descends into the lower atmosphere. It should also be mentioned that roughly 50 percent of the cst 37 found in surface air during the fall of 1960 was pre-Hardtack in origin as deduced from activity ratios. Hence it would appear that the initial estimates of stratospheric residence time of 5 to 10 years based upon measurements of debris from Castle may not have been wide of the mark, The time dependence of stratospheric residence time coupled with the non-uniformity in the rate of removal of stratospheric debris adds greatly to the complexity of fallout mechanics; however, the proper consideration of these factors will not only improve the accuracy with which ground contamination from present stratospheric debris may be predicted, but will also further our understanding of the upper atmosphere. (A classified appendix to this paper appears in Part II of the Proceedings, SCR-421.) References 1. L. B. Lockhart, Jr., R. A. Baus, R. L. Patterson, Jr., and A. W. Saunders, Jr., Science 130, 2. T. Hvinden, D. Hvending, A. Lillegraven, andS. H. Small, Nature 185, 805 (1960). 3. This work was performed under the auspices of the U.S. Atomic Energy Commission. The generosity of Dr. J. Sedlet of the Industrial Hygiene and Safety Division of this Laboratory in providing air filters from his site-monitoring system is hereby acknowledged. 4, E. A. Martell, Geophysical Research Papers No. 65, "Global Fallout and its Variability'', October 1959, AFCRC-TR-59-268. 5. L. B. Lockhart, Jr., R. A. Baus, R. L. Patterson, Jr., and A. W. Saunders, Jr., NRL Report 5390, October 23, 1959. U. S. Naval Research Laboratory, Washington, D. C. 6. A. J. Cruikshank, G. Cowper, and W. E. Grummitt, Can. J. Chem. 34, 214 (1956). 161 (1959). 129-130