As you travel in potential avalanche terrain, make it a habit to test the snow throughout the day. A simple poke at the snow surface with your ski pole now and then can give you useful information (for instance, if the upper layers are changing as you progress up a slope). You should also perform more involved tests of the snowpack as detailed below.
The first step in evaluating snowpack safety is to dig a snow pit. It is critically important that the pit be dug on a slope that is the same aspect and angle as the slope you intend to travel, while still being sure to not expose yourself to an avalanche if one were to trigger. This is tricky and should be practiced with someone with lots of experience.
This "sliding block" test (pronounced ROOTSH–block) shows how much force is needed to make the snow slide on a given slope. It is usually performed by a person on skis, but can be done with someone on snowshoes or a splitboard style of snowboard as well.
The Rutschblock test is fairly reliable in predicting fracture initiation (how much force is required to start an avalanche). The Extended Column Test has become more popular because it not only predicts fracture initiation, it includes fracture propagation (how big the avalanche might be). The ECT is also easier to perform since the size of the isolated block is smaller.
Whether you perform the Rutschblock or the ECT, you should repeat the test at a minimum of a couple more locations and compare the results.
Deciding what to do with the results is not easy. Obviously, the earlier the block/column fails, the less stable the slope. There is no magic number at which you will be safe! You must use ALL your observations along with multiple stability tests to reach the conclusion that is right for you and your group on whether to ski/ride that slope.
There is another observation to consider as well: When your block/column fails (or shears), note the quality of that shear. This is recorded simply as:
Q1: clean and fast shear, high energy release.
Q2: clean or average shear, sluggish release or little movement at all.
Q3: uneven, stepped or broken shear.
A Q1 shear is of more concern to the backcountry traveler than a Q3 shear.
Next up: Avalanches, Part 3: Reducing Risk.
Go back to Avalanches, Part 1: The Basics.
By Geoff Irons
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Last updated: Thu Aug 16 14:53:08 PDT 2012
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