by Thomas Cleary
Over the last 20 years, there has been an average of 6 avalanche deaths in Colorado and 22 nationally per year. Of those national deaths, forty percent were skiers and snowboarders in the backcountry or just outside of ski area boundaries (http://geosurvey.state.co.us/avalanche). This winter a patroller at Wolf Creek was caught and killed. In the ‘90s, the avalanche forecaster for the La Sal mountains was killed in an avalanche in his home range, leaving behind a wife and 2 small children. I was the first to find his gear melting out of the debris that next spring. One summer I backpacked through an area of old growth timber piled like Pick Up Sticks® from an avalanche that had roared down one flank of a valley, across half a mile of flat bottomland and another half mile UP the other flank of the valley. And yet I continue to venture into the winter backcountry, because of the exceptional opportunities for solitude, beauty, and deep powder.
The Colorado Avalanche Information Center is the local expert on avalanche, and the longest established center in the country. The CAIC website (listed above) is my number one source for regional mountain weather forecasting and the preeminent source for snowpack stability forecasting.
This article summarizes the three natural components of the Avalanche Triad: Terrain, Weather, and Snowpack, as described in Avalanche Awareness, by John Moynier, and the human components such as recognizing hazards, collecting information, routefinding, and selecting safety gear. It will focus on avoidance rather than rescue. Avalanches travel at speeds over 100 mph and are impossible to outrun. Victims buried without a beacon (avalanche transceiver) have a mere 30% survival rate, and ANY victims buried for 35 minutes have only a 30% survival rate.
The avalanche triad
Ninety percent of avalanches happen on (or below) slopes between 30 and 45 degrees and 98% between 25 and 50 degrees. A slope meter (inclinometer) is your first and best tool to avoid avalanches. The second best tool is a compass. Slopes that face north and east tend to slide more often and bigger for most of the Colorado winter. In the warmth of spring, south and west facing slopes are more prone. The roughness of the terrain also affects the snow’s adherence to the slope but only if objects in the terrain protrude through all the snow layers and are close together.
Wind, and the temperatures, rates, and amounts of snowfall are the primary weather factors that contribute to snow stability. I was once buried alive in a snow cave as wind transported a mere inch of snow covering a windward basin to the leeward location of my shelter, burying me several feet deep. Snow similarly deposited on a 35-degree slope is a deathtrap waiting to snap. Snow tends to bond better in warmer conditions (near freezing), but when cold snow falls on a cold base it bonds poorly, and is prone to slide. And when snow falls quickly, it doesn’t have time to settle. When new snow accumulates deeply, its weight destabilizes the slope.
Colorado weather sets us up for unstable snowpacks, which is why Warren Miller and other extreme skiing/riding video producers rarely film here. A snowpack tends to stabilize when its internal temperatures are consistent throughout its depth. Deeper/thicker snowpacks can absorb larger temperature variances without destabilization. Ground temperatures tend to be near zero degrees Celsius, so when air temperatures hover near freezing, the snow pack becomes more stable. However, when we get our usual cold snaps during November and December, which is when our snowpack tends to be shallow, the snow crystals become large, poorly-bonded, sugary and unstable. When the bigger midwinter snows fall on top of this weak foundation, avalanches are produced. This explains why colder, north facing slopes are more prone to early season slides.
Routefinding in avalanche terrain necessitates recognizing potential hazard including looking for signs of current instability (cracking, settling, and recent slides; checking out professional avalanche forecasts from http://geosurvey.state.co.us/avalanche), and historical activity (avalanche paths, trees with broken off uphill branches, or debris piles). The most important travel consideration is to stay off of slopes steeper than 30 degrees (and don’t travel under them either). Travel on ridges or rock ribs that would shed avalanches off to either side. Watch out for terrain traps, areas where avalanche debris would accumulate (with you at the bottom), such as gullies or below funneling terrain. Treed areas will generally provide protection but a big slide can rage through old timber, and if the trees are open enough to ride they are open enough to slide.
To assess snowpack stability directly, a snow study pit can be dug in a safe area that has similar conditions to the potentially dangerous terrain you want to travel through or ride down, such as slope angle, aspect (compass direction), and roughness factors (rocks, trees, etc). This shovel shear test and the Rutsch Block test are more thoroughly described in Avalanche Awareness, by John Moynier. When travelling in suspect terrain your group needs to work as a team, including travelling one at a time between safe points to minimize exposure.
The gear that I have mentioned throughout these 2 articles includes (in order of importance): inclinometer/slope meter, compass, and shovel. Additional items, and the knowledge of how to use them, could include: snow saw, snow study kit, probe poles, and avalanche transceiver/beacon. These items are available at most backpacking/snow sport stores.
Please consider this article as the simplest of introductions. If you have even an inkling that you may be travelling near avalanche terrain, seek out a course and get proper training; start at the Colorado Avalanche Information Center listed above. Have fun, but be careful out there by knowing and avoiding the risks.