than the PSP, you need to experiment with the fog settings.
Also you should have some knowledge on mapping / using Worldcraft or VHE!!
THIS WILL NOT WORK IN VANILLA QUAKE UNLESS YOU ADD THE TRIGGER_COMMAND INTO THE QC SOURCE!!
1st: Just make a big block around your map, use your trigger texture... Should look like this:
2nd: tie to entity, use "trigger_command" and add your prefered fog value in.. should look like this:
3rd: Compile your map and run it in your game (Quake, Kurok or NZP)...
You will now ALWAYS have the nice fog effect on your map, also this SHOULD
bypass the annoying hardcoded NZP fog..
Example pics with the fog command used to change the look of the map and simulate a sky:
Desert look a like (simulating sandstorm):
Winter look a like: