of residual radioactivity caused by the rainout. Of the four patterns of measurable residual radioactivity remaining in and around the two cities upon the arrival of the occupation troops, the most significant was in the vicinity of the Nishiyama Reservoir outside Nagasaki, indicated in figure 19. This area, outlined by the contour of x’s, had a slightly greater radiation intensity than the other areas. Inside the contour, the intensity rose gradually to a high of about one milliroentgen per hour at the time of the troops’ arrival. Outside the contour, the intensity fell to background levels very quickly in the reservoir area and in the direction of Nagasaki. Moreover, this pattern east of Nagasaki was the only one of the four that included slight levels of plutonium in the radioactive mixture. The terrain was, however, remote and rugged, characterized by steep slopes and heavy forests, with few trails or roads and even fewer buildings. The Japanese population was sparse, and there were no occupation forces and little need for military patrols in the area. The small rainout pattern west of Hiroshima, shown by the oval of x’s in figure 18, had an intensity of 0.03 milliroentgen per hour. The exception was the center of the oval, which registered a high of less than 0.05 milliroentgen per hour when the occupation troops reached this part of Japan. By the time of the occupation, the intensity of the ground radioactivity caused by rainout had dropped to less than a thousandth of the intensity 1 hour after shot-time. of fission products. The main reason for this was the rapid overall decay For example, a given intensity of radioactivity 1 hour 158 :