Sunday 18 March 2018

Avalanche feared to have killed four skiers in Swiss Alps.

The bodies of two skiers have been recovered from under about six meters of snow, and a further two are missing, following an avalanche in the Vallon d'Arbi area near Riddes in the District of Martigny of Canton Valais in Switzerland on Friday 16 March 2018. All four skiers are said to be aged between 20 and 25 and from the Alsace Region of France.

The scene of an avalanche in Vallon d'Arbi on 16 March 2018. Valais Canton Police.

Avalanches are caused by the mechanical failure of snowpacks; essentially when the weight of the snow above a certain point exceeds the carrying capacity of the snow at that point to support its weight. This can happen for two reasons, because more snow falls upslope, causing the weight to rise, or because snow begins to melt downslope, causing the carrying capacity to fall. Avalanches may also be triggered by other events, such as Earthquakes or rockfalls. Contrary to what is often seen in films and on television, avalanches are not usually triggered by loud noises. Because snow forms layers, with each layer typically occurring due to a different snowfall, and having different physical properties, multiple avalanches can occur at the same spot, with the failure of a weaker layer losing to the loss of the snow above it, but other layers below left in place - to potentially fail later.

 Diagrammatic representation of an avalanche, showing how layering of snow contributes to these events. Expedition Earth.

The Alps have seen a number of avalanche related incidents this winter, largely due to high levels of snowfall. This is, in turn caused by warmer conditions over the Atlantic, which leads to higher rates of evaporation over the ocean, and therefore higher rates of precipitation over Europe, which falls as snow in cooler regions such as the Alps, where the moist air meets cold air fromt the east. This situation is likely to get worst this week after a significant cold front from the east brought plunging temperatures across Europe this week, which is expected to be replaced by a new wet front from the west over the weekend, depositing precipitation as more snow across the now cooler continent. 

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