There are four seasons in Sons of the Forest, but that's not the only factor. Specific mountain ranges are frigid and can be problematic for staying warm. It's best to acquire a winter jacket so you aren't in a panic throughout your game. There is a massive mountain in the middle of the map that you'll likely stumble on at some point, and these temperatures are consistent year-round. Do yourself a solid and pick up the jacket to traverse these areas and gain an advantage.