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Know the Snow: Spring time

Posted: April 12, 2013 - 12:01am

Back when I was growing up in Israel, the arrival of spring was marked by blooming flowers, warm temperatures and long sunny days.

This is not the case for me anymore.

As I write this column, I look through the window and see large amounts of rain falling from the skies and the temperatures are falling.

Other people may recognize changes looming in the trees, the birds’ behavioral patterns or the different footprints in the snow.

For me, the arrival of spring is marked by a sharp transition in avalanche types. Not that avalanche types are confined to specific times of the year, but when I start to see the scars of more and more glide avalanches’ on the slopes, I know that spring is here.

If in the winter dry slab avalanches are the main concern, in the spring, the majority of avalanches tend to be wet slab, glide or cornice fall avalanches.

The differences between dry slab and warming-induced avalanches are clearly highlighted by the statistics of avalanche accidents. In almost all (90 percent) dry slab avalanche accidents, the avalanche is triggered by the victim or by someone from their party. On the other hand, in almost half of wet slab avalanche accidents, the avalanches were natural avalanches.

In his book, “Staying Alive in Avalanche Terrain,” Bruce Tremper adequately describes the difference in the mechanisms that lead to dry and wet slab avalanches. Tremper wrote that dry slab avalanches result from excessive loading on a weak snowpack, to the point where it fails. Wet slab avalanches result from weakening of the snowpack, to the point where it can no longer carry its own weight. Tremper’s differentiation implies that warm temperature-driven avalanches are both harder to forecast and harder to artificially trigger.

Glide and cornice fall avalanches are the ones I struggle with the most. Both avalanches do not necessarily occur as a direct result of weather events. They may occur with or without additional loading; they can occur during both warming and cooling trends, and on sunny and overcast days. They can happen when the snowpack is cold and dry, or warm and wet. They also develop cracks above the gliding zones, but these cracks can appear weeks or seconds before the avalanche releases.

The last couple of years I had time-lapse cameras taking images of glide avalanche-prone slopes. I hoped that reviewing the changes in the snow cover at, and around, gliding zones would help me identify a few clues and signs that precede a glide avalanche release. By looking at images of avalanches again and again, and by reading a new paper by Perry Bartelt and other avalanche scientists from Switzerland, my waiting eventually paid off. Other than reconfirming the notion that I am a geek, I also noticed that shortly before a glide avalanche release, new cracks in the snow cover start to appear, this time at the bottom of the gliding zone. Clearly, nonappearance of these cracks is not a guarantee of slope stability because these cracks may appear minutes or seconds before the avalanche releases.

One thing is certain: I sure as hell would not come close to a slope that had glide cracks both at the top and at the bottom of the glide zone.

The sharp differences between the aforementioned types of avalanches indicates we need to change the way we do business when seasons change.

The question is: how?

Many folks I know (including me) tend to widen their margin of error as uncertainty increases. Also, I don’t trust slopes if there are any clues they may be unstable, even if they got heavily skied, snowboarded or snowmobiled by others. I rarely trust other people’s tracks, and I trust them even less when I deal with warm weather-type avalanches.

Now that I see scars of glide avalanches appearing on the slopes as I drive up to Eaglecrest Ski Area, I know that my wife is right. It is time to start planning our summer vacation.

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

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