From Valve Developer Community
The $maxfogdensityscalar when specified in a VertexLitGeneric material modulates the amount of fog the material receives.
Tip:$maxfogdensityscalar is useful for modulating lit materials in foggy maps. For example in L4D2's Hard Rain campaign, this is used on the gas station in the 3D skybox to make it visible as a landmark through opaque fog.
Only tested in L4D2, support in other games not verified. Not known if its cheap or expensive.