Find some deep snow and you have the building material for a winter shelter that is not only warm, but also lots of fun.
Snow is a great insulator, due to all the air trapped between and within the flakes. The thick walls of a snow cave, or “Quinzhee,” trap enough ground and body heat to stay incredibly warm in relation to the outside temp. My brother Chuck and I once slept in a quinzhee that stayed a balmy 15 degrees, 50 degrees warmer than the minus 35 outside--and with a wide open doorway (we all fell asleep without blocking the door with a backpack, as usually).
Not far below the surface, the ground maintains a fairly constant 50 degrees year-round. Bare winter soil will freeze several feet down, but covered with deep snow, it stays thawed. You can trap and use that heat by building an insulated bubble over it. Add a couple of heat-generating human bodies (100 BTU's/hr per person) and some candles, and you’ve got a pretty cozy room.
To start your quinzhee, pile up snow with a scoop shovel (or use a snowshoe as a shovel) to the size you desire. The inside will be quite a bit smaller due to the thick walls, so keep that in mind. Be sure to avoid a flat top. Cones and domes are self-supporting, but a flat roof may cave in.
Six feet high by nine or ten feet in diameter will do. Don’t worry about packing your snow pile. When snow is disturbed, pushed, or piled it “sinters,” or interlocks. After sitting for a while it becomes incredibly strong. Believe it or not, five of us played King-On-The-Hill on a three day old snow cave no more than four or five inches thick at the top. Usually a few hours is long enough to wait, but real cold, dry snow must be left longer, maybe even overnight if piled late in the day.
Start digging it out at the door, working your way in and up. A useful trick is to push sticks in from the outside to the desired wall thickness, a foot or so at the bottom, tapered to four or five inches at the top (the thicker, the warmer). When you hit a stick, move on! A pot lid works well to dig out and shape the inside while your partner shovels away from the door.
In theory, you’re supposed to keep the top of the door lower than your sleeping platform for a proper warm air trap, but don't worry about that. It’s almost impossible unless you’re on the right hillside (I did it once). Just plan on stuffing a pack in the doorway. And by the way, wear wool, fleece, or even a rain suit (preferred by me) when digging out, or you may come out soaked to the skin!