Fog

The main setting for controlling the 'fog' effect on your world is

*settings → Landscape → Fogging Mode

0 = Amount of fog varies according to the current time of day and weather settings
1 = Static level of light grey fog
2 = Static level of black fog
3 = Static level of white fog
4 = Static level of mid-grey fog
5 = Dense mid grey fog
6 = Very dense mid/dark grey fog
7 = Dense black fog
8 = Dense white fog
9 = No fog effect
10 = Varies according to the current daytime and weather but the fog distance is further.

When using modes 0 or 10, the amount (and colour) of fog will be determined by the current weather - check here for details of the weather settings