Technical information, news, research, and opinion on avalanches, snow safety, and winter backcountry travel.

Friday, January 14, 2011

Q & A

Answers to questions sent to me via email.

Why is buried surface hoar so dangerous?
Buried surface hoar is dangerous because surface hoar crystals form a thin layer that contains a lot of air. This means that the layer on top of the surface hoar is mostly supported by air. When crushed, surface hoar crystals have the capacity to rearrange themselves into a much smaller space, which causes the layer above to fall. As the layer falls, it provides energy that causes the crushing to spread. After the crushing process is complete, you're left with two layers of snow that have no attachment to each other-delamination has occurred. Finally, gravity, which is always in effect, pulls the detached layer downhill.

All the persistent forms ( facets, depth hoar, surface hoar, and combinations of these with crusts ) are dangerous for this reason. Crusts, when found alone, are dangerous for slightly different reasons, mostly related to poor bonding at the interface between the crust and the layer above-poor bonding increases the risk of catastrophic delamination, and catastrophic delamination is required for avalanche formation. The answer provided here intentionally does not discuss weak layer parameters such as anisotropy or variations in grain morphology, which are more related to why the layers are persistent than to why they are dangerous.

What is the significance of snow crystal size?
There are two key factors related to crystal size: the first factor is that large crystals have a lower number of bonds per unit volume. This is part of the reason why the persistent forms, such as facets and surface hoar, are so weak. Networks of large crystals can be quite strong, but they are almost always relatively weaker than networks of smaller crystals.

The second factor is grain size mismatch between layers. Grain size mismatches result in weaker bonding between layers, because the smaller grains overlay the pore space between the larger grains. This means a lot of the smaller grains are simply "touching the air". Furthermore, such configurations tend to concentrate stress at the interface between the layers.

What wind speed produces wind slab?
This depends almost entirely on the condition of surface snow. It takes little more than a stiff breeze to move dry, loosely packed snow. On the other hand, hurricane force winds will have little effect on dense, frozen corn snow.

What is the source of uncertainty in persistent weak layers?
With respect to persistent weak layers, uncertainty arises from a few key factors:
  • Where is the weak layer?
  • What is its depth below the surface?
  • How weak is the layer and surrounding interfaces?
Picture a mountain valley. You know that there is buried surface hoar in some places. You also know that its depth below the surface varies. You also know that its degree of weakness varies by location.

Now that we've discussed what you know, think about what you don't know.

No comments:

Post a Comment