Last weekend we were in the Chablais. There was quite a crust on the surface early in the days with power snow underneath. What affect will this have on the snowpack stability? If we break the crust could the whole slope go?

Crusts are really only nasty when they are buried. In this case you can get a temperature gradient across the crust - this is point where the temperature in the snow changes rapidly over a short distance. This can lead to the formation of faceted and then depth hoar crystals which will present a weak layer. These can form either below or on top of the crust so that when the snow above is overloaded, say by the passage of a skier, the whole slope down to the crust breaks away.

Just as nasty the crust, or simply a layer of icy snow, can itself can form a sliding surface when it has fresh snow sitting above.

What would worry me is what is below the crust. The crust itself may actually bridge over a weak layer. Of course a thick wind crust is effectively a large slab of snow (windslab) which is sufficiently loaded, normally by a group, can break and slide on the surface below. But I assume what you saw was a thinnish crust formed overnight by the freeze/thaw cycle.

Given the length of time since the last snowfall at the weekend the powder may have been facetted snow - basically snow crystals that have turned into powder by the action of the cold. Nice to ski when the crust melts.

davidof - 05 March 2008 03:18 PM

The crust itself may actually bridge over a weak layer. Of course a thick wind crust is effectively a large slab of snow (windslab) which is sufficiently loaded, normally by a group, can break and slide on the surface below. But I assume what you saw was a thinnish crust formed overnight by the freeze/thaw cycle.

Very good point that a thick wind crust is effectively a thin wind slab. Or maybe not so thin. Once the crust is not thin enough to be breakable, you really don’t know how thick it is.

I almost got into trouble a couple of months ago when there was a transition from a thin breakable wind crust over most of the slope I was climbing to a hard surface near a terrain feature. I was trying to pound my ski edge into the surface so I wouldn’t slip, and all of a sudden the snow broke loose and took me 10m down the hill. Fortunately it wasn’t over a cliff. I was real surprised when I got up looked around and saw that I was surrounded by some blocks of hard snow over one foot high. Turned out to be a wind slab 3m x 4m in area—and over 30cm thick.

My analysis:
Wind crust is a sign of lots of wind. Where there is lots of wind, there can be wind slabs. Then it becomes a question of how well bonded the slab is to what’s underneath it. My lesson: If the breakable wind crust unexpectedly becomes non-breakable, and I don’t know how thick and heavy it is, and don’t have good reason to think it’s well-bonded to the underlying snowpack—then I should carefully gently step backwards down my track - (and surely not start aggressively pounding on the snow).

Ken