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Know the Snow: The benefits and limitations of snowpits

Posted: January 6, 2012 - 1:01am

A few weeks back, I was standing on top of a slope, Ben Stewart ways, thinking to myself: should I dive into it right away and ski the slope or should I take the time to dig a quick snowpit or two?

I had plenty of reasons to believe this slope was stable. It had been calm weather for several days, and after a short avalanche cycle during the last snowfall, the snowpack was getting stable fast. Neither I nor people I talked to had noticed any avalanches in the past three or four days. Ski patrol had been testing the slope with explosives for several days without substantial results, and my training in assessing the snowpack led me to believe that there were no persistent weak layers that may have survived the last cycle. If that wasn’t enough, I knew skiing was going to be spectacular. My time was limited and skiing the slope right away would leave me time for another lap. But what if my assessment was wrong?

Before we dive into why, why not and when we should dig snowpits, we should look into how the most dangerous avalanche type — dry slab avalanche — comes about. A dry slab avalanche starts from a fracture along a weak snowpack layer. If the fracture undercuts a large enough area of the slope, the slab above the weak layer breaks free from its surroundings and avalanches.

How do these weak layer fractures happen? Snow fractures, like most material. Almost everyone who has a car in Juneau has had or knows someone who has a crack across his or her windshield. If you think about how this crack developed you most likely remember that it started small, usually from a rock. Over time, the crack expands more and more as you drive over bumps on the road. As the crack grows bigger, it needs smaller and smaller bumps to expand. Eventually, it gets large enough that with one small bump the crack is propagated across the entire windshield.

The same goes with snow. A crack in a weak snowpack layer starts to expand when it is overloaded. This additional load can come from a person, snow machine or new snow. If the crack keeps expanding far and wide, an avalanche will occur.

Unfortunately, figuring out if a slope is safe or not is not as simple as the impression I may have left with the reader. There’s no way to tell how many flows or preexisting cracks there are in the weak layers, where they may be on a slope and what size they are. The snow only “informs” us when it is unstable; we are not aware of clear signs of stable snowpack. We assume the snowpack is stable from lack of evidence that suggests the opposite. Even ski or snowmobile tracks on a slope are no guarantee for stability. The slope may be primed for avalanching, but the folks that played on it just managed to miss all the weak snowpack layer flaws.

Think about it this way: when kids try to hit a piñata, the fact that the first or second kid didn’t hit the unlucky parent that was standing in the wrong place doesn’t mean that the third or fourth would not change the course of the party. The same goes for playing on a slope, the fact that the first or second person didn’t trigger an avalanche doesn’t necessarily mean that the third or fourth won’t either.

A snowpit may not reveal how likely a person traveling on snow is to start a crack propagation. However, it can tell us other important information about the snowpack that is hiding under the surface. For example, a snowpit can tell us how likely a propagating crack is to expand over a large area of the slope. A good look at the snowpack layers can also give us a good idea of the distribution and size of weak layers’ flaws, the depth and size of a potential avalanche or if the added load of a person could penetrate deep enough into the snowpack to start crack propagation in weak layers, and more.

There are many good reasons to dig snowpits. As a professional, I dig snowpits to identify and track weak snowpack layers. I want to know where, why and when they form. Are they getting stronger or weaker? How long are they likely to remain a problem? How much additional load would it take to turn a weak layer into a reactive weak layer that causes avalanches? I dig pits to follow up and see if my conclusions from older pits were right. I dig pits to back up my decisions in the field with solid snowpack data. However, when I recreate in the backcountry, I dig snowpits when a search for signs of instabilities comes up empty, leading me toward the conclusion that the snowpack is stable. Then I check to make sure there are no weaknesses lurking under the surface. Only then, after I am happy with what I see above and under the snow surface, do I ski the slope.

*In my “Essential tools for venturing into avalanche country” I forgot to mention Sea Dog’s substantial contribution in getting the avalanche beacon training park to Eaglecrest.

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

 

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