Cold Weather Survival

All characters have a number of cold points equal to 1+their Constitution modifier (minimum 1).
Whenever the temperature is at or below 0 degrees Fahrenheit, a creature exposed to the cold must succeed on a DC 10 Constitution saving throw at the end of each hour or lose one cold point. The saving throw DC increases by 1 for each hour spent in the cold. Once a character has lost all of their cold points, every failure against cold weather causes the character to gain one point of exhaustion.
Creatures with resistance or immunity to cold damage automatically succeed on the saving throw, as do creatures naturally adapted to the cold. 
Creatures wearing cold weather gear (thick coats, cloaks, gloves, etc.) have advantage on the saving throw. Creatures soaked in water have disadvantage on the saving throw - regardless of whether or not they are wearing cold weather gear.
Creatures that take adequate shelter from the cold (such as in a large cave, or in a tent) are unaffected by the cold, do not need to make saving throws, and the saving throw DC does not increase. If a creature in shelter spends 1 hour near a fire or other similar source of warmth, the saving throw DC against cold weather decreases by 1, to a minimum of 10, and the creature regains one cold point.