Abstract
A physical explanation of the intensity and polarization of diffusion skylight in the plane of the sun’s vertical is attempted by an approximation method which takes into account the effects of multiple scattering. Various model atmospheres are considered. The scattered radiation field is assumed to be composed of (1) the scattered radiation due to the molecular atmosphere resulting from all orders of scattering; (2) the scattered radiation due to the turbid atmosphere including primary scattering only; and (3) the scattered radiation due to the turbid atmosphere resulting from secondary and higher-order scattering. The contribution due to the first term can be computed from the tables applied by Coulson et al. The contribution due to the second term is found in the Mainz tables. It is assumed that the contribution due to the third term arises mainly from secondary and higher-order scattering by the smallest aerosol particles. This assumption makes it possible to evaluate the relative contributions of secondary and higher-order scattering by these particles to the scattering function for a turbid atmosphere by subtracting the contribution due to primary scattering from the values tabulated in the Coulson et al. tables (from consideration of equivalent Rayleigh optical thickness). The decomposition of the radiation field into component parts makes possible the discussion of the relative contribution of secondary and higher-order scattering to the total radiation field and its depolarization. The intensity and degree of polarization of diffuse skylight have been computed by this approximation method for three wavelength regions, in which measurements were made, for five zenith distances of the sun, and for three turbidity factors of the atmosphere. The theoretical computations are compared with measured values, and the usefulness of this approximation method in the evaluation of intensity and polarization of diffuse skylight is discussed.
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