The Attack of Depth Hoar

The beginning of winter 2015/16 has lived up to forecaster’s predictions, with a strong El Niño cycle bringing an onslaught of snow across the western United States. With great amounts of snow, however, comes risk. A persistent weak layer has formed deep within the snowpack and can be found from the Wasatch Mountains of Utah to the Rocky Mountains of Colorado and Montana. This layer can be reactive and could pose a safety concern for backcountry travelers in these regions of the west.

What can we thank for this weak layer?

In Crested Butte, Colo. the Crested Butte Avalanche Center recently reported that the weak layer is comprised of depth hoar, a type of snow crystallization that they explain is formed when there are “large temperature gradients between the warm ground and the cold snow surface. [Its formation] usually requires a thin snowpack combined with a clear sky or cold air temperature. [Depth hoar] grows best at snow temperatures from -2° C to -15° C.”

Crested Butte Avalanche Center report. | December 30, 2015

Why is this weak layer dangerous?

The Crested Butte Avalanche Center goes on to explain that depth hoar is dangerous because it “[b]ehaves like a stack of champagne glasses. Relatively stronger in compression than in shear. Fails both in collapse and in shear. Especially nasty when it forms on a hard bed surface. Commonly propagates long distances, around corners and easily triggered from the bottom—your basic nightmare.”

The Gallatin National Forest Avalanche Center released a video from December 30, 2015 where they talk about the likelihood of triggering a side on this persistent weak layer. They explain that while the chances of triggering an avalanche on the weak layer are growing smaller, the consequences of an avalanche are high due to the large amount of snow that has recently fallen in these western regions. While thick slab can bridge the weak layer, at shallower locations there is still a chance for human triggered avalanches that can propagate throughout the deeper snowpack. These results are reflected in the Crested Butte Avalanche Center video from the same day.

Gallatin National Forest Avalanche Center report. | December 30, 2015

What can you do to stay safe?

Make sure to check with your local avalanche center and find out if the snowpack near you is suffering from this persistent layer of depth hoar. If it is, follow the suggestions of your avalanche center on how to best travel during periods where persistent weak layers may trigger avalanches. Stay informed on all backcountry forecasts and incident reports from your region and never go out without the proper safety tools, information and training.