A difficulty in the interpretation of certain solar radiation measurements

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Abstract
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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