From AMS Glossary
Polarization in the clear daytime sky can be quantified by the degree of linear polarization P and by the vibration ellipse orientation for skylight's polarized component.
For Stokes's parameters, I (the scattered irradiance), Q, U, and V, skylight's linear polarization P is
thus ignoring its minimal degree of circular polarization,
. P is zero at neutral points and has local maxima along a celestial great circle that is ∼90° from the sun. At sunrise and sunset, this great circle is a sky meridian perpendicular to the clear sky's principal plane.