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

Wednesday, December 15, 2010

Q & A

I have been hyperbusy with personal stuff, so thanks for your patience. Today, I'm going to answer a few questions that have been sent my way over the past few weeks.

( Promise a full post tomorrow or the next day-I've managed to read through most of the ISSW 2010 papers, and I've got a few additional remarks that I'd like to make about some recent conversations in which I have participated. )

Do avalanches originate in tree-covered areas?

Good question. Trees definitely alter the snowpack by anchoring snow, intercepting snowfall, intercepting radiation, and by depositing "bombs" and moisture that break up the snowpack.
  1. According to The Avalanche Handbook, avalanches in tree covered areas are infrequent, but they do happen. If you travel in mountainous terrain during the winter, you can rely on very thick tree cover for safety only when there is no avalanche terrain anywhere above the trees.
  2. Avalanches frequently initiate above tree-covered areas and flowing snow travels easily through the trees, even if the trees are thick. Have you ever noticed snow plastered on the uphill side of a tree? This is a good indicator of avalanche activity, and can also be used to determine the height of flowing snow. Remember that forest clearings may be especially dangerous because of instantaneous changes in snow conditions. This means that you can go from safe to unsafe in just a few steps.
  3. Gladed areas are, for all intents and purposes, the same as open slopes. The consequences of taking a ride through the trees are severe. How severe? Think about what might happen if you have a high-velocity encounter with a tree. You can express such an encounter with the following formula: NOTFUN.
  4. You may not be safer in forested areas, and if conditions are very poor, you might actually be less safe. As always, you must use the current terrain, weather, and snowpack to determine the likelihood of avalanche formation, and you must choose terrain appropriate for conditions.
  5. According to researchers, people in North America are much more likely to suffer traumatic injuries during avalanches than their counterparts in Europe. Trees are definitely a contributory factor.
  6. Don't forget about tree wells.
Other facts about trees and the snowpack.
  1. Thick trees anchor the snowpack, which can prevent avalanche formation. On the other hand, trees often serve as trigger points or fracture points. It's a good idea to keep this in mind.
  2. Trees intercept incoming shortwave radiation, which explains why you can find decent snow in the trees several days after a storm.
  3. Trees intercept outgoing longwave radiation and reflect it back into the snow cover, which prevents heat loss. This is why surface hoar doesn't form readily beneath trees, and it's also why you're less likely to find near surface facets underneath the trees unless the temperatures are really really cold.
  4. Trees drop bombs and liquid water onto the snowpack. Large snow bombs break up slabs, and liquid water turns the snowpack into solid concrete when it freezes.
How are buried facets formed?

This is a bit more complicated.
  1. Facets can form inside the snowpack when the air is much colder than the ground on which the snow sits. The strong temperature difference means that water vapour moves quickly toward the snow surface, and a constant replenishment of water vapour means that crystals can grow rapidly. Rapid growth produces angular crystals.
  2. Facets can form inside the snowpack itself when a crust creates a vapour barrier that traps moisture. Over time, with vapour supply and the right temperature gradient, fast crystal growth occurs. Once again, rapid growth produces angular crystals.
  3. Facets can also form at the snow surface. This happens with very cold ambient air temperatures, or when the snow surface becomes intensely cool from longwave radiation loss.
  4. Radiation recrystallisation can also produce facets. This happens when sunlight strikes very cold snow, and usually happens on south faces at at very high elevations.
  5. Finally, facets can form when cold, dry snow falls on wet snow. The wet snow contains heat that sets up a strong temperature difference when it comes into contact with cold, dry snow. Again, as with above, the strong temperature gradient moves moisture rapidly, and constant replenishment of water vapour means that crystals can grow rapidly.

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