Why is the max. polarization of the blue sky at 90 degrees angle with the sun? Looking at the blue sky using my polarizing sunglasses I notice an effect (details below), that seems to match well to this answer, where it is said:

The greatest degree of polarisation occurs when the scattering angle
is through 90 degrees (since one of the polarisation states of the
incoming solar light is effectively "unseen")

My question is about the reason for the effect.
Experience:
At early morning, if I look at the West using my polarized sunglasses, and tilt my head up, the darkest blue colour is about the zenith. And if I start looking at the North or South and repeat the head movement, the zenith is brighter. It suggest that the light coming from the zenith is the most strongly polarized.
When I do the same test at noon, but now looking at the South, (the sun about 30 degrees North), the darkest region is somewhere between the zenith and the horizon.
 A: The root cause is that you see no radiation from an oscillating electric dipole when looking along the axis of oscillation.
Unpolarized light can be considered an equal mixture of two perpendicular linear polarisation states (with random phases). Such light causes electric dipoles in the atmosphere (i.e. molecules) to oscillate in those two perpendicular directions and emit the Rayleigh-scattered light. If this scattered light is observed along a direction parallel to one of these perpendicular oscillation axes, then no light will be seen from that oscillating component, leaving just pure linearly polarised light from the other oscillation component.
Clearly, this happens when the light is scattered through 90 degrees.
The situation is more complicated in Earth's atmosphere, because the scattering takes place at a range of heights, there is the possibility of multiple scattering, and there are also particles larger than the wavelength of light (aerosols and dust) that have differing polarising properties.
