On the afternoon of Feb. 1, seven friends set out on skis from the Opus Hut, a backcountry lodge located high in Colorado’s San Juan Mountains. The sky was blue, the temperatures relatively warm, and a recent series of storms had left a sparkling blanket of fresh snow on the alpine slopes, just begging to be etched with fresh ski tracks. After a climb, the crew dropped one by one down a sparsely treed, northeast-facing slope called The Nose. Minutes later, four of them were swept away by an avalanche. Only one would be rescued alive.
That was on a Monday. Before the week was over, avalanches would kill another 10 people in snowslides across the West, including four 20-somethings in a single Utah avalanche, putting this winter on track to be among the deadliest in Colorado and Utah since 1950.
*SNOTEL or Snowpack Telemetry sites are part of an automated system that measure snow, operated by the U.S. Natural Resources Conservation Service. There are more than 730 sites in 11 Western states, including Alaska.
This may seem surprising, given that this winter has been one of the skimpiest, in terms of snowfall, for those same states. And yet it’s this very lack of snowfall, along with its timing and the weather between the storms, that has created an especially dangerous snowpack, according to Andy Gleason, a snow scientist on the NASA SnowEx project and a lecturer on geosciences at Fort Lewis College.
11: The number of avalanche-related fatalities in Colorado in 2020-’21, as of Feb. 25. The record number of seasonal deaths since 1950 was 12.
Late last autumn, storms covered the region’s high country in a couple feet of snow, which fell as intricate crystalline flakes that resembled the paper cutouts you made back in grade school. But during the long, cold dry spell that followed, differences in temperature throughout the snow sent vapor coursing through it, whittling the flakes into large faceted grains that were unable to bond with one another. This is known as depth hoar, which is distinguished by its sugar-like consistency.
While this phenomenon can occur in a variety of conditions, it tends to be exacerbated when the snowpack is shallow, because the temperature gradient is greater. “When this weak layer gets buried by new snow,” Gleason said, “it is easy to trigger an avalanche, because the lattice structure of the depth hoar is so weak compared to well-bonded (new) snow.” Even if the slide is triggered on an upper layer by a skier’s weight, for example, the deeper, weaker layers may cause the weight of the slide itself to “step down” into the depth hoar, entraining, or incorporating, all of the layers, making it that much bigger — and more deadly.
Jonathan Thompson is a contributing editor at High Country News. He is the author of River of Lost Souls: The Science, Politics and Greed Behind the Gold King Mine Disaster. Email him at [email protected] or submit aletter to the editor.Follow @jonnypeace
Infographic design: Luna Anna Archey/High Country News Sources: Colorado Avalanche Information Center, Natural Resources Conservation Service.
A depth hoar, a large, faceted grain that is unable to bond to other grains of snow.
Agricultural Research Service, U.S. Department of Agriculture
