Local differences in the attenuation of solar radiation over Britain

Abstract In relation to other British stations, radiation totals at Aberporth on the Welsh coast are anomalously high. On the basis of recorded sunshine, the annual mean intensity of direct radiation at Aberporth is 31 mw cm −2 compared with 25 mw cm −2 elsewhere. Allowing for the change of extra‐terrestrial radiation with latitude, the effective transmission of the atmosphere is greatest at Aberporth and Lerwick, less at Eskdale‐muir, and least at Kew, consistent with differences of air pollution. Measurements at a station installed near Aberystwyth suggest that the relatively clean air over Aberporth may be confined to a very narrow coastal strip. At Kingsway, the direct radiation per hour of sunshine increased from 21 mw cm −2 in 1957 to 26 mw cm −2 in 1963 consistent with the decrease of smoke in central London. Kingsway and Kew now record about the same intensity of radiation attenuated at roughly 1 per cent per 10 μg m −3 of smoke. Applying this analysis to the radiation balance of Britain, previous estimates of potential evaporation in north Scotland were raised from 13 to 17 inches per year, removing most of the difference in estimated evaporation over Britain.