- ll - In Lima, Peru, the total fallout of Sr90 in January, 1956, was 0.7 mc/mi2. The annual precipitation in Lima averages only 1.89 inches, though there is a considerable amount of ground fog and mist. . It seems clear from these results and the reasonableness of the mechanism of deposition advanced by Mr. Greenfield that there is valid reason to believe that world-wide fallout is small in the absence of precipitation. Also, it is clear from this mechanism. that the fallout should not be strictly proportional to total rainfall. Frequent light rains or mists would be expected to be more efficient than occaSional heavy rains. So, the importance of rain is © only to be revealed by a study of desert areas and a careful investigation of the scavenging mechantsn - itself. This work may well prove to be of considerable - importance in meteorology, as well as to fallout studies. One should note that the local fallout due to larger particles which descend in the first hours ‘ probably does not need rain to precipitate it and occurs in the absence of the precipitation of moisture, although rain may well be able to increase even this fallout. The importance of precipitation as a scavenging mechanism raises the possibility that different regions will be subjected to varying intensities of fallout, depending upon the weather conditions. It will be important to test whether this is so and whether it is a major effect in populated areas. We have evidence showing that extreme aridity greatly reduces the longrange or world-wide fallout as explained above. The evidence to date does not indicate that it is a major. “ mo, effect for normal climates in the sense that it does not appear to amount to more than a factor of 2. Regions in which people live normally have enough precipitation so that differences in precipitation appear not to affect the fallout by more than such ‘a factor. Careful study of the data appended and (more) 13K