Winter Climbs in the Cairngorms. Allen Fyffe

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      Sgor an Lochain Uiane and An Garbh Choire from Ben Macdui (Cairntoul/Braeriach Amphitheatre)

      A slab avalanche, on the other hand, occurs when a cohesive slab of snow slides on a weak layer. They are the most common, least predictable and, therefore, 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 СКАЧАТЬ