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What causes an avalanche? |
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| Avalanches occur when the snowpack fractures. As a result, any factor that will reduce the snowpack's cohesion (increase in temperature, etc.) and/or increase its load (abundant snow, increase in its humidity, skiers go by, etc.) increases the risks of there being an avalanche. |
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The different kinds of avalanches |
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| There are three types of avalanches: |
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 Loose powder avalanches
These are the most spectacular and also the fastest. They are caused by an important accumulation of light snow with little internal cohesion. |
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Slab avalanches
They occur when a strong, stiff layer of snow known as a slab fractures and slides on to another slab. These avalanches travel more slowly but result in a snow displacement that is quite dense.
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Wet wind or slush avalanches
These occur when a temperature increase leads to a change in the snowpack's weight and its cohesion characteristics. |
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Risk factors |
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| There are many risks factors that can trigger an avalanche and all should be taken into account. |
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Temperature and slope exposure
The temperature and the exposure to the sun can transform the snow by increasing its load and modifying its cohesion characteristics. |
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Slope shape
Convex slopes (those shaped like a bump) are more dangerous than concave slopes (those that form a dimple).
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Types of terrain
Some terrains give the snowpack better adherence. A rocky terrain will provide more support. On the contrary, a grassy slope will provide a more limited adherence.
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Slope steepness
The steepest slopes are of course the most dangerous ones. |
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Wind
The wind, as it allows the snow to accumulate on certain types of slopes, is an important avalanche risk factor. If the previous days were windy, avoid entirely any downwind slopes. |
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Avalanche danger ratings |
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1.LOW
Generally stable snowpack on most slopes. Natural avalanches are very unlikely, they can occur but only when there is a heavy load (3) on some very rare steep slopes (2). Only snow slides or small avalanches may occur spontaneously.
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2. MODERATE
Moderately unstable snowpack on some steep slopes (1). Elsewhere, the snowpack is stable. Possibility of avalanches being triggered, especially if there is a heavy load, on a few slopes. Natural big-scale avalanches unlikely. |
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3.CONSIDERABLE
Moderately to very unstable snowpack on many steep slopes (2). Triggering of avalanches possible, even if there is a light load (3), on many of the slopes. In some cases, possible triggering of natural medium-size to big-size avalanches. |
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4.HIGH
Very unstable snowpack on most slopes (2). Probable triggering of avalanches, even if there is a light load (3), on most sufficiently steep slopes. In some cases, medium-size to big-size avalanches are expected. |
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5.EXTREME
Unstable snowpacks overall. Many widespread natural or human triggered big-sized avalanches expected, including in not so steep terrain. |
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(1) Slopes particularly avalanche prone, due to their downslope, the terrain's make up, their proximity to crests ...
(2) Characteristics of the slopes are usually detailed in the risk reports : altitude, exposure, land topography ...
(3) Indication of load: heavy load (for example a group of skiers) or light load (for example an individual skier, or a pedestrian).
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Safety rules |
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| The odds of a skier surviving when caught in an avalanche are very slight. Here a few guidelines that will hopefully help you avoid such a situation. |
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Check the latest weather reports
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Ski with a guide
The guide will know which slopes are avalanche prone and the other mountain traps that could exist (crevasses, etc). |
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Always ski in a group
When in a risky area on a slope, the group should ski down one by one so that the others stay safe. |
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Avoid crossings
Favour the ascents and descents in the fall line of the slopes. |
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Gear up
An ARVA and a folding snow shovel per person are the minimum safety equipment you should have with you.Take your cell phone (make sure the battery's charged), it can come in very handy to call for help. Test the ARVAs before you leave. If you can, also take an ABS bag with you. |
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