Commands:

fog_color fog_color [R] [G] [B] This command sets the color of fog, if enabled. The value provided should be an RGB value (separated by spaces).
fog_colorskybox fog_colorskybox [R] [G] [B] This command sets the color of the fog skybox, if fog is enabled. An RGB value separated with spaces should be provided.
fog_enable fog_enable [0 / 1] This command enables fog if set to 1. Note that you will need to set fog_override to 1 if your map's settings disable fog.
fog_enable_water_fog fog_enable_water_fog [0 / 1] This command enables water fog if set to 1. If set to 0, water fog will be disabled. Note that you need to override your map's fog settings with fog_override 1.
fog_enableskybox fog_enableskybox [0 / 1] This command will enable (1, default) whether or not fog should be applied to the skybox.
fog_end fog_end [Distance] This command will set the "end" of the fog in relation to your player. Higher numbers will make the fog more distant. -1 will set to default. Fog will be shown in the distance between fog_start's value and this value.
fog_endskybox fog_endskybox [Distance] This command will set how far you can see into the sky before fog appears.
fog_hdrcolorscale fog_hdrcolorscale [Scale] This command sets the fog's HDR color scale. Specify a decimal value between 0 and 1.
fog_hdrcolorscaleskybox fog_hdrcolorscaleskybox [Scale] This command sets the fog's skybox HDR color scale. Specify a decimal value between 0 and 1.
fog_maxdensity fog_maxdensity [Density] This command sets the maximum density of the fog, given as a decimal percentage. 0.5 would be 50% transparency, 1 would be completely opaque.
fog_maxdensityskybox fog_maxdensityskybox [Density] This command sets the maximum density of the fog, given as a decimal percentage. 0.5 would be 50% transparency, 1 would be completely opaque.
fog_override fog_override [0 / 1] This command will allow you to override the fog settings of the map if set to 1. Default is 0 (meaning you will by default use the map's fog settings).
fog_start fog_start [Distance] This command sets the distance from the player at which the fog will start being rendered.
fog_startskybox fog_startskybox [Distance] This command sets the distance from the player at which the skybox fog will start.
fogui fogui This command will toggle the visibility of the fog control GUI.