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A very useful video looking at the causes and effects of avalanches

What are avalanches?

Avalanches are masses of snow, ice and rocks that move downhill at speeds of up to 300kph. They occur naturally in mountain environments and only pose a hazard when they impact on people or human activity, such as transport routes or houses.

There are two main types of avalanches:

Loose snow avalanche - this type of avalanche usually starts from a single point on the hillside and involves loose, powerdery snow

Slab avalanche - this tends to be a more deadly type of avalanche. It involves a large slab of ice and snow shearing away from a hillside and moving rapidly downhill, carrying rocks and trees as it does so

Causes of avalanches:
  • Heavy snowfall - this adds weight to earlier snowfalls. Uneven freezing, together with occasional melting, can create disticnt layers within the snow and ice making slab avalanches more likely to occur as one layer slips over another
  • Steep slopes - avalanches are more likely to occur on steep slopes in excess of 30 degrees
  • Tree removal - the removal of trees for ski developments enables avalanches to move downhill unimpeded. When present on a hillside, trees can break up an avalanche and prevent it becoming too large
  • Temperature rise - sudden rises in temperatures and associated melting often lead to avalanches in the spring
  • Heavy rainfall - this can lubricate a slope and trigger an avalanche
  • Human factors - almost all deaths from avalanches kill the people who actually triggered them. Off-piste skiing is a major cause of avalanches because it often involves skiing in areas of fresh snow which have not been assessed for the avalanche rise
Avalanches as hazards
  • Death
  • Injuries
  • Property damage
There has been an increased number of avalanches due to the growth of winter sports and the expansion of ski resorts to cator for increasing numbers of visitors.

In Switzerland an average of 40 people die each year from avalanches, over 80% of whom are involved in winter sports. In France between 2007-2008, 15 poeple died, four were climbing and the rest were skiing or snowboarding.

Avalanches will increase and the death toll will rise as a result of climate change

http://www.coloradodaily.com/news/2009/jul/11/boulder-climbers-memorial-copp-johnson-dash/ - an article detailing the death of rock climbers due to an avalanche
http://news.bbc.co.uk/1/hi/world/europe/8031312.stm - six killed in Austrian avalanche
http://www.bbc.co.uk/weather/world/news/02012009news.shtml - a deadly start to the avalanche season