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The same scattering process that creates sky colors also alters the skylight polarization, which is the tendency for the electric field associated with the light to oscillate preferentially in a particular direction. Sunlight is randomly polarized, with no preferred orientation for its electric field. This light becomes partially polarized when it is scattered by atmospheric gas molecules or aerosols. In fact, an ideal Rayleigh scattering event creates perfectly polarized light 90° from the sun. However, gas molecules are not ideal Rayleigh particles, and the light undergoes multiple scattering events, so there is a band of highly, but not perfectly, polarized light stretching across the clear sky, 90° from the sun. A measurement of the sky polarization pattern is shown in Fig. 10.1 at a 670- nm wavelength for a clear sky. This all-sky image was recorded by an instrument we designed and built at Montana State University in Bozeman, MT. The outer circle is the horizon, the center is the zenith, and the colors at the right of the image denote the degree of polarization, ranging from 0 to 1 (0% for random polarization to 100% for total linear polarization). The yellow region is the band of maximum polarization, located 90° from the sun (hidden behind the occulter at the lower left). If you wore polarized sunglasses and rocked your head back and forth while looking at the highly polarized part of the sky, you would see the sky alternating between bright and dark. But it would never go completely dark, as it would if it was 100% polarized. Throughout the day the band of maximum polarization moves across the sky, always oriented 90° from the sun. Its maximum value changes with aerosols and surface reflectance. The obstructions near the top of this image are a large radio antenna and two penthouses on the roof of the building where the all-sky imager was mounted.
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