As I've been asked for I just want to add this little fog script which had been long times ago published by SAS Colster but which didnt work anymore with orangebox. This one does work again (tested with dod: source).
BUT: The map you are playing on should have an env_fog_controller - else it wont work.
Installation
1. Put file into folder: addons/eventscripts/sib_fog
2. Put es_load sib_fog in your autoexec.cfg
3. Restart server
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