An avalanche is a phenomenon of nature. It is linked to snow. When there is too much snow on a mountain, some of the snow may fall, causing damage to things in its path. This is called an avalanche. People talk about avalanches because they may be dangerous to skiers and other people doing winter sports.
Large avalanches may carry stones, boulders and trees with them. They may bury people under them. If the people are not found rapidly enough by rescue teams they will die of suffocation (not getting enough air) or of hypothermia (freezing cold).
The chance of surviving an avalanche is as follows:
- 92% if found within 15 minutes
- 30% if found within 35 minutes (victims die of suffocation)
- nearly zero after two hours (victims die of injuries and hypothermia)
European avalanche risk table
In Europe, the avalanche risk is widely rated on the following scale, which was adopted in April 1993 to replace the earlier non-standard national schemes. Descriptions were last updated in May 2003 to enhance uniformity pdf.
|Risk Level||Snow Stability||Avalanche Risk|
|1 - Low||Snow is generally very stable.||Avalanches are unlikely except when heavy loads  are applied on a very few extreme steep slopes. Any spontaneous avalanches will be minor (sluffs). In general, safe conditions.|
|2 - Limited||On some steep slopes the snow is only moderately stable . Elsewhere it is very stable.||Avalanches may be triggered when heavy  loads are applied, especially on a few generally identified steep slopes. Large spontaneous avalanches are not expected.|
|3 - Medium||On many steep slopes  the snow is only moderately or weakly stable.||Avalanches may be triggered 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 to be triggered on many slopes even if only light loads  are applied. In some places, many medium or sometimes 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.|
- Generally described in more detail in the avalanche bulletin (regarding the altitude, aspect, type of terrain etc.)
 additional load:
- heavy: two or more skiers or boarders without spacing between them, a single hiker or climber, a grooming machine, avalanche blasting.
- light: a single skier or snowboarder smoothly linking turns and without falling, a group of skiers or snowboarders with a minimum 10 m gap between each person, a single person on snowshoes.
- gentle slopes: with an incline below about 30°.
- steep slopes: with an incline over 30°.
- very steep slopes: with an incline over 35°.
- extreme steep slopes: extreme in terms of the incline (over 40°), the terrain profile, proximity of the ridge, smoothness of underlying ground.
|Size||Runout||Potential Damage||Physical Size|
|1 - Sluff||Small snow slide that cannot bury a person, though there is a danger of falling.||Relatively harmless to people||length <50 m
volume <100 m³
|2 - Small||Stops within the slope.||Could bury, injure or kill a person.||length <100 m
volume <1,000 m³
|3 - Medium||Runs to the bottom of the slope.||Could bury and destroy a car, damage a truck, destroy small buildings or break trees.||length <1,000 m
volume <10,000 m³
|4 - Large||Runs over flat areas (significantly less than 30°) of at least 50 m in length, may reach the valley bottom.||Could bury and destroy large trucks and trains, large buildings and forested areas.||length >1,000 m
volume >10,000 m³
The toe of an avalanche in Alaska's Kenai Fjords.
A crown fracture from a slab avalanche near the Neve Glacier in the North Cascades mountains. Extensive fracture propagation is evident.
Loose snow avalanches (far left) and slab avalanches (near center) near Mount Shuksan in the North Cascades mountains. Fracture propagation is relatively limited.
15 cm deep, soft slab avalanche triggered by a snowboarder near Heliotrope Ridge, Mount Baker in March 2010. Multiple crown fracture lines are visible in the top-middle of the image. Note the granular characteristic of the debris in the foreground that results from the slab breaking up during descent.
United States Forest Service avalanche danger advisories.
Snow fences in Switzerland
Avalanche blasting in French ski resort Tignes (3,600 m)
Avalanche for Kids. Kiddle Encyclopedia.