METEOROLOGICALEFFECTS 45 



the consensus is that a poleward transfer takes place in the lower tropical stratosphere and 

 that removal of both equatorial and polar stratospheric radioactivity occurs primarily in 

 temperate and polar regions. 



V. Future Fallout 



The long-lived fission products in the atmosphere in November, 1958, can be estimated 

 in two ways, each involving approximations and assumptions which cannot be completely 

 verified. The estimates are in fairly good agreement. The usual approximation concerning 

 the production of Sr'° is: 



1 megaton of fission yield = 0.1 megacurie of Sr"" 



= 0.5 millicurie per square mile of Sr"" if uniformly distributed over the globe. 



The Atomic Energy Commission has announced that as of November, 1958, a total 

 fission yield of 92 megatons or 9.2 megacuries of Sr"" has been released into the atmosphere. 

 Of these, it is estimated that about 4.0 megacuries are contained in local fallout and are there- 

 fore unavailable for world-wide distribution. This estimate is uncertain and represents the 

 weakest link in the computation of the amount still in the atmosphere. Using the 1958 soil 

 analysis in figure 1, it is estimated that 3.0 megacuries have already been deposited over the 

 world exclusive of local fallout. This leaves a residual of 2.2 megacuries (less decay since 

 time of formation) still in the atmosphere as of November, 1958. An alternative computation 

 involves an estimate of the atmospheric content based on balloon and aircraft observations. 

 From the limited data available, it is estimated that in July, 1958, there were 1.0 megacuries 

 of Sr'° in the stratosphere. In the period from July to November, it is estimated that 1.5 

 additional megacuries were injected into the atmosphere by the U.S.S.R. test series, resulting 

 in a total of 2.5 megacuries in the atmosphere in November, 1958. 



The future distribution of the debris still in the stratosphere can be predicted from a 

 knowledge of the distribution of debris already deposited. The dotted curve in figure 1 shows 

 the expected distribution of Sr"" on the ground in 1962-1963, assuming no additional injec- 

 tions after November, 1958. This curve is based on the aircraft and balloon data (which in- 

 dicate a distribution of 0.7 megacuries of Sr'" in the northern hemisphere and 0.3 in the 

 southern hemisphere in July, 1958) ; on an assumption that virtually all of the 1.5 megacuries 

 added in the fall of 1958 remains in the northern hemisphere; on the expectation that future 

 stratospheric fallout will parallel that in the past; and with an allowance for radioactive decay. 

 Almost all the debris now in the stratosphere will have been deposited by 1962-1963. The 

 predicted average peak of 70 to 75 millicuries Sr*" per square mile in the 40°-50°N latitude 

 band may be increased by as much as 35 millicuries per square mile in the area downwind of 

 the Nevada Test Site. The equatorial minimum is expected to be about 10 millicuries per 

 square mile and the secondary peak in the southern hemisphere about 20 millicuries per 

 square mile. Cesium- 137 fallout should be similarly distributed, with the actual values about 

 80% greater. 



The average concentration of Sr™ in ground level air reached a peak of about 1 to 15 

 disintegrations per minute per 1 00 cubic meters at some northern hemisphere stations in the 

 spring of 1959. These are undoubtedly the highest values that will occur if no further testing 

 takes place and will decline substantially from that time on. A smaller but detectable peak 

 should occur in the spring of 1960 if theories concerning a spring maximum are valid. 



