Calculations and measurements of the spectral radiance of the solar aureole

The application of the theory of primary scattering to describe and interpret the spectral distribution of the sky radiance is discussed. It is shown that within the solar aureole the influence of the scattering of higher order can be neglected. Theoretical calculations of the spectral distribution of the sky radiance, carried out by Bullrich et al. (1965) based on an exponential aerosol size distribution with an upper limiting particle radius r = 10 µ, have been extended to r = 150 µ. The detailed study of the influence of these "giant" particles revealed that aerosol particles of r >30 µ have no effect on the sky radiation any more. Representative measurements taken at Mainz, Germany, at the research station Jungfraujoch in the High Alps, Switzerland, and on the island of Maui, Hawaii, are analyzed and interpreted by comparison with theoretical values. Some measurements required a modification of the basic model of the turbid atmosphere. The exponent of the aerosol size distribution generally a constant has to be considered a function of the particle size. Furthermore, the exponent of the aerosol size distribution was found to be a function of height at least in case of subsidence. The exponent increases with height.