Avalanche

An avalanche is caused when a build up of snow is released down a slope, and is one of the major dangers faced in the mountains in Winter. An avalanche is a type of gravity current.

Table of contents

1 Human Survival 2 Causes 3 Contributory Factors 3.1 Snow 3.2 Terrain 3.3 Weather 4 Avalanche Avoidance

Human Survival

Even small avalanches are a serious danger to life, even with properly trained and equipped companions who avoid the avalanche.

Research carried out in Italy based on 422 buried skiers indicates that survival drops very rapidly from 92% within 15 minutes, to only 30% after 35 minutes, and near zero after two hours - Nature vol 368 p21. Consequently it is vital that everyone surviving an avalanche is used in an immediate rescue operation, rather than sending for help.

(Historically, the chances of survival were estimated at 85% within 15 minutes, 50% within 30 minutes, 20% within one hour).

Causes

Avalanches occur naturally when the weight of snow becomes so heavy that a layer of snow looses its bond with the layer beneath. However most avalanches that involve humans are caused when the additional weight or the action of the equipment (e.g. skis) of the traveller causes the loss of bond.

Contributory Factors

There are very many factors that cause a snow slope to be avalanche prone:

Snow

• 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.

Terrain

• Steepness - slopes under 25 degrees and over 60 degrees have a low avalanche risk. Slopes in the danger zone are unfortunately ideal for skiing.

• Direction - slopes facing south are most dangerous in the sun and during the spring. In mid winter, the north facing slopes can be more dangerous.

• Profile - convex slopes are more likely to avalanche than concave.

• Shelter - slopes sheltered from the wind are more dangerous, since snow settles there to a greater depth.

Weather

• Temperature - if the temperature is warm 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 freezing point may cause the whole slope to avalanche, especially in spring. Persistent low temperatures means the snow does not gain stability from the freeze-thaw action.

• Wind - anything more than a gentle wind can both contribute to rapid build up of snow on sheltered slopes, while the wind pressure can also destabilize slopes.

• Heavy snowfalls - cause instability, both through the additional weight, and because the snow has insufficient time to bond.

• Rain - also causes instability

Avalanche Avoidance

Due to the complexity of the subject, winter travelling in the backcountry (off-piste) is never 100% safe. Risks can best be minimised by employing the service of a local qualified mountain guide, by carrying the right equipment and knowing how to use it properly, and by minimising the number of people on the avalanche prone slope to ensure that, should an avalanche occur, immediate help is available.

Equipment

• Beacons - known as ARVAs or under trade names including Ortovox and Pieps are essential for every member of the party. They emit a signal in normal use, but can be switched to receive to locate a buried victim.

• Probes - can be extended to probe into the snow to locate the exact location of a victim at several yards / metres in depth.

• Shovels - essential for digging through the snow to the victim.