Hmm, good point. I imagine back a hundred years ago a man could make a camp when it got so cold that the surface was well below zero but by using snow to cover a tunnel you could sleep under the tree like an igloo at 32F or around there with your body heat being kept in the open area under the tree preventing it from getting below zero where death can come fast thought a fire is likely a stupid idea that far down since the smoke could suffocate you and the heat would possibly bury you when the snow caved in or the ground became wet from snow melting so you'd be worse off wet at above 32F than dry at 32F. Just a guess on all this.