One of the great things about trying to post daily is that it is forcing me to notice things. We went to walk along the mid section of Hadrian's Wall yesterday. Not much Physics to comment on but I began to notice through the gaps in the clouds that the sky was always a paler blue further away towards the horizon. I know that the sky is blue because oxygen and nitrogen molecules scatter the shortest wavelengths of light. This is called Rayleigh Scattering. The darker blue is closer to indigo and will have a shorter wavelength. The paler blue has a longer wavelength. Now the reason given for the sky being red at sunset is that more colours have been scattered as the light has to come further through the atmosphere, leaving only red visible. But surely this would need bigger particles. It is true that there are aerosols of larger particles closer to the ground. This is the reason given on one website I found for the paler sky colour http://www.webexhibits.org/causesofcolor/14D.html I was a bit dubious about this source at first but it claims to be part financed by the US Dept of Education which sounds OK. They suggest Mie Scattering, but that should lead to white like clouds, not paler blue. I'm wondering if it is just because the light has travelled obliquely across the sky a longer distance and had more chance for the shorter wavelengths to be scattered away. It is true that to reach my eye the photons will have come through aerosols closer to the ground when coming from nearer the horizon. I will check out their claims that the sky goes a deeper blue after rainfall and that it is a deeper blue at altitude.
Here is the photographic evidence.
I like this picture because you can pick out the Cheviots in the distance.
When I got home I photographed the sunset. Note that the sky is bluer higher up where the light has followed an oblique trace. I suppose you'd argue that there aren't the aerosols up there. And you can also pick out the lights on the Anthorn aerials, as mentioned on Monday.
