opacity : This is the distance, in game units, until the fog becomes totally opaque, as measured from the point of view of the observer. By making the height of the fog brush shorter than the distance to opaque, the apparent density of the fog can be reduced (because it never reaches the depth at which full opacity occurs).
The fog volume can only have one surface visible (from outside the fog).
Fog must be made of one brush. It cannot be made of adjacent brushes.
Fog brushes must be axial. This means that only square or rectangular brushes may contain fog, and those must have their edges drawn along the axes of the map grid (all 90 degree angles).
I like working around stuff. Besides this doesn't do it the way i want it. Let's say i want 300 units of vision horizontally but only 100 vertically. No shader will do that. Hell i doubt a trick will do that but i've got time to try.
It isn't for realism it serves the purpose of the map. Like scourge's saying, quake isn't something that needs to be modelled to reflect the real world if it doesn't fit what you think.