-15- strontium-90 fallout which has occurred in the United States to USSR high-yield tests. Underlying the study of tungsten-185 is the hope that by determining the contribution of a single equatorial test series to the fallout in all parts of the world, it will be possible to distinguish among these different models. It may be that each is partially correct or that the actual situa- tion is considerably different from any of them. In any case it seems quite clear that any gain in understanding the route and origin of the observed fallout must be accompanied by a gain in understanding the circulation of a big wadTePolyte pprrd the stratosphere. The rhodium-102, having a half-life of 210 days, altitude | a high . From available estimates of turbulent diffusior coefficients (Ref. 17) the time required for the rhodium-102 to spread down tc the tropopause would be many years. completely for practical purposes. By that time it would have decayed away The main motivation for this experiment i: to find out whether there is some unknown transfer process which would return such fine particulate matter to the lower stratosphere more rapidly than woul: be expected on the basis of known mixing processes. If such a mechanism exis! with a time scale of weeks or months, there is a chance that the tracer would be detectable in some of our samples by sensitive analytical techniques, although far below the concentration of other radioactivity already present. 6. Summary The radionuclides being studied in the various projects which have been discussed are listed in Table III. Many others, both natural and artificial, are present in the atmosphere and could undoubtedly be studied with profit. However, at the present stage much more must be learned before. Oe ARCHIVES assured that we are making full use of the considerable number now being measured. (1