Direct sunlight is unpolarized. The electric vectors of its radiation point in random directions around the ray direction. Light becomes polarized, or partially polarized, when the electric fields or vectors have non-random orientations. Plane polarized light (there are other types) has the oscillating electric fields all in one plane.
Skylight is sunlight scattered by individual air molecules. The scattered light is richer in short wavelengths giving us a blue rather than black sky and the light is also polarized.
Blue sky is most strongly polarized in directions 90° to the sun. The polarization there is never 100% - (1) because air molecules are not perfect dipole oscillators, (2) there is scattering by dust, moisture and aerosol and (3) the is some multiple scattering.
Unaided eyes hardly notice sky polarization but it is no secret to photographers who have long exploited it to increase contrast between sky and clouds. Light scattered by clouds is unpolarized and a polarizing filter selectively darkens the background blue sky.
Near horizon sky is less polarized because there the sunlight is scattered several times and the polarization direction is lost.
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