Facts about avalanches

Fact: mountain slopes between 20° and 60° + snow = avalanche.
  • The most frequent and devastating avalanches occur on slopes of between 30° and 45°.
  • Most (80–90%) avalanches occur during or immediately after a heavy fall of new snow; a new fall of snow is the most common ‘trigger’ for an avalanche.
  • The majority of avalanches are surface slides involving only the upper layer of snow cover, but less common airborne avalanches also occur.
  • The freeze–thaw cycle and the surface-hardening effect of the wind give rise to crusty surfaces, providing potential sliding surfaces for subsequent snow layers – either fresh snow or windblown snow ‘slab’.
In the Cairngorms, avalanche conditions are aggravated by:
  • Large numbers of climbers and walkers. In winter, the Cairngorms National Park is far more accessible, even than Ben Nevis. In some corries there may be 400–500 people going to climb in a single day. Each person can be a potential trigger for an avalanche.
  • The topography of the corries, particularly Coire an Lochain, and their ability to catch snow.
  • The position of the headwall of Coire Cas above the ski slopes. The snow is monitored throughout the ski season. Preventative blasting is carried out to avoid any serious hazard for skiers.
In Coire an Lochain, the full-depth, spring-time avalanche on the Great Slab is a well-known feature of the Scottish calendar and spring melt conditions – somewhere between early April and mid-June. Conditions are monitored and avalanche risk is forecast, but luck still has a part to play as you will read in A Cautionary Tale