Hyde
Legend
The drier the atmosphere, the more vapor you lose from respiration - especially breathing through your mouth. And obviously if you’re layered up to stay insulated, inevitably you will be sweating with physical activity. So it definitely dehydrates you more than moderate weather.I’m pretty sure the cold can dehydrate you as well.
If anyone’s ever cutting weight in a sauna, breath out of your mouth - it’s very substantial, like 3/4lb an hour more on average or something in a sauna.