between 60, 000 and 70, 000 feet in the latitude band 30°N-30°S, After June 1959, when the effects of individual shots had been eliminated by mixing of debris, the points closely fit a decay curve representing the 293 day radioactive decay half-time of the ratio. Most of the debris in the lower tropical stratosphere after the sum- mer of 1958 had probably been produced by the Hardtack test series and UK tests in the fall of 1958 (as can be 185,, 90 /Sr ratios). shown by data on the concentrations of tungsten-185, a tracer for early Hardtack debris, and W Since all megaton tests in the Hardtack series were surface bursts (with the exception of Teak and Orange) it is a near certainty that cerium-144, which has no rare gas precursor, was to a large degree fractionated from sr? which has a gaseous precursor, and was deposited as local fallout through the scavenging effect of the soil picked up by the fireball, As a result the Ce 144), / Sr’ 90 ratios in the lower tropical stratosphere were lower than would have been expected if the weapons which produced the nuclides had been detonated in the air. By early 1959 mixing between the northern polar and tropical stratosphere had carried debris from Hard-. 144 /sr°° tack into the polar stratosphere at altitudes between 60,000 and 70, 000 feet, and had raised the Ce ratio in the debris in that region to a value close to that in the tropical stratosphere (see Figure 3). Below 60, 000 feet the ratio was even higher, doubtless due to a residue of debris from the autumn 1958 Soviet tests. By mid-1959, the fallout of Soviet debris from the lower polar stratosphere together with further vertical and 144 757,90 meridional mixing had resulted in the establishment of a fairly uniform Ce ratio throughout the lower stratosphere of the Northern Hemisphere, 9 Rather suddenly during December 1959 there was an increase in the Ce 144 16. © vatio between 60, 000 and 70, 000 feet in the northern polar stratosphere, probably as a result of the movement into that region of unfractionated debris from some past air burst. A similar increase occurred between 40, 000 and 55, 000 feet in April, had passed through a maximum during January, according to its radioactive half-life. 1960. After the Ce 144 Jsr?? ratio 1960, in the 60,000 to 70, 000 feet altitude layer it decayed There can be little doubt that the admixture of "unfractionated" Teak and Orange nuclear debris from the high stratosphere (>70, 000 feet) was the cause of the sudden increase of the Ce 144 9,90 ratic. in the polar stratosphere, for rhodium-102 concentrations rose at the same time as the 144,, 9 Ce 4 /Sr 0 ratio and there is no evidence that significant quantities of debris from Soviet air bursts rose above - 60,000 feet. The general picture, then, is one in which the nuclear debris from the upper atmosphere entered the lower stratosphere, predominantly in the polar regions, during the winter months and penetrated down almost to the tropopause by June, 1960. Estimation of Teak and Orange Debris in Lower Stratosphere We may estimate the contribution of Teak and Orange to the total lower stratospheric burden of fission products during early 1960 by considering the strontium-90 concentrations together with the Ce in the stratospheric debris. 144,. /Sr 90 ratios The strontium-90 concentrations (in dpm/1000 SCF) at three altitudes, 60,000 feet, 65,000 feet, and 70,000 feet, are plotted against time for the latitude bands 90°N-30°N, 30°N-30°S, and 30°S-60°S in Figures 4, 5, and 6, respectively. sr® ratios. These plots indicate the same general trend as do the cel #4) While the strontium-90 concentrations in the tropical stratosphere were decreasing after the middle of 1959 (about a factor of two by June 1960), those in the northern polar stratosphere remained essen- tially constant and those in the southern polar hemisphere increased. In the period July to August 1959 the strontium-90 concentrations at 65, 000 and 70, 000 feet in both tropical and northern polar stratosphere were 58