You're in the mountains above the snowline. You didn't bring a tent with you because you thought you would be back in the valley before dark - but you are not going to get down in time. The wind is getting up and you are starting to get cold. There is nothing around you apart from snow and maybe a few rocks but you have to find some shelter for the night. What are you going to do? Well one option is to dig yourself a snow hole.
In fact building a snow shelter doesn't need to be an emergency decision; if you have the right equipment, it can be a reasonably comfortable way to spend a night or two in the mountains. Snow offers good protection from the wind and is a good insulator. Even with body heat alone, a snow hole will be noticeably warmer than the outside temperature.
If you are planning to build a snow shelter you will need at least one snow shovel. It might also be worth taking a snow saw if you are planning to build a deluxe habitation and you think the snow will be hard enough. A candle or two would also be handy both in terms of raising the temperature and illumination.
There are several different types of snow shelter possible, from the very basic snow grave to the igloo. In fact the igloo, despite popular perception, is probably the least useful of all as it will take you all day (unless you are an Inuit) and is vulnerable to a sudden thaw. Probably the best option is the snow hole, sometimes also called the snow cave.
Where to Dig Your Snow Hole
The first thing to do is to look for a slope. The steeper it is the easier it will be to dig your hole, but the more difficult it will be moving around your camp. A 30° slope would be about ideal. If you can find a slope next to a flat bit, then that is even better. If you can avoid having the entrance into the wind then that is another plus point. It is sometimes easier to dig below a cornice, as the snow is softer. Be careful about breaking through to the other side of the ridge though...
The second thing to do is to check that the snow on or above the slope is not going to avalanche. Check the consistency of the snow, think about the wind direction relative to the slope, dig an avalanche test pit. Have a look upward to see what might fall down on you. While you are doing this check there is enough snow. Two metres or more would be ideal.
Construction of the Snow Hole
Now you are ready to dig. You need to start with an entrance tunnel, which should be about one metre long and no wider than you need to dig effectively. When you have got one metre in you can start hollowing out the hole sideways. Make it a couple of metres long so you can get equipment inside. Unless you are going to be there for a while, there is no need to make it much higher than what you need to sit up. Make sure you leave at least 60cm of snow in the roof and walls otherwise they will cave in and you will have to start again. If there are two of you, either one digs and the other moves the excavated snow away, or you can dig parallel entrances and then fill one up when you have completed the hole. When the hole is large enough, smooth the snow on the inside off to avoid drips when you get inside and the temperature heats up. Put a rucksack in the entrance to keep the wind and any drifting snow out. All the rest of your equipment needs to be spread out on the floor to offer the maximum insulation between you and the snow.
If you have time you could dig a sleeping size alcove on the other side of the shelter from the entrance. This decreases the risk of being buried in the event of a cave-in. This works best where you have plenty of snow and where the hole is for one person only. Another way to increase the comfort of your hole is to dig a hole in the floor of the shelter, off to one side. The cold air will flow into it and pool there, leaving the main room warmer.
It is very important to maintain adequate ventilation in the shelter particularly if you are going to light a stove inside. Carbon monoxide poisoning is a real danger otherwise. Poke a hole in the roof with an ice axe if you can feel a headache coming on. Finally, don't forget to mark the top of the shelter in some way, crossed ski poles being one possibility. You might not be the only one on the hill and this will avoid other mountaineers 'dropping in on you'.
It is worth bearing in mind that all this will take you about two or three hours hard work. In an emergency, there might be more profitable things you could do with that time. It depends on the context. You can save time and gain stability by using natural features such as rock formations or small trees and branches, plus any spare equipment you might have, in the design of your hole or mound.
Other Types of Snow Shelter
The Snow Mound
Known as a quincie in Canadian English, the snow mound is for dry and cold conditions when the only snow you have is soft and powdery and you don't have access to a slope. Pile as much snow as possible into a heap at least two metres high and three metres in diameter. Leave it for at least two hours to consolidate. Then tunnel in at the bottom as above.
A time-saving trick for the construction of this type of shelter is piling up your equipment and then making the snow mound over it. When it has hardened you can tunnel in and pull your equipment out (carefully). If you adopt this strategy, make sure you don't leave anything in the pile of equipment that you might need while your mound is hardening. Another good hint is to collect a few twigs that are the same length as the desired thickness of the wall and poke them in from the outside. That way, when you are digging from the inside and reach the twig, you will know to stop and will not cave the roof in.
For an igloo you need to cut blocks, so you need really good hard snow. Make the blocks as large as possible. Do a first circle, two to three metres in diameter and then shape the next layers inwards so that you end up with something that is, well, igloo-shaped. When you've finished the dome cut an entrance hole and do a short entrance tunnel to give protection from the wind. If you're feeling flash and there's a river nearby, why not cut yourself an ice block window? Get one with a frozen fish in it and you are the king of the igloo builders...
The Snow Grave
The snow grave is strictly for emergency only, but if you are on flat ground, there is not much snow and you have to find shelter, it could be your only option. It only works for one person. Mark a rectangle about 30cm longer and wider than the lucky future resident. Cut blocks from this rectangle and carefully keep them. Enlarge the resulting hole to about coffin size, leaving a sill to balance the blocks on. Put all the blocks apart from the ones at the head end back on the sill, leaving a slot you can slide into. Some reference works recommend sliding the last two blocks in position over your head, but this Researcher would suggest simply sheltering your head from the elements with a rucksack to lessen the risk of getting buried by drifting snow. Not one for claustrophobes anywise.
The Snowball Shelter
In very damp snow, it is apparently possible to roll giant snowballs and then put these together to form a crude shelter.
The Debris Shelter
The debris shelter is an option where you have access to trees and other plant material.
Tent or Snowhole?
A snow shelter is warmer than a tent but a lot slower if you are in a hurry. You can combine the best elements of both options by partially burying the tent in snow, or building a snow block wall on the windward side of the tent.