This is fully Volumetric fog. You can increase reselution if you want. I would suggest lowering the resolution for a higher FPS, since this lowers mine to 1... I found that you could use layered faces in all directions to be able to create a fog effect. This is very heavy on the CPU, so if you know of any better way of doing this, with the same HD effect, please comment and call me an idiot. Well, enjoy your foggy day.
I agree that the clipping is a good idea, but the mist in the world settings isn't really "volumetiric", or at least, it is a very pathetic volumetiric.
Can you explain what settings did you tinker, cause I have no idea how to re-create this effect in order to use it.
I dont know if my way is "better" but it actually increases FPS:
Light; I use a hemisphere as light (0.1 strength)
Clipping; I go to camera panel -> and make clipping "end 40m". This makes the camera not see beyond 40m. This gives much better FPS and lets u work with a lot more polygons. This lets me have loads of grass with butter-smooth fps. Longer clipping isnt needed anyway since the fog prevents longer vision.
Fog; I go to world panel -> enable mist (start 15m, depth 25m) - this must match the clipping settings (15+25 = 40) and I play with the ambient/horizon colors. I make them gray, with the horizon at about 0.4 and ambient at about 0.2 ;-)
hope it helps