Polarization of the Sky

an atmospheric optical phenomenon observed in the daytime in cloudless weather and at night in the moonlight. The radiant energy striking the earth’s surface in the form of light scattered by the atmosphere is in fact partially polarized. Polarization of the sky was discovered in 1809 by the French scientist D. Arago. It cannot be detected by the naked eye and must be observed with a polariscope.

The polarization at each point in the sky is characterized chiefly by two quantities: the degree of polarization, which is the ratio of the completely polarized flux of radiant energy to the entire radiant flux emanating from a given part of the sky, and the position of the plane of polarization, which is determined by the dihedral angle formed by this plane with the vertical plane. Polarization of the sky has been studied most thoroughly for the vertical plane passing through the sun. The maximum degree of polarization generally is observed at the point of the vertical-plane that is separated from the solar disk by 90°, where the level of polarized rays may reach 85 percent, and the plane of polarization coincides with the vertical plane. Polarization decreases in both directions from this point and reaches zero at the neutral points, known as the Arago and Babinet points. This type of polarization is subject to diurnal and annual variations and depends on weather conditions, the geographical location of the site, and other factors. Light scattered by large particles is totally unpolarized; even slight cloudiness therefore greatly reduces the amount of polarization. An increase in atmospheric turbidity as a result of dust, smoke, volcanic ash, and other such impurities alsocauses a sharp decrease in polarization. The degree of polarization can therefore serve as an indirect indication of atmospheric transmittance.