Maybe I could try so ... firstly add a layer with a gradient from gray (where the fog is thickest, usually above horizon) to transparent (nearest to camera position) which simulates the fog's thickness the greater the distance is. Then erase some parts manually with different opacities, sizes and shapes of the brush which simulates that the fog isn't homogeneous. Finally, try to add two overlaping, somewhat perspective cones of light with light gray (or light blue which helps to simulate xenon lights), negatively multiplied.
After all you should bring up a similar result as I've attached.
Thank you very much, that's very pleasant. Nevertheless I try to develop myself farther and hope to always come up with new things - btw. the thing with the fog was the first time I tried that. I'm suprised it came out so well.