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Home > Avalanche

• Chevrolet has produced a car named Avalanche
• The Avalanches are an Australian dance music group
• An Avalanche diode is an electronic device

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 an example of a gravity current consisting of granular material.

In an avalanche, lots of material or mixtures of diffrent types of material fall or slide rapidly under the force of gravity. Avalanches are often classified by what they are made of, for example snow, ice, rock or soil avalanches. A mixture of these would be called a debris avalanche.

1 Causes

Avalanches occur when the load on the upper snow layers excedes bonding forces (bonding to layer benath, support from anchors such as rocks and trees, stress support from tom or bottom of slope).

Critical load may be exceeded naturally by adding new snow or by rapid loading, by falling ice, cornices and similar means. Avalanches are also triggered by humans - because of the additonal weight, kicks during skiing (e.g. during jumps) or intentionaly by explosives, slope-cuts and other means.

More than 90% avalanche victims worldwide are due to avalnches triggered by victim or someone in the victims party.

2 Contributing factors

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

2.1 Terrain

• Steepness - slopes under 25 degrees and over 60 degrees have a low avalanche risk. Distibution of avalanches by slope has shar peak between 35 to 45 degrees. Fo example 38 degrees in Alps. Unfortunately, slopes with the most dangerous steepness are favourite for skiing.

• Direction - snowpack evolution is influenced by solar heating and wind. In mid latitudes on northern hemisphere more accidents happen on shady slopes with northern and north-eastern aspects. Slopes sheltered from the wind tend to gather more snow.

• Profile - convex slopes are statistically more dangerous than concave. Reasons are partly in human behaviour.

• Surface - base avalanches are more common on slopes covered with grass surface than slopes with dwarf pine.

2.2 Snow

Structure of the snowpack determines avalange danger. Unfortunately relations between easily observable properties of snow layers (strenght, grain size, grain type, temperature) and avalanche danger are complex and yet fully understood. Additionaly snow cover varies in space and so does stability of snow.

2.3 Weather

Weather determines the evolution of snowpack. Most important influneces are heating by solar radiation, radiational cooling, temperature gradients in snow and, obviously, snowfall.

• 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 stabilize slopes.

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

• Rain - in short-term causes instability

3 Avalanche avoidance

Due to the complexity of the subject, winter travelling in the backcountry (off-piste) is never 100% safe. Good avalanche safety a continous process, including route selection, examination of the snowpack, weather conditions, and human factors. Several well-known good habits can also minimise the risk

3.1 The group

• Traversing - is to be avoided to minimise the 'cut' made across the slope.

• Risk exposure - minimise the number of people on the slope. Ideally one person should pass over the slope into an avalanche protected area before the next leaves protective cover.

• Group size - the party should be large enough to perform a rescue, but additional people will increase the disturbance to the slope.

4 Human survival and avalanche rescue

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

Research carried out in ItalyThe Italian Republic or Italy ( Italian: Italia is a country in the south of Europe, consisting mainly of a boot-shaped peninsula together with two large islands in the Mediterranean Sea: Sicily and Sardinia. To the north, where it borders France, Switzer based on 422 buried skiers indicates how the chances of survival drop

• very rapidly from 92% within 15 minutes to only 30% after 35 minutes (victims die of suffocationSuffocation can mean two things: Suffocation, or Asphyxia, is a medical condition where the body is depraved of oxygen. Suffocation is a death metal band from New York.)
• and near zero after two hours (victims die of hypothermiaHypothermia is a medical condition in which the victim's core body temperature has dropped to significantly below normal and normal metabolism begins to be impaired. This begins to occur when the core temperature drops below 35 degrees Celsius (95 degrees)

- Nature vol 368 p21. Consequently it is vital that everyone surviving an avalanche is used in an immediate search and 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).

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