Environment
Light & Obstructions. Your vision is affected by the availability and quality of light (unless you have darkvision) or as well as by the presence of obstructions.
Terrain. Difficult terrain slows you down. Some terrain is challenging to traverse, such as a sheet of ice or a tight-rope. Some terrain is dangerous, such as a cliff wall you try to scale or treacherous waters you attempt to swim. Tracking a creature is more difficult in certain terrain. See DMG 244.
Weather. Weather conditions can affect you in various ways: Snow rain, or fog can affect your sight. Hot weather can increase your need for water, the lack of which could cause you exhaustion. Strong winds could slow you down (house rule: treat as difficult terrain).
Other Nature. Natural hazards include dangerous terrain as well as larger phenomena. See also eldritch storms, supernatural regions, and planes.
Disease & Poison. Given their often contagious nature, diseases are included here. Poisons can (rarely) be part of the environment.