Stolen without permission from DM oKOyA
Cold Dangers
Cold and exposure usually deal nonlethal damage to the victim. A character cannot recover from the damage dealt by a cold environment until she gets out of the cold and warms up again. (ie not just rest and refit). Once a character has taken an amount of damage equal to her total hit points, any further damage from a cold environment is converted to lethal damage.
An unprotected character in cold weather (below 5° C) must make a Fortitude save each hour (DC 15, +1 per previous check) or take 1d6 points of nonlethal damage. A character who has the Survival skill may receive a bonus on this saving throw and might be able to apply this bonus to other characters as well (see the skill description). Characters protected from the cold, by cold weather outfits for example, do not need to make fort saves at all.
In conditions of severe cold or exposure (below -20° C), an unprotected character must make a Fortitude save once every 10 minutes (DC 15, +1 per previous check), taking 1d6 points of nonlethal damage on each failed save. A character who has the Survival skill may receive a bonus on this saving throw and might be able to apply this bonus to other characters as well. Characters wearing a cold weather outfit only need check once per hour (DC 15, +1 per previous check) for cold and exposure damage.
Extreme cold (below –30° C) deals 1d6 points of nonlethal damage per minute (no save) to unprotected characters and they must make a Fortitude save (DC 15, +1 per previous check) or take 1d6 points of lethal damage as well. A character who has the Survival skill may receive a bonus on this saving throw and might be able to apply this bonus to other characters as well. Characters wearing a cold weather outfit only need check once per 10 minutes for nonlethal damage (DC 15, +1 per previous check).
A character who takes any damage from cold or exposure is beset by frostbite or hypothermia (treat her as fatigued). These penalties end when the character recovers all damage taken from the cold and exposure.