In the cold lab, however, where the temperature is 8 degrees below zero, the focus is on a one-square-meter panel, brilliantly lighted by an artificial sun and watched over by an icy artificial sky that can be widely varied to replicate different winter conditions. Wearing his puffy down jacket, wool hat and sunglasses, Dr. Adams shows how he can reproduce the wide range of conditions found on mountain slopes and create different types of snow. “We want to understand what conditions cause the change in the crystalline structure and the bonding between crystals,” he said. It is the missing part of the puzzle of understanding avalanches.
Once he and his students and colleagues have created the snow crystals under certain conditions, they put them under the microscope to see what conditions made for the strongest or weakest layers. Snow layers are the key to predicting avalanches.
The biggest cause of avalanches is a weak layer of snow on a slope covered by solid layers, Dr. Adams said. “The weak layers are faceted crystals, very smooth and unbonded to each other,” almost like ball bearings, he said. Strong layers have stronger bonds between crystals, which makes them more stable.
“It’s like a layer cake with very weak frosting,” Dr. Adams said. When something causes the weak layer, usually less than an inch thick, to give way, the strong layer or layers—there can be dozens, some of them feet thick—go with it. Even skiing at low altitudes can fracture a weak layer and set off an avalanche far above. Contrary to conventional wisdom, sound, unless it is from an explosion, does not set off avalanches.
Some ski areas offer skiers free skiing in exchange for “boot packing” — trampling weak layers with their boots to harden them.
The key to improving forecasting, Dr. Adams said, is understanding the surface layer, where sun and cold cause the snow crystals to change. Understanding the energy transfer on the surface can provide information about what is going on underneath.