I know I'm not mistaken when I say a polarizer wont work under grey skies. I didn't make it up, it's true! :~) A clear blue sky has lots of polarized light.... as DeviousMoose has pointed out you must be at a 90 degree angle from the sun to obtain the maximum effect from a polarizer. A gray over cast sky polarizes virtually no light, and so it will be hardly affected by a polarizer... notice I said hardly! So I'm not saying that Simon is completely off base... if there is some polarized light coming through it may be possible to see some darkening of clouds... Also when shooting glass, water, glossy things..... 30 degree is the right angle for maximum elimination of reflections. A polarizer does not work on metals.......

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