It is difficult to imagine a Siberian or northern winter without frost. Actually, no one is trying, and -30 in January is quite the norm. However, in such weather, as we know from childhood, it is cold and there is a Christmas tree in the forest. A person is a much more thermophilic creature and in an unheated room, unlike a Christmas tree, will not survive in winter. And at -30 in Siberia, and at +2, say, in Italy.