There are two scenarios when a snowhole might be used . The first is when you are caught out or 'benighted'. In this situation the group will have limited resources except what they normally carry on a winter's day out. In this situation the shelter will be very simple. Just enough to get everybody down and out of the wind. Possibly a covered over pit or a basic cave. It will be down to the leader whether to spend the time constructing the shelter or retreating from the mountain.
The second scenario is a planned overnight or multi day outing. The group will be fully prepared with tools for digging and the extra kit needed. In this situation the snowhole offers a few advantages over a tent. The first being that you do not have to carry it around with you. It offers much better protection from the elements and far greater insulation ( if constructed properly). The disadvantages are that it is a little more time consuming and tricky to put up. It takes 45mins to 2 hours to construct a good cave for 2-4 people.
First you must choose a suitable sight for your cave. A bank of snow or a drift is ideal. A cave can be constructed on the flat but the snow pack must be quite deep. Test the depth of the snow with an avalanche probe if you have one. It wants to be deeper than your probe.
There is no dry way to dig a snowhole, at least not for the tunneller. I suggest that you work in pairs one being the tunneller and the other one the snow shifter. The tunneller should wear just his waterproofs and a thermal so as to keep his other clothes dry. Don't worry about being cold, digging is quite hard work. To start, tunnel into the base of your bank of snow at a gently rising incline ( since hot air rises this will stop all the heat being lost out of the entrance tunnel). Although the tunnel wants to be quite tight, it is easier to make it a bit wider now and fill it in later. At first you can just lift the snow out but as the tunnel gets deeper all you can do is push the snow behind you and down the tunnel. You will end up in the situation where you have snow in front of you and snow behind you. This is not a great job for someone with claustrophobia. The snow shifter stands at the tunnel entrance and moves the snow away. During the whole construction that is all the snow shifter does, keeps clearing the entrance tunnel of snow. Building your cave on a slope will also make it easier for the snow shifter to move the snow away. Once the tunnel is about 5 feet long, you begin to shape the inner chamber. I turn left 90 degrees from the entrance tunnel and expand the chamber horizontally. The chamber wants to be wide enough and long for two people to lie down side by side, and high enough to sit up comfortably. On the other side of the entrance tunnel I cut a platform for the cooker and someone to sit. The idea is that the person working the cooker sits with his legs in the entrance tunnel while the other sits in the sleeping area.
Once you are happy with the size and shape of the cave, you need to smooth the walls with your shovel. Water will drip from any points but will run down the side of a smooth wall, so make it really nice to stay dry all night. You might not think that there will be much water inside a snow cave but the temperature inside could reach 0 C if you have built your air trap properly. Add all the water vapour that you breathe out during the night and there is a lot of moisture floating round.
Ventilation is very important in a snowhole and has been the cause of a number of accidents. It is wise to put a small ventilation hole in the roof. Also stoves and candles burn oxygen and emit a lot of CO2. Try to use torches instead of candles. If you have a headache or have trouble lighting stoves and lighters this is a sign that there is too much CO2, you should increase ventilation.
A backpack or two can be used to partially block the entrance tunnel if it is too wide but do not seal it because of ventilation. If the tunnel is drifting in then you should clear it at least every two hours.
Try to avoid brushing against the sides or roof of the cave as this dislodges snow that your body heat will melt, also remove as much loose snow as possible when you enter. If the walls are thin there is also a danger of collapse, especially if your cave is really warm, in this case increase ventilation to reduce the temperature.
Everything not inside your sleeping bag will freeze solid during the night, including your boots. I take my inners into my bag but leave the outers to freeze. Any clothes that you are not wearing but are slightly damp are best off inside your bag. Do not put your head inside your sleeping bag to warm it up when you first get in, as this adds moisture to the bag that will ultimately make it colder for you.
Apart from yourselves you will the same equipment as for a short camping trip. Cooker and pan for eating and drinking. Some high calorie food to keep you warm all night. Spare clothes, torch and first aid kit. Don't forget to tell someone where you are going and when you expect to be back. Choose somewhere that should things go wrong you could get home even at night.
A good snowhole offers such good protection that a full gale can be blowing outside and you wouldn't even notice until forced outside to take a leak! If you pick a full moon night you can even have a short hike and ride at midnight.