The blue sky was observed near 90° from the sun, in the vertical plane through the direction of the sun, with the sun above and just below the horizon. The observations were made with six filters ranging from 3250 to 9400 Å. For a normal clear sky at the McDonald Observatory with the sun 11°.5 above the horizon, 75% polarization was found at 5500 Å. The agreement of the Rayleigh—Chandrasekhar theory with the observations is good. The difference of the above 75% with 100% polarization appears to be due to multiple scattering (6%), molecular anisotropy (6%), and reflection by the ground (5%), while a residual 8% is presumably due to aerosols. At the McDonald Observatory, the polarization of the daytime sky has a maximum near 5500 Å, with a decrease toward longer as well as toward shorter wavelengths. The decline in the ultraviolet is caused by multiple scattering, while in the infrared it is mostly due to ground reflection, which is especially strong when green plants are present.
THOMAS GEHRELS, "Wavelength Dependence of the Polarization of the Sunlit Sky," J. Opt. Soc. Am. 52, 1164-1173 (1962)