I meant to say "Clothing, Cold Weather". From Frostmaiden
This outfit consists of a heavy fur coat or cloak over layers of wool clothing, as well as a fur-lined hat or hood, goggles, and fur-lined leather boots and gloves.
As long as cold weather clothing remains dry, its wearer automatically succeeds on saving throws against the effects of extreme cold (see “Extreme Cold”).
It should have been in the PHB, it is odd that we needed a supplement, it was an oversight.
Extreme Cold is defined simply as "Below 0 degrees Fahrenheit" because D&D simply doesn't get to levels of detail some people seem to want. D&D is many things. An accurate and detailed survival sim is not one of them, it never has been.