Ummm I think I found the problem.
GetSkyColor and GetSkyColorAtm are using different functions, which makes sense, but they should be almost the same.
GetSkyColorAtm is used for the sky we see, and is using a very nice sky rendering algorithm.
GetSkyColor is used for extinction, and is using a different sky rendering algorithm.
GetSkyColour doesn't need the 'sun spot' rendering in it because that would then be visible inside objects that the sun passes behind while driving. Due to the sun's very high intensity it would show up even at short distances to objects.
I'm pretty sure this could be fixed quite easily.
I also think the 'inputs' to the decent new sky (the one we see in the actual sky) are quite similar to mie/ray too.
Ie, we should simply be able to adjust this shader to use the ray/mie TOD curves, among some others... rather than hard coding the 'sky' values in atmosphere.ini.
So there is no wonder the sky was doing different things to the extinction
/*
// Ideal fix
Indeed the two outputs could be the same, just don't render the sun spot.
The sun spot should be added to a car via speculars, not the envmap (reflections in shadow still get specular otherwise, and it doesn't conserve energy either!)
The sun visually should then be represented by a 'flare' rendered as a post effect.
*/
Dave