One experiment I tried to get some atmospheric effects was to create a second ring, an air ring, just inside the scrith ring. My idea was to apply shaders to the air ring so that I could view it from the outside, looking in, and perhaps get a view showing what the ring would look like to a daytime observer. I was never able to get reasonable results with this, mostly, I think, because I wasn't trying the right kinds of shader.
I did get one effect I liked from the air ring (it is very tempting to call it the "air plane"). At Ken Musgrave's suggestion, I tried varying the opacity of the ring based on the angle between the normal and the incident ray. The idea is that when looking at the atmosphere from a low angle, you are looking through more air, which would fog up the landscape more. This lead to air.sl, the air shader. I found that scaling the opacity linearly with the angle didn't drop off quickly enough, so I squared it and got better results. In actuality, the effect this gives in fairly subtle. Comparing the image above to the image on the previous page, you will see that the night time band at the bottom is just a bit lighter in the image above, because of the air ring. This is the only image in this set of Web pages that incorporates the air ring. The image below shows just the air ring, so that you can see the opacity fall-off more easily.