Fog allows the developer to create ambient scenes in which distant areas are obscured by a fog-like effect.
http://janusvr.com/docs/build/roomtag/index.html#Room
fog - (default "false") - Whether or not to use fog in the webspace.
fog_mode - (default "exp") - Defines the weight function to use, choices are: exp, exp2, linear.
fog_density - (default "1.0") - The coefficient which applies to the "exp" and "exp2" fog_mode.
fog_start - (default "0.0") - The coefficient which applies to the "linear" fog_mode (the distance at which "fog begins").
fog_end - (default "1.0") - The coefficient which applies to the "linear" fog_mode (the distance at which "fog ends/is maximal").
fog_col - (default "0 0 0") - Defines the colour of the fog.