be due to selective absorption in the near infra-red ? One would expect that as the airmass increases, the region left for extra absorption between molecular bands would decrease.Miss E. M. SHAW : (In reply to a previous question, Mr. Hamilton noted on the blackboard that water vapour and ozone were the major absorbers of radiation).In view of the fact that water vapour is of first importance in the absorption of radiation, QJ., 1967, pp. 263-269). Whatever the horizontal visibility may have been, there does seem to have been some effective scattering agent in suspension, possibly at considerable heights. Two years ago, my fixation about the intense radiation recorded on the Welsh coast (sometimes referred to as the ' excesses at Aberporth ') drove me to take a small solarimeter on holiday near Caernarvon. I was rewarded with a spell of wonderfully clear and cloudless weather over the Whit weekend. The ratio of diffuse to total radiation was often about 10 per cent, similar to the smallest values recorded at Aberporth. One would expect the air over Wales to be clearer than over southern England, but why should it be clearer than the air over the Shetlands 1 As a Scot, I find this an unpalatable deduction and I hope MI. Hamilton will be able to reassure us that there are some occasions at least when the diffuse/total ratio at Lerwick was less than on the days he selected in his paper. Dr. H. L. PENMAN : Re-arranging, the extra absorption is and from Table 1 Dr. Penman's rapid algebra shows clearly that if in fact the variation in A is not true, it must result from errors in Go and Do, or else from errors in G and D arising from instrumental errors. In the paper we have discussed in detail the possible magnitude of errors in Go and Do and shown that they are much too small to give rise to the observed anomalous absorption. In reply to Professor Monteith the ratio 15 per cent of diffuse to total radiation was rather high for a clear sky at Lerwick, though such values occur quite often. I have looked at the values of this ratio for near-noon summer hours at both Lerwick and Aberporth, for 1957-1963, and I think it would be a fair summary to state that at both stations the lowest values attained in each summer is normally in the range 9 to 11 per cent -in the model atmosphere with no aerosol but 1.5 gm cm2 water vapour the value wouId be about 7 per cent.
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Measurements of global and diffuse radiation were made at Lerwick on a number of cloudless summer days, most of them in a very clear atmosphere. Even on the clear days about 10 per cent of the global radiation appears to be absorbed, in addition to that predicted for a model atmosphere and the amount of scattered radiation is about twice the computed Rayleigh scattering. Whereas extra scattering increases with air mass as expected, the apparent absorption decreases with increasing air mass. No physical explanation of this absorption anomaly is apparent, and no reasonable adjustment to the assumed coefficients of scattering and absorption removes it. It would be removed if the sensitivity of the Kipp solarimeter were assumed to vary with the angle or intensity of the incident radiation by as much as 10 per cent over the range concerned but special tests, carried out on Kipp solarimeters at Kew, do not provide evidence to support this assumption.
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