The second feature of the deposition process concerns the variation of the rate of depog; is 1. ~ tion during the course of a shower. Experiments carried out at Harwell show that this effect. For some years, the fission product activity in the troposphere has been sampleg re al larly by fitting filters to aircraft carrying out standard meteorological flights from Aldergre” Northern Ireland.! At the same time, measurements have been made of the gross activity in ¥e, ground level air and of the activity of rain water at Harwell. It has been shown from the air measurements that, except when fresh dust clouds are crossing the U. K. for the first time the mean gradient of activity in the lower troposphere following nuclear explosions in the “normal” size range is comparatively constant for different series of tests. It is possible therefore, to scale the rain water activities following such tests to correspond to a fixeg activity level in the troposphere. Normalized figures for the specific activity of rain wate, samples calculated in this way have been separated into groups according to the amountof Pain which fell during the collection of the samples and an average figure computed for each group, The results are plotted in Fig. 1 which clearly shows that the specific activity of rain wate, de. TTT PPT TTP? TT yt tS SPECIFIC ACTIVITY OF RAIN (ARBITRARY UNITS) ! 3 | “met 80 }-— — 70 be 4 \ DOTTED CURVE: DATA CORRECTED FOR ORY DEPOSITION 60 + 50 40 r— woLtt 0 it | tt 5 pt El 10 tt tt 15 20 SIZE OF SHOWER (MM) Fig. 1— Variation of specific activity of rain water with size of shower. creases with the amount of rain that falls. The effect of dry deposition on this graph, greatest at low rainfalls, has been computed using the slightly exaggerated value of 0.1 cm/sec for the deposition velocity, and the corrected curve is also shown in Fig. 1. It is clear that the observed effect is not due primarily to dry deposition and it follows that the specific activity of rain water is a decreasing function of the amount of rain which falls. Further experiments are being carried out on this aspect of deposition. 9 RESULTS OF DEPOSITION MEASUREMENTSIN THE U. K. The results obtained from the radiochemical analysis of the monthly samples collected at Milford Haven are given in Table 2 in which the activities are expressed on the last day of each sampling period. The Sr®? data are given both in terms of the specific activity of the rain water ee, 234