Agreed with prior commenters. The effect of a CPL filter on a blue sky is greatest at about 90 degrees to the sun and will reduce more and more as the angle changes to either side of 90 degree street . But the CPL will still have a good effect on highlights from leaves or water or from nonspectral reflections from glass, metal, etc. Steve Perry at Backcountry Gallery, a member of UGG, has a very clear YouTube video on using CPL filters that I highly recommend. jak
I need to check out that video.