Avalanche Causes Contributing Factors Terrain Snow

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

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.

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).

4.1 Equipment

Chances of being found and rescued are increased by using standard avalanche equipment.

Beacons - known as ARVAs, avalanche transceiverIn computer networking the term transceiver (sometimes abbreviated to TCVR) is a device that performs, within one chassis, both transmitting and receiving functions that is in a common housing, sometimes designed for portable or mobile use, uses common cis or under trade names including Ortovox and Pieps are important for every member of the party. They emit a 457kHz radio signal in normal use, but can be switched to receive mode to locate a buried victim. To be effective, beacons require significant amount of practice.

Probes - can be extended to probe into the snow to locate the exact location of a victim at several yardThis article is about the unit of measure known as the yard''. For other definitions, see Yard (disambiguation). A yard (abbr. yd is an imperial unit of length, defined as 3 feet or 36 inches, which is exactly 0. 9144 metres, presuming international inches / metreFor other uses of "metre" and "meter", see Metre (disambiguation). The metre is the basic unit of length in the International System of Units (SI: Systeme International d'Unites). It is defined as the length of the path travelled by light in absolute vacus in depth.

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

Other rescue devices are proposed, developed and used, such as Avalanche Ball, Avalung vest and avalanche airbags.

5 European avalanche risk table

In EuropeFor the band of the same name, see Europe (band . Europe is a continent forming the westermost part of the Eurasian supercontinent. Europe is bounded to the north by the Arctic Ocean, to the west by the Atlantic Ocean, to the south by the Mediterranean Se, the avalanche risk is widely rated on the following scale.

Risk Level	Snow Stability	Avalance Risk
1 - Low	Snow is very stable on most slopes.	Avalanches are unlikely except when heavy loads are applied on a few steep slopes. Any spontaneous avalances will be small.
2 - Limited	On some steep slopes the snow is only moderately stable. Elshewhere it is very stable.	Avalanches may occur especially on a few generally identified steep slopes when heavy loads are applied. Large spontaneous avalanches are not expected.
3 - Medium	On many slopes the snow is only moderately or weakly stable.	Avalanches are possible on many slopes even if only light loads are applied. On some slopes, medium or even fairly large spontaneous avalanches may occur.
4 - High	On most steep slopes the snow is not very stable.	Avalanches are likely on many slopes even if only light loads are applied. In some places, many medium or fairly large spontaneous avalanches are likely.
5 - Very High	The snow is generally unstable.	Even on gentle slopes, many large spontaneous avalanches are likely to occur.

6 See also

Cornices
Crevasses

7 External links

Natural hazards

8 References

D. McClung, P. Shaerer: The avalanche handbook, The Mountaineers 1993, BooksEnthsiast.com

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