On the surface, winter is inevitable. Unlike in the Underground, where you could just move to a different location to avoid the cold, on the surface, the cold spread all around the place. The only way you could get away from the low temperatures was if you went inside a place with heat. For monsters who lived in mostly neutral and hot places, this was very unusual. But no fear! Papyrus and Sans, Skeletons of Snowdin were here! (Cover credit: https://mymodernmet.com/frozen-lake-baikal-siberia/ )