Aerosol and solar radiation in Britain

The irradiance of the solar beam was measured on cloudless days at Sutton Bonington in the English Midlands and at sites in north‐west Scotland. Total and diffuse fluxes were also measured on some days. An attenuation coefficient for aerosol τ a was defined by\documentclass{article}\pagestyle{empty}\begin{document}$$ s\left({\tau _a} \right) = s\left(o \right)\exp \left({- \tau _a \,m} \right) $$\end{document}relating the measured flux at normal incidence S(τ a ) to the flux calculated for a dust‐free atmosphere when the air mass number is m . Changes of τ a from day to day were related to changes of air mass origin; local sources of aerosol were relatively unimportant. In maritime air, τ a ranged from 0.05 to 0.15, and in continental air, from 0.1 to 0.5. In a tropical maritime air mass, τ a decreased from 0.13 at sea level to 0.07 at 1,340 m. The fraction of (ultra‐violet + visible) to total radiation was (0.54 – 0.28 τ a ) and the ratio of diffuse to total radiation ( m < 2) was (0.1 + 0.7 τ a ). The ratio of total scattering to absorption by aerosol decreased from 4 at m = 1.1 to 0.5 at m = 2. Mean monthly values of τ a at four Meteorological Office stations were calculated from records of solar radiation and hours of sunshine and corresponding values of total and diffuse flux were tabulated for ‘isolated’, ‘rural’ and ‘urban’ sites.