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An avalanche is a phenomenon that causes a number of deaths yearly. Avalanche is a phenomenon every skier should be aware of, especially backcountry skiers. Avalanches are common phenomena in the mountains, even though most of the on-piste skiers never see them. Even though you might now see an avalanche, it is very important to understand what it is, how it behaves and how YOU should behave in case of an avalanche.

This article series provides you information on avalanches as a phenomenon, behavior of an avalanche as well as instructions to avoid avalanches, and what to do if you or your fellow skier gets buried by an avalanche. This first article describes an avalanche as a phenomenon of the nature.

As stated in Wikipedia “An avalanche is a flow of snow or rocks down a mountainside. Avalanches are among the biggest dangers in the mountains for both life and property“. There are several factors that together trigger an avalanche. Most of the avalanches are triggered by the weather conditions, but sometimes dangerous avalanches are triggered by humans.

How avalanches get triggered?

Very big avalanches have incredible power. They can easily destroy forests and structures and bury whole villages. Avalanches that destroy villages and kill people are a real threat in the mountains where people live, such as the Alps and Rocky Mountains. Sometimes these accidents are very dramatic, such as in 1999 in Montroc in France (near Chamonix) where an avalanche killed 12 people.

Dangerous avalanches typically are born in slopes that are 25-45 degrees steep, but an avalanche can be triggered on a 15 degrees steep slope if the conditions are optimum for an avalanche to form. In slopes steeper than 45 degrees the danger is not that big because the snow usually falls before there is snow enough for an avalanche to form.

In case of avalanche disasters, a human being is usually the one who triggered the avalanche. A human being living in a risky area triggers an avalanche which falls down to a village. This is a usual case in ski resorts where buildings are built in the bottom of a valley. When people go skiing the backcountries, every now and then someone triggers an avalanche. Sometimes the avalanches are huge in size and have power to fall down to valley burying houses, roads, railroads and people.

Different forms of avalanches

The most form of an avalanche is slab avalanche that can be wider than one kilometer and several meters thick. In optimal conditions a slab avalanche can gain speed over 320 km/h. A slush avalanche moves slower but has more destruction power due to bigger mass.

An avalanche is triggered when forces keeping snow from falling become weaker than the gravitational force of the snow mass. The forces pulling the snow masses down are the gravitational force of the snow and the mass of a skier or animal or such. The forces keeping snow from falling are friction between snow masses and the underlay, stress of snow crystals (cohesion) and “anchors” such as rocks, trees and man-made avalanche fences.

There are several types of avalanches, from which slab and slush avalanches were already mentioned. Other types of avalanches are powder snow avalanche and ice avalanche. As also mentioned, the most dangerous avalanche type is slab avalanche. In order to form a slab avalanche, the snow must be layered due to frequent snowing. A high risk slab avalanche is formed when the snow is layered due to wind. When a slab avalanche is triggered, a big slab of snow starts falling, usually in a large area. In the place where the slab was detached, one can usually recognize a typical “crown” formation on snow.

In the next article we will discuss how to find out the risk of an avalanche. Below is an example of a big powder snow avalanche as a series of still pictures.