northern hemisphere have been adjusted using the mean of the values obtained at Milford Haven and Abingdon as reference but for the southern hemisphere stations, Ohakea has been used. Figures 5a and 5b have the same general characteristics, with a pronounced minimum near the equator, a strong suggestion of a maximum in the middle latitudes in the northern hemisphere and a possible smaller maximum in a similar region of the southern hemisphere. The low point plotted at 63°N on each diagram was obtained from the analysis of a snow core from Norway; this point might be artificially low as there has undoubtedly been some melting of the snow in the summer and there is a possibility that the percolating water might remove Sr°” preferentially. An interesting results is demonstrated in Fig. 6 in which the specific activities of the quarterly samples at Milford Haven and at Ohakea have been plotted. The Milford Haven curve 3 Te —_— 6 — 5r | 2 MILFORD HAVEN — aL 2 = = ee, - _OHAKEA ~ saLLLLILitLitittiitititirte tl i pity MAM J 9 AS ON DJ) Y FMAM JJ AS ON DIL FMAM JS 1955 1956 1957 J A Fig. 6—Seasonal variation of Sr’® content of rain water at Milford Haven and Ohakea. is a portion of the curve in Fig. 3, showing the pronounced seasonal effect. The New Zealand curve also shows a seasonal effect but of the opposite phase to that in the northern hemisphere. This result will be discussed further when all the data are examined in terms of a model of general circulation of the atmosphere. 7 THE RATIO OF Cs!" TO Sr** IN RAIN WATER From radiochemical analysis and gross activity measurements of several samples of stratospheric dust collected at various times after the 1954 thermonucleartests, a value of 4% has been obtainedfor the effective fission yield of Sr® in such tests. Cs'*" is situated in one of the peaks of the fission product yield curve and its yield should therefore be about 6% and comparatively insensitive to the type of fission. Since the half lives of Cs'*" and Sr®® are nearly equal, one might therefore expect to find Cs'*"/Sr*® ratios of the order of 1.5 in airborne dust and in rain water. The actual values obtained from the quarterly rain water samples are given in Table 4 together with values of the mean determined from the ratio of the 241