STRONTIUM -90 ON THE EARTH’S SURFACE. IU 367 to explain this phenomenon, which occurs consistently in the soil data. It was evident in the earlier surveys and in the incremental depositions between 1960 and 1963. Among the explanations of the cause ad- vanced have been the mean storm tracks over the United States, the effect of the Rockies in creating vertical motions, and the position of the jet stream. None of these explanations has been rigorously tested, due principally to lack of adequate data. It is interesting to note that the Public Health Service Radiation Surveillance Network data indicate that the ground-level air in the Great Plains states does not contain a higher concentration of gross beta activity than air in other sections in the country. The HASL precipitation collectors do indicate somewhat higher deposition per inch of rainfall, but the total deposition during the 1960 to 1963 period is lower than that in the soil increment. (Unfortunately, there are no directly comparable soil sites and precipitation collectors in this area.) Although the northeastern states receive about three times more (45 vs. 15 in.) annual rainfall than does, say, Rapid City, S. Dak., in the late spring months, the period of maximum deposition, the average rainfall at Rapid City is quite comparable to that in the northeastern states. There is a pronounced spring maximum of rainfall in the Great Plains states, whereas there is a relatively uniform distribution of precipitation throughout the year in the north- eastern states. Recent studies have also shown that the precipitation in the Great Plains states occurs from clouds extending to much higher altitudes than those in the northeastern states.® Precipitation clouds extending about the tropopause are quite rare in New England, but they are a common phenomenon in the spring thunderstorms over the Great Plains. This may contribute to the high deposition rate althoughit cannot serve to explain all the difference. Another suggestion is that dry deposition is possibly more important over the Great Plains region. Surface winds over the Great Plains are, on the average, higher than over any other large region of the country. This may result in greater impaction of particles on growing vegetation. CONCLUSION The global soil survey provides useful information on the total deposition of *Sr on the surface of the earth and on the characteristics of its distribution. The observed distribution seems to correlate principally with latitude and precipitation amounts. Mechanisms re- sponsible for deviations from the expected pattern are being studied. REFERENCES 1. L, T, Alexander, R. H. Jordan, R. F. Dever, E. P. Hardy, Jr., G.H. Hamada, L. Machta, and R. J. List, Strontium-~90 on the Earth’s Surface, USAEC Report TID-6567, February 1961.