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
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.
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The amount of polarization at 1/λ = 1.89, if the observation had been made at exactly
phase, is estimated under “Est. Pol. at
.” The values in italics are smaller than the amplitude of the polarization cosine curve. Parentheses are used for observations at 1/λ =1 · 26.
Table VI
Averages of percentage polarization, normalized to 61.1% at 1/λ = 1.89.
The amount of polarization at 1/λ = 1.89, if the observation had been made at exactly
phase, is estimated under “Est. Pol. at
.” The values in italics are smaller than the amplitude of the polarization cosine curve. Parentheses are used for observations at 1/λ =1 · 26.
Table VI
Averages of percentage polarization, normalized to 61.1% at 1/λ = 1.89.