Light Scattering in the Atmosphere and the Polarization of Sky Light*

The characteristic properties of scattered light in the atmosphere can be derived from the equations of radiative transfer, conveniently formulated in terms of four Stokes polarization parameters. For the case of Rayleigh scattering, by a method outlined by Chandrasekhar, it is possible to obtain an exact solution of this equation in the sense that the effect of all orders of scattering is included. With the use of this method, the intensity and the polarization of the diffuse sky radiation in a pure, molecular atmosphere were computed. These theoretical values are compared with the results of measurements of sky-light polarization, performed by means of a new photoelectric polarimeter specially constructed for such measurements. The deviations of the observed sky-light polarization from that of a molecular atmosphere are discussed with respect to the degree of atmospheric turbidity. These deviations show a systematic character which can be deduced already from the form of the equation of radiative transfer for a turbid atmosphere. The method of solving the problem of radiative transfer for a turbid atmosphere is outlined, with emphasis on the possible determination of the type of scattering from the measurements of sky-light polarization.