Snow Blindness
Northerners believe that snow blindness is caused by light glittering on snow or ice, and sometimes by malevolent spirits. The real cause is intense ultraviolet rays burning the eye.
Snow blindness becomes a danger after 4 hours of exposure to a brightly sunlit snowscape. Starting at the end of the 4th hour of exposure, a character must make a Constitution saving throw at the end of every hour; the DC equals 10 + hours exposed beyond 3 (or, for simplicity, 7 + hours of exposure).
Travelers have devised effective precautions. Characters can avoid making a check by taking a Short Rest after 3 hours, or by wearing dark goggles or snow visors (similar to goggles but made from an opaque material such as bone with a narrow, horizontal slit instead of tinted lenses). Monsters native to the Environment are immune, as are Constructs and Undead.
Failing the saving throw imposes the first stage of snow blindness: the character has disadvantage on Wisdom (Perception) checks that rely on sight, weapon and spell ranges (for visually targeted spells) are halved, and all Attack rolls are made with a –2 penalty. A second failed saving throw brings on the second stage of snow blindness: severe pain in the eyes, sight range is limited to 15 feet, and disadvantage on Attack rolls and Wisdom (Perception) checks that rely on sight. Spending two days in a dimly-lit Environment reduces snow blindness by one stage. Sight is restored completely by Lesser Restoration or comparable magic.
