United States to meteorological conditions and to local and tropospheric fallout as a result of the proximity of the Nevada Test Site. The generally higher concentrations in the north tem- perate latitudes were attributed to prevailing meteorological conditions and their effects on tropospheric fallout from tests in the USSR and at the United States Pacific Proving Grounds. Machta* proposed a model of stratospheric fallout which differs in some respects from Libby’s. He postulated that stratospheric mixing is slow and that stratospheric distribution of fission products is still nonuniform. He feels that a major portion of the nuclear debris is still in the northern portions of the northern hemisphere, rather than uniformly spread over the entire globe or even uniformly dispersed in the northern hemisphereitself. He feels also that stratospheric movement of the fission products is largely by direct transport from west to east in the general latitude of the point of injection with very slow vertical mixing. Slow polewards circulation of stratospheric air from equatorial regions provides some mixing toward the poles. The higher concentration of fallout in the temperate latitudes is explained on the basis of air exchange between the stratosphere and troposphere through the break in the tropopause in the vicinity of the jet streams. A large part of the higher concentration of Sr™ found in the northern part of the United States may result from preferential stratospheric leakage in the vicinity of 30°N to 40°N latitude instead of the proximity of the Nevada TestSite. Qualitatively, both models predict the same general distribution of fallout. Quantitatively, the Machta model predicts a greater degree of nonuniformity of fallout over the earth with higher deposition of fission products in the north and south temperate latitudes from nuclear debris still in the stratospheric reservoir. Figure 1 shows the essential features of the Machta model and the present general world-wide surface distribution pattern of sr, 3.2 Average Maximum Surface Deposition Levels (a) Present Levels (1956-1957), A crude indication of latitudinal distribution of the integrated Sr® surface deposition levels as of June 1956, derived from soil data, is shown by the lower curve in Fig. 2. This curve is essentially the same as the one given by Machta’* except a few points have been added and the peak concentration in the north temperate latitudes is drawn slightly higher to allow some weighting for average Sr® levels in United States soils. These data suggest a level of about 13 mc/sq mile for the north temperate latitudes. No soil data are available yet for mid-1957. Fallout data from pot collections in New York and Pittsburgh, however, showed that cumulative Sr® fallout increased by about 50 per cent from June 1956 to June 1957.5 The upper curve in Fig. 2 represents estimated latitudinal fallout dis- tribution in June 1957. Some of the increase in New York and Pittsburgh fallout could have been tropospheric contribution from Russian tests, which would result in over-prediction of the Sr®™ levels in other areas. This and other criticisms, however, seem minor compared to the uncertainty in the primary soil data. Estimated deposition levels in June 1957 show a total Sr®™ fallout ‘of about 19 mc/sq mile for the north temperatelatitudes, 3 to 4 mc/sq mile for the equatorial regions, and about 5 to 6 mc/sq mile for the south temperate latitudes (Fig. 1). Data from pot collections in the New York area suggest total sr* deposition levels of about 35 mc/sq mile in the northern United States in mid-1957. The rapid build-up of Sr®™ in the northern states in the spring of 1957 cannot be attributed to tropospheric fallout from Nevada tests, since Operation Plumbbob had not begun. It may be due to tropospheric fallout from spring test operations in the USSR and to preferential stratospheric fallout from past tests. The total amount of Sr® deposited over the earth’s surface (from both tropospheric and stratospheric fallout) as of mid-1957 can be estimated from the upper curvein Fig. 2 by replotting the data in terms of Sr® deposition/degree times the earth’s area/degree. This ' calculation suggests a world total deposition of 1.64 megacuries, which gives a world average surface level of 8.2 mc/sqmile. Libby’s” estimates of Sr™ surface deposition levels for the fall of 1956 were 22 mc/sq mile for the northern United States, 15 to 17 mc/sq mile for similar latitudes elsewhere,* and 3 to 4 mc/sq mile for the rest of the world. These values are in good agreement with those *The north temperate fallout band was indirectly defined as the region between 60°N-10°N latitude. It is assumed that the surface deposition of 16 to 17 mc/sq mile applies to this area. 285