Eburacum wrote:My colleague Stephen Inness, at OA, has written this essay on sky colour,, if it is of any help
Nice. I think he goes astray a little with the green/yellow/red skies from Rayleigh scattering as the atmosphere thickens, however.
Here's what goes on:
A lot of the blue sky light you see comes from ~10km away ... you can see it being scattered between your eye and distant hills, for instance. Red light is also being scattered, but only 1/16 as much, so the blue predominates.
The same thing is happening in a parcel of air 40km away, with one difference ... blue light scattered towards your eye at that distance is likely to be
scattered again before it reaches your eye. The small amount of red light, however, is so poorly scattered that it is likely to reach your eye directly.
So you can imagine there's a layered effect going on ... blue light generally comes from close to, green light from farther away, red light from the far distance.
The Earth's atmosphere is sufficiently thick that blue light predominates if you look straight up ... the air is just too thin at high altitude to introduce a red component. Looking out horizontally, though, you "see" >30 times as much air. All wavelengths are being scattered towards your eye from somewhere out there, so the horizon looks white, even in the absence of Mie scattering.
To get a sky coloured other than blue, you need to somehow eliminate the nearby "blue air". This happens if dense cloud or an eclipse shadows the air nearby, allowing you to look a long way out to sunlit sky on the horizon. Under these circumstance you can see the colour of distant air, and you'll often see red, or a desaturated greenish yellow (red+green) coming to your eye from afar.
But with open air and no shadows all that will happen if you make the atmosphere denser is that the whiteness of the horizon will close upwards towards the zenith. As the air gets less dense, the zenith will get darker and the horizon will become bluer.
No other colours from Rayleigh alone, I'm afraid.
Grant