The colour of sky 天空的颜色（转载）

    If we look at the sky on a perfectly fine summer‘s day we shall find that the  blue colour is the most pure and intense overhead，and when looking high up in adirection opposite to the sun. Near the horizon it is always less bright，while  in the region immediately around the sun it is more or less yellow. The reasonof this is that near the horizon we look through a very great thickness of thelower atmosphere，which is full of the larger dust particles reflecting whitelight，and this diluter the pure blue of the higher atmosphere seen beyond，And inthe vicinity of the sun a good deal of the blue light is reflected back intospace by the finer dust，thus giving a yellowish tinge to that which reaches usreflected chiefly from the coarse dust of the lower atmosphere. At sunset andsunrise，however，this last effect is greatly intensified，owing to the greatthickness of the strata of air through which the light reaches us. The enormousamount of this dust is well shown by the fact that then only we can look full atthe sun，even when the whole sky is free from clouds and there is no apparentmist.