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Know the Snow: Fast changing problem

Posted: November 16, 2012 - 1:02am

Earlier this week I headed into the hills on a short ski tour. Conditions and snow depth changed quickly as I gained and lost elevation. The tour started and ended on thin and wet snow cover at the trailhead, but at higher elevations we found more than five feet of mostly dry snow. The rapid changes in snowpack over a relatively short change in elevation reminded me how complex and varied the avalanche problem can be in one short ski tour.

Over the last few years I have been brutally reminded several times that I cannot fall into complacency and should remain alert to drastic changes in the avalanche problem, sometimes over the course of a single ski run. In several cases, I started a ski descent with dry slab avalanches on my mind. My thoughts quickly changed to wet loose avalanches mid-slope, and then to wet slab avalanches as I approached lower elevations.

When we ski in the backcountry, we look for different clues and signs for different avalanche problems. We manage each avalanche type and the hazard associated with it differently.

Loose snow avalanches (wet or dry) need a steep slope to run on. They usually don’t pose a direct hazard; they are often too small to bury a person. The main hazard from loose snow and sluff avalanches is getting swept over terrain fixtures like rocks, cliffs, long steep slopes or into a crevasse. We manage these avalanches using a method which involves triggering the avalanche from the top of a steep section of the slope and letting it “run”.

Dry slab avalanches are a different story altogether. These avalanches are blamed for the vast majority of avalanche accidents. They can be very powerful, destructive and sometimes they can start from above the person traveling on the slope.

In contrast to a popular belief, avalanches don’t strike without a warning.

Luckily, most dry slab avalanche cycles are confined to time and place and clear signs of increasing danger are easy to observe. Signs like recent avalanches, additional loading (from new snow or winds), rapid warm-up, collapsing of the snowpack under skies, shooting cracks in front of skis or poor snowpit results are all indicators that it’s important to reevaluate goals and scale down the terrains to travel on. Good travel rituals can also prevent avalanche accidents. In about nine out of 10 dry slab avalanches accidents, the avalanche is triggered by the victim or someone in the victim’s party. Skiing one at a time, choosing good escape routes and good communication will come handy in a pinch.

Wet slab avalanches are rarely triggered by people traveling on the slope. They tend to involve the weakening of the snow cover due to free liquid water flow between snow layers. These avalanches usually “travel” from warmer sunnier slopes to colder shadier slopes over time. Carefully observing what slopes are susceptible to wet slab avalanches at a given time or on a given day can make the different between telling a good story in the bar, or finding oneself in the hospital.

During our tour earlier in the week, we saw about 10 inches of dry, but heavy new snow that fell in the last few days. This layer was getting heavier and wetter as we descended. Under this layer we found a crust layer that formed after the rain from the first weekend of November. As we descended, we noticed it became increasingly easier for the top layer to slide over the crust and down the slope. The main avalanche hazard was getting pulled by the heavy, slow sliding snow and swept over cliffs. We managed this hazard by ski cutting and pushing the loose snow and letting it slide and clearing the hazard further down the slope ahead of us.

• Ron Simenhois is an avalanche forecaster who lives in North Douglas. You can contact him at ron.simenhois@gmail.com.

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