I observed a strange phenomenon the other day which led me to ask this question. I was in Bahamas on a cruise, where it is very bright. This might have enhanced an otherwise more subtle effect. (OK, get the drinking jokes out of your system now... :tongue:) I was outside, right next to the hull of the ship on a tender. It was about 11AM, so sun was very high in a virtually cloudless sky. Yet I was so close to a 12 story structure that I was in shadow. It was clear and there was no haze. I looked up at the sky directly above me, so I was looking through sky that was in ship-shadow into cloudless sky. The sky directly above me was so dark that I thought for a good minute that the sky was completely overcast and grey with cloud cover, like it was about to rain very heavily. Anywhere else I looked the sky was bright, saturated blue, but right above the ship it was this medium foreboding grey. I'm pretty familiar with sky and weather phenomena, so I have a lot to compare to, but this was odd. It took me a few moments of analysis, and some small clouds to drift into the grey before I could convince myself that the sky I was seeing was actually clear as a bell like everywhere else, and not overcast. Obviously what was happening was that I was looking at the shadow of the ship. But shadows are a lack of light, which means I should have been looking through dark at bright sky beyond. In a clear hazeless sky, I should not be able to see an absence of light. It leads me to wonder if most of the scattering due to sunlight actually occurs very near the observer, not across the entire expanse of sky. In a sense, the implication of this is that most the brightness and blueness of a sky is actually being generated less than a 50 metres from the observer.