Light and vision, important for any adventurer, and well covered in the WSG.
Overland movement in reduced visibility.
Clear moonlight, 2/3
Darkness snow, 1/4
Pages 72-75 a whole section on Vision and Visibility
Table 41: Outdoor Range of Normal Vision
Clear daylight 500 yards to make out a man sized creature. Double for L, half for S.
Twilight 300 yards, moonlight 50 yards, darkness 25yards.
And rules for overcast; moderate fog; heavy fog, rain or snow; heavy snow with wind; blowing sand/dust.
While perhaps more options than needed, the 500 yards for M size figure - actually very helpful.
Rules for Infravision (page 73-74) quite good descriptions, and would have been helpful as a DM, who always struggled with infravision.
Some interesting rules about torches being effected by wind velocity.
Wind 11-25 mph, illumination 30' instead of 40' and will burn out in 4 turns as opposed to 6.
It takes 1 round to light a torch, plus 1 round for every 10mph wind velocity.
A lighted torch can be seen 200 yards away in moonlight and darkness.
Fighting in poor visibility
eg moonlight -1 attack, and -1 all saving throws related to dodging or evasion
In darkness, if the modified attack roll is 0 or less = will have struck another creature or object (must save vs crushing blow).