Thus a large fractionation relative to fission products and radioactive carbon dioxide occurs. Of course, there probably is some entrainmentof fission products on the surfaces of the falling ice crystals by the Greenfield Brownian motion accretion mechanism. In fact, it is known that about 1 per cent of megaton yield off-site fallout occurs in the early banded tropospheric man- ner. This may be due to this entrainment and thus one would expect that the latitudinal distribu- tions of early tropospheric fallout of both fission products and tritium water from megaton yield bombsfired in the troposphere!! should be identical. No satisfactory data are now available to check this point. In the calculations in this paper the figure of 1 per cent for tropospheric contribution from megaton yields has been used. 3 CONCLUSION The more recent data, particularly on bomb C"‘, when taken together with the earlier data on bomb fission products and tritium, give us some confidence in our present understanding of the fallout mechanism. All of these observations and considerations afford unprecedented op- portunities for the study of meteorology and geophysics, particularly in an international cooperative effort such as the International Geophysical Year. REFERENCES 1. Hearings before the Special Subcommittee on Radiation of the Joint Committee on Atomic Energy, The Nature of Radioactive Fallout and Its Effects on Man, May 27-29, June 3-7, ® w Bb 1957, Parts 1 and 2. U. S. Government Printing Office, Washington, 1957. . E. A. Martell, Project Sunshine Bulletin #12, August 1, 1956, AECU-3297(Rev.). . E. A. Martell, The Chicago Sunshine Method, May, 1956, AECU-3262. . W. F. Libby, Radioactive Strontium Fallout, Proc. Natl. Acad. Sci.42, 365-90 (1956). on . W. F. Libby, Current Research Findings on Radioactive Fallout, Proc. Natl. Acad. Sci, 42, 1m 945-56 (1956). . W. F. Libby, Radioactive Fallout, Proc. Natl. Acad. Sci, 43, 758-75 (1957). . W. F. Libby, Dosages from Natural Radioactivity and Cosmic Rays, Science 122, 57-8 (1955). 8. Merril Eisenbud and J. H. Harley, Radioactive Fallout in the United States, Science 121, 677-80 (1955). 9. Merril Eisenbud and J. H. Harley, Radioactive Fallout through September 1955, Science 124, 251-55 (1956). 10a. J. L. Kulp, W. R. Eckelmann, and A R Schulert, Strontium-90 in Man, Science 125, 219-25 (1957), 10b. J. L. Kulp, W. R. Eckelmann, and A. R. Schulert, Strontium-90 in Man, H, Science 127, 266-74 (1958). 11. L. Machta, R. J. List, and L. F. Hubert, World-wide Travel of Atomic Debris, Science 124, 474-7 (1956). 12. “World-wide Effects of Atomic Weapons, Project Sunshine,” August 6, 1953, R-251-AEC (amended). 13. F. J. Bryant, A. C. Chamberlain, A. Morgan, G. S. Spicer, Radiostrontium in Soil, Grass, and Bone in U.K.: 1956 Results, AERE HP/R 2353 (1957). 14. The Hazards to Man of Nuclear and Allied Radiations, British Medical Research Council (1956). 15. ¥. Hiyama, A Measure of Future Strontium-90 Level from Earth Surface to Human Bone, Gakujutsu Geppo 10, 27-43 (1957). 16. Y. Hiyama, Radiological Data in Japan Il, Gakujutsu Geppo 10, 1-17 (1957). 17. E. Dahl, The Dangers from Fallout of Strontium-90 after Atomic Bomb Explosions, Tek. Ukeblad (July 1957). 18. N. G. Stewart, R. N. Crooks, and E. M. R. Fisher, The Radiological Dose to Persons in the U.K. Due to Debris from Nuclear Test Explosions Prior to January 1956, AERE HP/R 2017 (1956). 19. O. Haxel and C. Schumann, Selbstreinigung der Atmosphire, Z. Physik 142, 126-32 (1955). 266