Summarized List: Weather
Thick smoke or fog scatters light, making it nearly impossible to spot distant objects.
In foggy areas, visibility is reduced to a maximum of 30 feet in any direction. You take a -5 penalty to Perception checks using your sight.
Objects shrouded in clouds of smoke or fog have their Perception difficulties increased by 10. If you're standing in a cloud of thick smoke, visibility is reduced to a mere 6 feet in any direction, and you take a -10 penalty to Perception checks using your sight.