Attenuation of solar radiation: A climatological study

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Abstract
Daily totals of direct and diffuse radiation transmitted by a cloudless atmosphere are calculated from the absorption and scattering coefficients given by Houghton (1954). Estimates of total (direct plus diffuse) radiation (T1) agree well with extreme maxima recorded at several British stations, but during spells of cloudless summer weather in south‐east England attenuation by aerosol decreases total radiation on average by 10 per cent, and increases the ratio of diffuse to total radiation by about 8 per cent of T1. The ratio of mean monthly radiation at country stations T2 to estimated T1 can be expressed as a function of cloud reflection (ρ), cloud absorption (ϕ), and surface reflection (α); and the values ρ = 0·50, ϕ = 0·16 are consistent with surface and aerial measurements. With α = 0·20, T2/T1 is approximately (1−0·61c) (0·6 < c < 0·9) where c is fractional cloudiness. The diffuse component beneath cloud can be estimated separately. Absorption by man‐made aerosol reaches maxima of 26 per cent at Kew and 30 per cent at Kingsway in January, and varies seasonally with measured smoke. In the country, absorption by pollution is negligible in summer but may reach 5 to 10 per cent in winter. At Kew, absorption by cloud and total back‐scattering (local planetary albedo) are about 9 per cent and 41 per cent of extra‐terrestrial flux respectively, with little seasonal variation.

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