STATUS OF GLOBAL FALLOUT PREDICTIONS 381 that is, it practically never precipitates from the rare clouds in the stratosphere. Early estimates of the mean residence time given by Libby ranged from five to ten years based on estimated stratospheric burdensof "Sr and pitifully incomplete deposition information. In general, most estimates in recent years have been for shorter times than the original values and are based on much more complete information. The sim- plicity of the method of describing and forecasting stratospheric fallout has led many persons to employ the exponential formulation, even today, despite its limitation." Problems with the use of a single mean residence time for debris introduced anywhere in the stratosphere arose in the late 1950’s. The Sr from the autumn 1958 U.S.S.R. tests appeared to return to the tro- posphere with a mean residence time of one-half year, whereas the ex- periment with radiotungsten, whose source was the lower equatorial stratosphere, yielded a longer mean residence time. Furthermore, it was felt that virtually no ‘*Rh from the Orange rocket shot of Aug. 12, 1958, at Johnston Island had reentered the troposphere in the first year after its injection at very high altitudes. Table 3, modified from a forthcoming report,'! summarizes some of the observed mean residence times in years. There are three methods by which the mean residence times may be computed, The first and oldest simply compares, say, the annual amount of fallout with the Stratospheric content at the beginning of the year. This is the counter- part of the forecasting procedure in which the future annual deposition is derived from the stratospheric content at the beginning of the year. A second procedure compares the stratospheric content at two times, say, the beginning and the end of a year. The decrease readily provides Table 3—SOME ESTIMATES OF MEAN STRATOSPHERIC RESIDENCE TIMES Yearof measurement 1959 ' 1960 1961 1963 1959 and 1960 1960 and 1961 Northern hemisphere 30S 2.0* 1.2* 2.1* 0.8t oo t 185 Southern hemisphere Vsy 185; 3.0% 3.8* 3.0* 0.6§ 2.9 oof 1.88 Global 30Sp 0.57 2.5* 1. 8* 2.0* 185wy 0.8§ *T, = 1/ln [I/(I— F)], where Lis the stratospheric inventory in January and F the annual deposit. +Contribution from 1958 U.S.S.R. tests. T,, = 1/l1n [I/(I— F)], where I is the stratospheric inventory in January and F is the annual deposit. tTm = 1/ln (F,/F,), where Fy and F, are the total deposits of one year compared to the next year. §Tm = 1/ln (I,/I,), where I, and I, are the stratospheric inventories at the Start of one year compared to the start of the next year. SO