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Know the Snow: Snowpack observation

Posted: January 20, 2012 - 1:01am
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The sugary looking crystals we call Facets crystals, these crystals create notoriously weak and persistent layers. Courtesy of the American Avalanche Association.  Courtesy of the American Avalanche Association.
Courtesy of the American Avalanche Association.
The sugary looking crystals we call Facets crystals, these crystals create notoriously weak and persistent layers. Courtesy of the American Avalanche Association.

Last weekend an arctic air mass crept south from the Yukon Territory into Southeast Alaska. Temperatures have been frigid all week.

When cold air lays over warm snowpack it initiates a progressing cold that advances deep into the snow. As the cold wave advances, it changes the snowpack’s structure. A sharp temperature change over a short distance inside the snowpack tends to change the snow grains into a sugar-like crystals. These crystals create a potentially reactive weak layer that can lurk under the surface for weeks. To monitor the advancing cold wave within the snowpack and its effect, I have to go and dig snowpits.

My previous column was on the why and when to dig a snowpit. In this column, I’ll touch on the how.

But, before I dive into the details, I’ll start with a basic description of what a snowpit is. A snowpit is a trench exposing a flat, vertical snow face from the snow surface down. It allows observers to study the characteristics of different snowpack layers. To the trained eye, a snowpit wall is like an open book. It highlights the details of the season’s weather events. For example, a heavy early season snowfall usually leaves a moist layer at the bottom of the snowpack; a cold fall with small snow fall events tends to leave sugary, weak crystals at the bottom of the snowpack. Warm or rainy periods leave wet grain layers that, with time and cold weather, turn into crust layers. Windy periods strip the snowpack on exposed areas and deposit it as a hard and chalky layer in gullies and lee slopes. Calm, cold and clear periods leave their mark in feathery frost layers, and so on.

Although, the layers in the snowpack can illustrate evidence of big weather events throughout the season, for stability evaluation, stability tests are the focal point of the snowpit. To a limit, stability tests, can give us a reasonable idea of the strength of the snowpack as a whole; they highlight the weak and reactive layers and, if done in the right place, they give us an indication of the overall slope stability.

There are several different stability tests and different folks prefer different tests. I’ll touch on the more commonly used tests in our tool box: the Rutschblock Test, the Compression Test and the Extended Column Test. All three tests involve isolating a column of snow and loading it with progressively increasing loads from the top until it fails. The Rutschblock Test is the grandfather of most stability tests. It was developed by the Swiss army in the late 1960s. It involves isolating a column the size of about 80 inches in the cross slope direction by 60 inches in the slope direction. The tester then steps on the block with their ski or snowboard and progressively loads the block with weight - a knee bend, a small jump, a large jump and three big jumps. Although this test indicates the effect of a person on the slope, it takes a considerable amount or time and effort to perform. The Compression Test was developed by snow rangers from Park Canada in the 1970s. This popular test is quick and easy to do. After isolating a one by one foot column, the tester places a shovel on the top of the column and loads it with 10 easy taps from the wrist, 10 taps from the elbow and 10 hard taps from the shoulder. The Extended Column Test first introduced in 2006, since then, it quickly gained popularity and become the go-to test for many professionals around the world. In addition to the information on load that is needed for the snowpack to fail, it also gives insight into the possibility of failure on a large area of the slope. This test involves isolating a column that is three feet in the cross slope direction by one foot in the slope direction. Then, the shovel blade is placed on one end of the column and loaded as in the Compression Test. In this test, the tester notes the number of taps it takes the snowpack to fail, but they also note if the fracture along the weak layer crosses the entire column or comes to an arrest mid-column.

Snowpack observation is a large topic to cover in one column. I recommend the interested reader pick up the new fifth edition of the Snow Sense book. This book is by two Alaskan avalanche legends: Jill Fredston and Doug Fesler. It was recently updated with the latest knowledge in the avalanche field and is a simple and easy to read.

Note: The Extended Column Test was developed by Ron Simenhois and Karl Birkeland.

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