A snow cave is a shelter constructed in snow by certain animals in the wild, human mountain climbers, winter recreational enthusiasts, and winter survivalists. It has thermal properties similar to an igloo and is particularly effective at providing protection from wind as well as low temperatures. A properly made snow cave can be 0 °C or warmer inside, even when outside temperatures are −40 °C. A snow cave is constructed by excavating snow so that the tunnel entrance is below the main space to retain warm air. Construction is simplified by building it on a steep slope and digging slightly upwards and horizontally into the slope. The roof is domed to prevent dripping on the occupants. Adequate snow depth, free of rocks and ice, is needed—generally a depth of is sufficient. Another kind of snow cave is the quinzhee, which is constructed of snow rather than created by digging a hole out of snow. Regardless of construction type, the snow must be consolidated so that it retains its structure. The walls and roof should be at least thick. A small pit may be dug deeper into one part of the cave floor to provide a place for the coldest air to gather, away from the occupants, and the entrance may be partially blocked with chunks of snow to deflect wind and retain heat, although it is vital to prevent drifting snow from completely plugging the rest of the entrance in order to maintain a constant air supply. Some prefer to place a ski pole or a ski up from the cave out through the roof or side wall. This can provide an emergency air hole should the main entrance become blocked. It is possible to sleep several consecutive nights in a snow cave, but care must be taken since a slight ice surface may develop on the inside of the cave from moisture in the exhaled air of the inhabitants. This is thought to result in reduced air ventilation through the snow cave walls and roof, and thus increase risk of suffocation. As a precaution it is common to scrape off a thin layerfrom the inside of the cave ceiling each day spent in the cave. A narrow entrance tunnel, a little wider than the occupants of the cave, leads into the main chamber which consists of a flat area, perhaps with elevated sleeping platforms, also excavated from snow. Most sources agree that using tools such as a shovel and ice axe are vital; digging by hand is for emergencies only. Digging a snow cave can be very physically demanding. In perfect conditions with good snow, digging a snow cave for 2 or 3 persons often takes 3 to 4 hours to complete. It can be useful to take turns in a group of climbers such that each person works in 5 minute intervals inside the cave, while the others help remove excess snow outside the cave and prepare food and warm liquids for the group.
