On your adventures you'll likely come across a variety of different cities, landscapes, terrains, and weather patterns. These Environmental Circumstances are aspects of the world around you that can have a positive or negative effect on checks you attempt. For example, you might have amazing aim, but that doesn't matter if you're in a dense fog and can barely see your hand in front of your face!
Changes in the terrain affect how easily you move.
In a clear and brightly-lit space, you can see most objects easily. Changes in visibility affect not only your Perception skill, but your ability to telegraph actions in combat.
Weather conditions can be mitigated by clothing, but extreme weather can impair your ability to use your skills effectively and even pose deadly hazards.