Avalanche

:This article refers to the natural event. For other uses, see Avalanche (disambiguation)

Contributing factors

Determining critical load which would cause a slope avalanche is a complex task involving evaluation of many factors. Some of them are:

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Terrain

• Steepness - slopes under 25 degrees and over 60 degrees have a low avalanche risk because of the angle of repose for snow. Snow does not accumulate significantly on steep slopes and does not easily flow on flat slopes. Distribution of avalanches by slope has a sharp peak between 35 to 45 degrees. That peak hazard lies at around 38 degrees. Unfortunately, slopes with the most dangerous steepness are favourite for skiing.
• Direction - snowpack evolution is influenced by solar heating and wind. In medium latitudes on the northern hemisphere, more accidents happen on shady slopes with northern and north-eastern aspects. Slopes sheltered from the wind tend to gather more snow. Cornices also accumulate on the downwind side of ridges, and can contribute to avalanche danger.
• Profile - convex slopes are statistically more dangerous than concave. Reasons lie partly in human behaviour, and the tensile strength of snow layers versus the compression strength.
• Surface - base avalanches are more common on slopes covered with grass surfaces than slopes with dwarf pines. Boulders or buried vegetation may create weak areas within the snow pack.

Snow

Structure of the snowpack determines avalanche danger. Unfortunately relations between easily observable properties of snow layers (strength, grain size, grain type, temperature) and avalanche danger are complex and not yet fully understood. Additionally snow cover varies in space and so does stability of snow.

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• New snow - has not had time to bond with the layers below, especially if it is light and powdery.
• Snow depth - snow that is above the layer of boulders and plants on the slope has none of these natural objects to help anchor it to the slope, and is therefore more dangerous. Naturally this is just the type of snow needed for snowsports such as skiing.
• Snow crystal shape - small ball-shaped snow crystals act as ball bearings and are potentially dangerous, while crystals that interlock are more stable.
• Snow compaction - compacted snow is less likely to move than light powdery layers.

Weather

Weather determines the evolution of snowpack. The most important factors are heating by solar radiation, radiational cooling, temperature gradients in snow, and snowfall amounts and type.

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Most avalanches happen during or soon after a storm.

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• Temperature - if the temperature is high enough for gentle freeze-thaw cycles to take place, the melting and re-freezing of water in the snow stabilizes the snow crystals. Temperatures rising significantly over the freezing point may cause the whole slope to avalanche, especially in spring. Persistent low temperatures cause the snow to not gain stability from the freeze-thaw action, and may contribute to an internal "depth hoar" frost layer, where there is a high temperature gradient within the snow.
• Wind - anything more than a gentle wind can contribute to rapid build up of snow on sheltered slopes (downwind), while the wind pressure can also stabilize other slopes. "Wind slab" is a particularly fragile structure -- heavily loaded, poorly bonded. Even on a clear day, wind can quickly shift snow-load to the snow pack.
• Heavy snowfall - cause instability, both through the additional weight, and because the snow has insufficient time to bond.
• Rain - in the short-term causes instability through additional load and possible lubrication of lower layers.