Snow blindness and hypothermia are ever-present dangers in the frigid regions, and not only because of the environment. They can also be triggered by some creatures and magical effects.
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.
Frostbite and Hypothermia
Basic rules for surviving cold weather can be found in the fifth edition rules. Characters who aren’t prepared for cold weather or who are exposed to extreme, unnatural, or magical cold can experience both hypothermia and frostbite. Under those conditions, characters must make Constitution saving throws to avoid harm; the GM determines when saving throws are called for and the DC for each one. Each failed saving throw increases the risk and damage, as described below.
Stage 1: Frostnipped. The skin itches, aches, and becomes numb. Yellow and white patches form. Frostnipped characters have a –2 penalty on Strength and Dexterity checks.
Stage 2: Shiverstruck. The character gains one level of exhaustion, shivers uncontrollably, and his or her teeth chatter. A shiverstruck spellcaster must make a successful DC 10 Constitution check (with disadvantage because of exhaustion) to cast a spell with a verbal or somatic component; if the check fails, the spell slot isn’t expended. Tasks such as getting into or out of armor take twice as long.
Stage 3: Kissed by the Ice Maiden (Hypothermia).
The character gains a level of exhaustion and can’t recover from exhaustion until after a long rest in a warm environment. The character’s skin develops black blisters but the character’s body is numb—which may initially seem a relief. A hypothermic character is only superficially aware of his or her surroundings and may trudge past shelter or blunder into monsters or hazards without realizing it.
Even simple activities such as drawing or sheathing a weapon, loading a crossbow, or retrieving something from a pouch or pack require a successful DC 10 Dexterity check (made with disadvantage from exhaustion) to accomplish without fumbling and dropping the item. This can be avoided if the character takes four times longer than normal to do simple things. More complex tasks, such as starting a fire with flint and steel, are impossible.
Stage 4: Frostbitten. The character gains a level of exhaustion and becomes incapacitated. Eventually organs fail, flesh dies and becomes gangrenous, teeth freeze and shatter, fingers and toes snap off, and other permanent injuries occur. The effect of such losses is determined by the GM. Every additional failed Constitution saving throw at this level of exposure adds another level of exhaustion. The character can survive and recover if rescued, but only magic can restore lost limbs and damaged organs.