@HeelX: Nice! Is the shader physically based in some way?

@Error: The fog does it's job. I would recommend you to use a simple ground fog too, though.
However, the fog is a good tool to give feedback about the lighting conditions in those scenes, f.i. your bright forest at day fog settings imply that there is a clear sky where for your night scene it feels like there is not much moon light reaching the ground (cause could be a thin cescent moon or clouds).
Darker fog for a day setting could therefore imply an overcast sky and rainy conditions, white fog for night scenes a bright full moon.