Sky Polarization over the Golden Horn, Istanbul ~ An evening panorama by M. Rasid Tugral taken using a polarizing filter from Galata Tower. The low, 7į high, sun is off screen at right. At centre, over the Galata bridge, the sky darkens. To the south east at left, towards the Hornís tip, it is light again. The central dark wedge is where the skyís light is most strongly polarized. See below for a labelled view of Istanbul. Images ©M. Rasid Tugral, shown with permission

Blue sky is most strongly polarized in a plane at 90° to the sun.

Unpolarized light can be represented as superposed horizontal and vertical polarized components.

The light scattered at right angles by the molecule is
always strongly polarized with its electric vector perpendicular to the incident light direction.
Vertically polarized light interacting with an air molecule. Its varying electric field induces oscillations in the molecule’s electron cloud.   They in turn produce outgoing light equally polarized.   There is little radiation vertically.

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.

Atmospheric
Optics
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Horizontally polarized light scatters light mainly upwards and downwards.
            

 

Why is the sky blue