‘… A massive shift in the amount of energy being absorbed by the snow’
By Bob Berwyn
SUMMIT COUNTY — Desert dust blowing on to the high peaks of Colorado is affecting stream flows and even changing tundra vegetation — and now it’s been traced as a cause of avalanches in the high country, researcher Chris Landry said Friday, addressing a packed house at the annual Colorado Snow and Avalanche Workshop.
Scientists have measured a significant increase in the number of dust-on-snow episodes in recent years. They’ve tracked the dust to it sources in the Southwest, where dry weather and disturbance to desert soils, including off-road use, agriculture and energy development have all been tabbed as contributing factors.
“Last year was the first time ski area operations were impacted by dust on the snow,” Landry said, showing a slide of A-Basin, where snow cats were used to scrape the dirty surface layer and push it into the trees. But since he was speaking to a group mostly interested in avalanches, he focused on how the layers affect snowpack stability.
First, he explained that clean white snow reflects about 95 percent of the incoming solar energy, while the dirty snow reflects only about 50 percent.
“That’s a massive shift in the amount of energy being absorbed by the snow,” he said, explaining that, at its peak, the dust makes an overwhelming contribution to snowmelt. And it’s not just guess work. In his San Juan study plots, Landry measures snowpack temperatures day by day and even hour by hour to track the effects. Read more at the Center for Snow and Avalanche Studies website.
“There are dramatic fluxes of energy going on in near-surface snowpack,” he said. Those changes often result in the formation of faceted crystals at the dust layers. Similar to other temperature-gradient conditions, the contrast between warm and cold layers is conducive to the formation of those pesky grains that don’t bond well with each other or the adjacent layers. Once the faceted crystals at the dust layer are buried by a subsequent snowfall, they can linger as an unstable layer.
In some conditions, the dust can also lead to the creation of a hard ice layer, which can also created a sliding surface for avalanches, he said, referencing a May 1 avalanche on Tenmile Peak in Summit County that injured a snowboarder. Based on observations after the slide, a dust layer from earlier in the spring played was a critical factor in the avalanche, as the snow below the dust layer was isothermal, or all the same temperature.
Landry said that not every dust incident is culpable in current or future avalanches. Sometimes the dust can blow in at night and be buried under new snow by morning, remaining benign within the snowpack.
But if it’s near the surface and exposed to the sun, the concentration of solar energy simply intensifies processes known to cause to avalanches.
“You have to be concerned about the massive amounts of extra energy going into this snowpack … Extreme conditions produce extreme avalanches,” he said, adding that he suspects dust layers can also cause massive cornice failures.