This is a tutorial for distance fog-brushes.

1. Create a brush at o­ne side of your map near the skybox. But make sure the brush isnt touching the skybox. Shift it down to the bottom.

2. Let the brush selected and press 'S'. Write the path of the fog-texture. The following fogs i know:

3. Now copy the brush for all remaining sides by pressing 'Space'. Move them so that they are touching their corners.

Now you have perfect fog. If you dont get it to work try the map-file here.