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Avalanche size categories renamed

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It is good practice to use uniform size designations when describing avalanches – for example in the avalanche bulletin, in the wake of accidents, or when reporting observations from the field. In the interests of clarity for winter sport participants, these names have been modified, effective from the start of the 2018/19 winter season.

 

On several occasions this week the avalanche bulletin stated, “These [avalanches] can reach a large size.” It is a powerful message, but what exactly does it mean? People’s perception of avalanche size is often subjective and dependent on the circumstances. Notwithstanding this inconsistency in practice, the European avalanche services officially defined avalanches sizes a long time ago, but the names they adopted were little known, partly because they did not reflect the perception of most men and women who engage in winter sports. Many fatal avalanches, for instance, were officially designated “small”. Modified designations have therefore been adopted as of the start of the 2018/19 winter, and typical skier-triggered avalanches are now described as “medium”:

 
Image 1 of 5
Size 1: Sluff or small avalanche (formerly sluff); length: approx. 10 to 30 m; volume: approx. 100 m³ (Photo: SLF)
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Size 2: Medium avalanche (formerly small); length: approx. 50 to 200 m; volume: approx. 1000 m³ (Photo: Rega)
Image 3 of 5
Size 3: Large avalanche (formerly medium); length: several hundred metres; volume: approx. 10,000 m³ (Photo: SLF)
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Size 4: Very large avalanche (formerly large); length: approx. 1 to 2 km; volume: approx. 100,000 m³ (Photo: J.-L. Lugon)
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Size 5: Extremely large avalanche (formerly very large); length: approx. 3 km; volume: greater than 100,000 m³ (Photo: SLF)
 

Why are definitions necessary?

The size designations are used regularly in the avalanche bulletin. This is because, alongside the probability of a release, the size of the expected avalanches has a crucial influence on the description of the danger situation. The avalanche bulletin can be interpreted properly only if these designations are understood correctly. In turn, the avalanche warning service can correctly assess reports from the field only if the received information about the size of avalanches is reliable. It is hoped that the modified designations for the five avalanche size categories will be understood more readily and used correctly to a greater extent than in the past.

An exact description of the avalanche size categories is available here: https://www.slf.ch/en/avalanche-bulletin-and-snow-situation/about-the-avalanche-bulletin/avalanche-sizes.html

 

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