On Snow Temperatures and Skier Triggered Avalanches

With the ever growing popularity of backcountry skiing and snowmobiling, and more recently, the explosion of people accessing the backcountry on snowshoes, understanding avalanches has become an important skill. For most of us heading off into avalanche terrain, our risk evaluation consists of a glance at the Avalanche Bulletin before heading off, some visual assessment of slopes, and maybe a few stability tests in the snowpack. These are all great things to do, but it is often easy to forget to monitor changing temperatures throughout the day, and how it is affecting the snowpack, and how we interact with it. At the end of the day, this is something that can be critical in determining snow stability and evaluating your risk in the backcountry.

McClung and Schweizer (1996) describe the stability of a snowpack as a function of metamorphism and mechanical strength. These two factors work at different timescales, and have opposite impacts on the stability of a dry snowpack. Consider a day that starts off at a chilly -15oC, but by lunchtime some warm air and sunlight has heated the area to -2oC. At colder temperatures, the snowpack is stronger, but as air temperatures rise, the snow surface heats up and loses a lot of its strength. Warm air softens the upper layer of the snowpack without being able to penetrate deep enough to destroy a buried weak layer (snow is a very good insulator!), significantly reducing the stability in the snowpack.

Let’s assume that this new (warm) air mass sticks around for a couple days and temperatures stay relatively close to the freezing level. Now that there has been time for some heat to penetrate deeper into the snowpack, there will be higher levels of metamorphism, which will consolidate the snow, allow for enhanced crystal bonding and sintering, and destroy  pesky weak layers close to the surface. All of this will end up making the snowpack more stable.

Immediate Effects (hours)

Delayed Effects (days)

  • Reduced stiffness – less resistance to deformation
  • Weaker snowpack – easier to fail material
  • Substantial heating at the top of the snowpack
  • Metamorphism  is enhanced – bond formation between layers
  • Enhanced creep – snow settling and densification
  • Smaller temperature gradient – would expect to see rounder, more stable snow grains
Decrease in snow stability; increased likelihood of avalanching. Greater snow stability; heat flow destroys weak layer.

According to the Avalanche Handbook (a highly recommended purchase!), roughly 80% of dry avalanches are triggered by humans, meaning we need to consider how we interact with the snowpack in order to fully address snowpack stability. As you ski/drive/hike on the snow, you are deforming the snow below you, creating a ‘stress bulb’. In order to trigger an avalanche, your stress bulb has to penetrate deep enough to strike a buried weak layer.  The size of your personal stress bulb is going to depend on your downward force (How much do you weigh? Are you landing a jump or skinning uphill? Are you on fat powder skis or a snowmobile?), but it’s also going to depend on the snow hardness, which we know from above is a product of temperature.

bulb

Taken from Forecaster’s Blog Jan 30,2013 (http://blogs.avalanche.ca/forecaster)

When you’re skiing your first runs of the day at -15oC, your stress bulb is going to be shallower than it will at 2pm and -2oC, meaning you’re more likely to hit that weak layer when the crust has warmed up (your weak layer can be, and most likely is, still cold). However, if it’s been warm for a couple days, its more likely that weak layers near the surface have been destroyed, leaving persistent weak layers deeper down in the snowpack.

Unfortunately, although weak layers are often relatively uniform across a large area, hardness is a lot more variable, depending on where the sun hits, what’s exposed to winds, and even the time of day. This means that a lot of the tests we do in the field give us an incomplete picture of the snowpack stability. Here’s a picture of Rutschblock tests along a slope (7 is stable and 1 is very unstable), and a maddeningly variable terrain that you’re sure to experience “out there”.

rutschblock

From Schweizer et al. (2006)

I’d be remiss if while talking about avalanches, I didn’t mention something about psychology, something that always seems to make its way into outdoor risk management. It’s really easy to ‘go through the motions’ and do your due diligence evaluating the snowpack in the morning, choose your slope, and ski for the rest of the day. Consider a scenario where you dig a pit at 9am, evaluate the slope, do a few runs, and rightfully so, have pretty high confidence with the slope stability at 10:30am. With each run you start moving into more and more aggressive terrain, however, all the time the snow is softening and you are getting closer and closer to hitting the buried weak layer. What makes this research particularly important is that it emphasizes how quickly things can change, and also how much variability can exist when trying to evaluate the snowpack. Anyone venturing in the backcountry has to understand that no matter the precautions and evaluations, there will always be some residual risk.

To review; initial warming of the snowpack is going to reduce its strength, reducing its stability. At the same time, your stress bulb is also going to be greater, meaning you can hit weaknesses lower in the snowpack. However, if it stays warm for a longer period of time, the snowpack densifies, metamorphoses, destroys weak layers near the surface, and generally becomes more stable. Much more eloquently put, in the words of avalanche research pioneer Ed LaChapelle:

Any rapid change in the mechanical or thermal energy state of the snowpack is a precursor to avalanching.  And I emphasize rapid.

OLYMPUS DIGITAL CAMERA

This was the third consecutive day of warm weather. We were skiing in mashed potatoes, but the stability was good!

Further Reading:

McClung, D. M., and Jürg Schweizer. “Effect of snow temperatures on skier triggering of dry slab avalanches.” Proceedings of the 1996 International Snow Science Workshop in Banff, Alberta. Canadian Avalanche Association, Revelstoke, BC. 1996. (link)

The Avalanche Handbook – David McClung and Peter Schaerer

Forecaster’s Blog – Canadian Avalanche Centre http://blogs.avalanche.ca/forecaster, great, well explained concepts – and entertaining too!

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