Avalanches are powerful and unpredictable disasters, a danger to all winter recreationists in mountainous terrain.

But these catastrophic events rarely happen by accident - most fatal avalanches are triggered by people. And their presence is usually by choice, not chance, as they seek out the best snow for their winter sports.

This winter has seen more avalanches simply because of the huge snow falls which the steep slopes cannot support. However, the fluctuating warm and cold spells have made the problem much worse, according to the UK Meteorological Office.

Warm Atlantic air and cold Arctic air have alternately invaded the Alps over the last few weeks. This causes the top layers of snow to thaw and freeze. They become more compact and heavy than the powdery snow insulated beneath.

This arrangement of heavy layers on lighter layers is unstable and make avalanches easier to trigger.

As the Eskimos say, there are a hundred types of snow. The type of ice crystals falling affects how the snow packs together and how well the layers stick to one another.

If the snow does not pack down firmly, or a hard snow layer overlies a softer one, the area is avalanche-prone. Every year, over one million avalanches happen.

Slab avalanches are the most dangerous kind, where a whole layer slips off a mountainside as one thundering mass. They can be triggered by the wind or a loud noise and pull in surrounding rocks and stones.

Sometimes the start of the avalanche is marked by the sound of snow cracking, something like gunshot.

Avalanches of looser, powdery snow start at a single point and eventually expand out to form an upside-down V-shape.

Some ski patrols use large guns to shoot explosives at the mountains when fresh snow has fallen.

This causes small avalanches when no one else is around, and makes the risk of a big avalanche less likely.

People also control avalanches by planting trees or building large snow fences in likely starting zones.

Avalanches mainly kill people skiing outside official boundaries, where the snow build-up has not been controlled.