the most dangerous type of avalanche. This type of snow, called windslab, is formed when wind-transported, and therefore damaged, snow crystals accumulate in sheltered areas such as lee slopes. Windslab consistency can vary from soft to hard, depending mainly on the speed of the wind that transported the snow. Hard slab, in particular, is very deceptive, as it can be firm enough to walk on without sinking in and so feel safe to the unwary. However, it does tend to have a dull, chalky appearance and may squeak or creak when walked on or probed with an axe. Although most common on lee slopes, particularly below cornices, it can build up in unexpected places, even on windward slopes.
Wet-snow avalanches usually occur during a thaw, when the snow or its top layers become saturated. The extra weight and lubrication of this water can weaken the bonds between different snow layers, or between snow and the ground, and may cause a large area to slide. Although easier to predict, this type of avalanche can be particularly descructive due to the high density of wet snow. Wet-snow avalanches can harden rapidly on settling, and so it can be extremely difficult to escape from them unless this is done immediately.
Avalanche avoidance
Before starting out
Avalanche awareness should start long before the mountains are reached. Monitoring the weather in the days prior to a climbing trip is a basic precaution, with heavy snowfall accompanied by wind being the main danger sign. Long periods of cold temperatures tend to make any danger last longer, while a freezing level that rises and falls rapidly usually quickly consolidates the snow pack. Be aware of the recent and forecast wind direction, as the greatest danger is likely to be on lee slopes, the place where windslab accumulates. Consult the snow and avalanche forecast, and consider possible alternatives should conditions turn out to be worse than expected. Seek expert local advice – avalanches tend to have local characteristics. Have the appropriate equipment and know how to use it.
In the mountains
Ongoing observations on the hill should be used to confirm, and refine or adjust, your initial hazard evaluation. Avalanches occurring or signs of recent activity are the most significant indicators of an avalanche danger and should not be ignored. On the approach observe the nature of the snow on the ground and in the air. The behaviour of snow underfoot can be very informative. Cracking and breaking away of snow blocks from beneath your feet is an indication of slab and instability. Look for signs of recent windslab deposition. Spindrift is usually a danger sign, as even fairly light winds can redistribute huge amounts of snow, packing it in as windslab on lee slopes and in other sheltered locations. Even a light surface drift can produce localised danger, especially in the tops of gullies.
Signs of a rise in temperature – such as rain, wet snow, sun balls rolling down the slope, strong sunlight, and melting snow and ice – can all be danger signs, especially if large cornices exist. If the thaw is due to strong sunshine, then different slope aspects may make a large difference, with perhaps danger on south faces but safer northerly aspects. In these circumstances, slopes of a similar aspect and altitude should be considered extremely suspect.
Although general observation is important, a simple snow pit can give further information about the snowpack and its stability. A snow pit dug in a safe but relevant position for the suspect slope is useful. It is dug down to ground level or, more usually, to the level of a stable consolidated layer. The back or sidewall of the pit is inspected for snow layers of differing hardness. Significant differences in hardness between adjacent snow layers can be a warning sign. A pit can be dug quickly with an axe and need not be elaborate to give relevant information. Even the resistance felt when pushing the axe into the snow may reveal much about the layers. However, due to local variations in the properties of snow, any snow pit or test will only give information about one point, and must be considered as part of the overall picture.
Route choice and safety precautions
A number of factors influence route choice in avalanche terrain (here route choice is not limited to a climb, but includes all movement in the mountains). Ridges, buttresses and flat ground are safer than open slopes. Windward slopes are generally safer than lee slopes, but even here local variations in topography can create a localised avalanche hazard. Convex slopes are more likely to slide than concave ones, and gullies or depressions tend to concentrate avalanche depth and power. Even a very small area of windslab can represent a hazard if the run-out is into rocks, over a drop, or into a terrain trap such as a stream-bed or similar hollow where the debris can be funnelled and build up to considerable depths. Some very serious accidents have occurred where the slab that released was only about the size of a mattress.
In very snowy conditions, ridges and buttresses usually provide the safest and best climbing conditions – here teams enjoy the Fiacaill Ridge
The need to travel on a suspect slope varies according to the circumstances. For example, it is seldom if ever necessary on the way up the hill in the morning, when you can vary the route or even retreat. However, it may be forced on you on the way down, in bad weather or in the dark. Travel one at a time between islands of safety and watch the person moving. Never assume that if one person crosses safely then there is no danger – this is not the case – but all use the same track. Tighten clothing, remove wrist loops and loosen rucksack straps so that it can be abandoned if necessary. If a slope has to be crossed, then traverse it high up rather than low down to reduce the amount of snow above, and thus reduce possible burial depth should it slide. It is better to be carried further but buried less deeply.
Ross Hewitt on Tough Guy, in the Eagle Ridge and Parallel Buttress Group (The Norhern Sector, Lochnagar) (photo: Sandy Simpson)
If you are caught in an avalanche
If you feel the snow slope around you move, then shout to alert others and try to delay your departure by using your axe. Attempt to roll out to the side. If you are carrying a heavy sack, get rid of it. A light sack, however, can provide some protection and will not drag you down too much – and if you are lucky enough not to be buried, you will still have your gear with you. During the avalanche, if possible get onto your back with your head uphill and try to swim or roll in order to stay on the surface. As the avalanche slows make a desperate effort to get to the surface, or thrust a limb out of the snow. Make every effort to create a breathing space in front of your face. As the snow comes to a halt it often hardens up very rapidly, so making escape even more difficult as time passes. If buried, try to remain calm so as to conserve oxygen and energy, and do not shout as the sound will not be heard on the surface.
If you see someone caught in an avalanche
Keep them in sight and note their starting position and where they were last seen. Check for further danger, and attract the attention of other people in the area. Mark the position where the victim was last seen, so as to indicate the most likely burial area. Make a thorough search of the debris for surface clues. Probe the most likely burial spots then conduct a systematic search, probing with ice axes or walking poles with the baskets removed. If there are a lot of people in the vicinity then send someone for help, but remember the initial search is vital. If a buried victim is not suffering any serious trauma injuries then he/she has about a 90 per cent chance of survival if dug out in the first 15 minutes. However, after 35 minutes this drops to only about 30 per cent. Therefore the importance of the first search cannot be stressed too strongly.
Avalanche information
There is much good written material on avalanches, and some reading on the subject should be considered as part of overall mountain safety. Avalanche reports are produced by the sportscotland Avalanche Information Service (SAIS) for much of the area covered by this guide. Much useful information and daily forecasts can be found on their website at www.sais.gov.uk. This website can also be a useful source of information for assessing climbing conditions. In addition the daily forecasts are often displayed in police stations, sports shops and hotels throughout the area. They are also posted at many popular mountain access points, such as the Cairn Gorm ski area car park and Aberarder farm.