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  • $\begingroup$ Interesting answer, I did read about Rayleigh scattering before your post, but to me it only explained the blue color of the sky as seen through the atmosphere. I do not understand the rest, even though I see what you mean as "the scattering off the dirt isn't Rayleigh scattering; it is more color blind". But I think your conclusion is what is verified in alemi's answer, right ? Blue color, then colorblind filter, then there is less blue, which appears gray but our peception tricks us. If it is what you meant, then I still have difficulties understanding why the filter is stronger when sunlit. $\endgroup$
    – PhilDenfer
    Commented Aug 12, 2014 at 9:12
  • $\begingroup$ Hi, I don't think it's right to talk about a "filter". The grey light you are seeing is not obtained by filtering the blue light from the sky - it wasn't reflected by droplets at random points of the sky first. Instead, the grey light is scattered white light that goes directly from the Sun, and changes the direction in the dirt on the glass. I tried to point out that your whole way of looking at the question is upside down from a physics viewpoint. The normal color is grey - the abnormal color that needs an extra explanation is blue. $\endgroup$
    – Luboš Motl
    Commented Aug 12, 2014 at 9:31