Fog Basics

Add a env_fog_controller point entity to your level. For basic fog, you need to set the Fog Enable to "Yes", change the Primary Fog Color to the color of the fog you wish. Set the Fog Start and Fog End values to a distance in world units for how far away the fog will render. The Fog Start distance is how far away the fog will appear from the player, and Fog End is the distance the for will appear totally opaque.

fog tute