Tuesday, February 5, 2013

Interpreting Snowpack Layers and Hardness

When the word avalanche is mentioned, images of curtains of snow cascading down the mountain come to mind. Specific conditions create avalanches. One of the basic elements, of course, is snow. The snowpack (the seasons snow on the ground) is a complex, but a key to understanding it is identifying layers and differences between them. Layers within the snowpack are a record of the winter’s weather. Like tree rings or strata of rock, layers can be traced to dates and conditions that formed them.

 One of the most important characteristics of a layer is its hardness. Harder snow is stronger and cohesive, while softer snow is weaker. For a slab avalanche we need a strong layer over a weak layer. We actually use a “Hand Hardness” scale by pushing our hand into each layer to determine the hardness. “Fist” hard snow is the softest, then “four finger”, then “one finger” snow being harder, and so on. No, there’s no “middle finger” on this scale. The greater the hardness difference of neighboring layers, the more likely we are to see avalanches at that interface.

The recent snowpack history from the Crested Butte backcountry is a great example of layer hardness relating to human triggered avalanches. Consider these three scenarios:

1) The last week in January brought up to 50” of snow over seven days. While this storm caused some natural avalanches, riders initially weren’t triggering many slab avalanches. Most of the snow came in light and soft. Digging in sheltered and shaded areas during the middle of the storm, you’d find very soft (fist hard) new snow on top of some slightly harder old snow.

2) The storm ended on Thursday January 31st. By then the wind had formed stiff slabs at all elevations but especially near and above treeline. Digging on some leeward slopes near treeline you could find one finger hard wind-loaded snow sitting over four finger hard facets. Many reports of natural and skier triggered slides came in from wind-loaded areas.

3) By Monday, February 4th, the storm had ended four days prior. Mild daytime temperatures had created settlement in the height of the storm snow and an increase in its hardness. I was skiing on sheltered easterly aspects below treeline.  These slopes had seen very little wind in the past week. Throughout the day, we felt many rumbling collapses and we remotely triggered two sizable avalanches. On these sheltered slopes we found storm snow that had stiffened to a hardness of four finger sitting over fist hard facets.

This storm and avalanche cycle in the late January to early February shows how layer hardness often relates to skier triggering. The most consistent skier-triggering occurred where we found the most change in hardness in adjacent strong over weak layer. If this talk of layers seems daunting, remember that you don’t have to figure it out on your own. The daily avalanche forecast is best place for current conditions. You can check out the "Observations" page to read what other backcountry travelers are seeing. You could even look at the "Snow Profiles" page for technical graphs of recent snow pits. From there you can investigate the snow as much or as little as you want. But most of all have fun in the mountains, make sound decisions, and stay safe doing it. Check out the daily forecast at www.cbavalanchecenter.org

CBAC Forecaster Josh Hirshberg

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