Avalanches > Weak Layers > Facetted Snow

Facets

If the snowpack is subject to a strong temperature gradient (anything above 10C per meter) water vapour gets driven through the snow and forms square looking crystals by a process of sublimation. That is, ice on the surface of the strong well bonded round crystal turns to vapour then recrystalizes. These crystals (faces planes in French) have the consistency of caster sugar. You will see the term sucre used in the French bulletins. Eventually this process can transform an entire snowpack, including a crust, into these crystals which are similar to fresh powder to ski. Remove the temperature gradient and these crystals will turn back into well bonded small snow grains but this takes time.

Depth Hoar

As the process continues the square crystals form layers in a pyramid structure. These are called gobelet in French and are like wine glasses. Quite strong in compression but extremely weak in shear, exactly the forces that a slab with some additional load such as a skier imposes. These crystals are extremely persistent. Only rain or spring melt will destroy them and they can be the cause of spring avalanches as the snowpack loses strength.

Surface Hoar

A temperature gradient at the surface of the snow will also form hoar crystals (frost) called givre de surface in French. These look like ferns and can be a weak layer when buried. Surface hoar needs a clear sky (to let the surface of the snow radiate way heat) and is accelarated by water vapour. So it often grows near streasm (you will find it a black ice on alpine roads) or at the top of inversion layers (cloud seas) in alpine valleys.

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