We will be producing this effect:
This is very easy technique, using a fog_controller & func_dustcloud, make sure you drag a brush over the intended area for the func_dustcloud.
Apply the func_dustcloud to a "nodraw brush"
The stats for the func_dustcloud are
Particles per second= 500
Min particle size = 100
Max particles size= 200
And don't touch any of the other settings for the func_dustcloud
Now the fog_controller
Just add the fog_controller and set active= yes
And that should about do it you should be ready to compile your map and run it..
Also can be used to make a nice ground fog by lowering the dustcloud in height.
Just lower the size of the particles to:
Min particle size = 50
Max particles size= 75
And lower the brush size to about 2 feet and move the brush about 2/3 under the ground leaving 1/3 above the ground to give the effect of ground fog.
I hope this helps