I'm still tackling cloud illumination in low sun. The basic failure of my current scheme is that cloud layers do not self-shade - every cloudlet is illuminated as if the others would not be there. The reason is of course that we can't do ray intersection tests between thousands of cloudlets in real time, so in rendering one cloudlet, we really don't know what the others do. Only the weather system knows where clouds are in relation to each other.
If the sun is high, that's not so problematic, as the weather system has the layer ordering available, and since the light comes from above, there's a good approximation scheme to get this right, so we can shade low cloud layers if there are closed layers above.
However, if the sun is low, the shading depends not only on the visible layers, but in addition also on assumptions about how layers which we haven't actually drawn yet would obscure the sunlight at the horizon. I've tried to capture this on average with a new parameter.
This is a broken cover in full glory illumination. - higher clouds still in light light up very brightly, the lower ones are already getting dark. I think under some conditions it can be seen like this...
...but normally the outcome would rather be like this if the layer would be continued to the horizon (as the weather system is likely to do if you go there):
(note also that the fog underneath the clouds has to be shaded as well).
The difference is in particular apparent when you look away from the sun: Full glory illumination for every cloudlet gives very bright contrast with the dark terrain...
...whereas this looks much more plausible if we apply the cloud self-shading correction:
Right now this is property controlled and the property is selected by hand - now that I know that it generates about the right appearance, it's down to finding a good function to set it from the weather system based on sun position and current weather conditions.
Since I was already tinkering with light all over the place, I finally got around to adding some moonlight correction as well:
This isn't *real* directional illumination (that'd be fairly expensive to do) but just an ambient light and fog color effect - but I think it does the job. Now someone needs to expose the moon phase somewhere, and we can have this automatically