Why the sky looks darker above?


When I wear polarized lenses and look directly above into the clear blue sky it is much darker than the rest of the sky. Also when I have sunglasses on I can see that the clear blue sky is not clear at all but is filled with wisps of vapor.
So my question is why does the vault of the sky even in broad daylight look dark through polarized glasses?

Thank you, I’m actually quite past age five, but I don’t know why this should be so.

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