Fog is a graphical effect of the game:
In ModWarfare every map has it's own fog settings, fog is set by server side and there is no way to change it by client side.
Client can only locally enable or disable the effect by setting the r_fog dvar to "1" or "0".
Please note:
r_fog is a cheat protected dvar, normal ranked servers does not allow you to change it (you can only do it in devmap).
To change the aspect of the fog, the only way is by a server mod script using the setExpFog function.
To change it in an already recorded demo, the only way is to use CoD4 Demo Manager (or by using a fog mod Like This ).
This because fog parameters are written inside the demo file and can't be changed at runtime.
Some mods (like promod) disable the whole fog by setting "0" as fog parameters.
So also if you set r_fog to "1" you will not able to see the fog.
But with Demo Manager you can restore the fog and set your own parameters.
To change the fog parameters of a demo simply type the setExpFog parameters (separated with blank) in the Fog Params field and modify the demo.
Params should look like this:
Now, if everything worked, go to COD4, set r_fog to "1" and play your fogged demo.
Please note:
promod server also force the r_fog dvar to "0" periodically during the game.
So, if during demo playback r_fog gets reset, you can always use the Remove Dvars functionality to remove any r_fog reset and have fog for the whole demo.