Sand

Avalanches > Weak Layers > Sand

Sand layers can sometimes be found on the surface or buried in the snowpack after storms that have passed over desert regions. Soot particles from large forest fires can also form a dark layer in the snow. Sand is quite common in Europe following storms that have been driven up from the south by a Foehn or Chinook wind. These storms sometimes pick up sand over the deserts of north Africa and Spain and this sand is deposited in the mountains.

The sand isn't a problem in itself but when light, both visible and invisible, reaches the layer, perhaps up to 10cm under the surface, it is absorbed because it is dark. This warms the snow and a temperature gradient is set-up. The heat in the layer drives water vapour into the colder snow both below and above the layer. This will lead to facetting of existing snow crystals surrounding the layer.

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