The Science of Avalanches

New Yorker – "Snow Science Against the Avalanche"

"We think of the snow on a mountain as a solid mass. In reality, it is a layer cake created by serial snowfalls, each layer distinctive and changeable. “The snow cover is never in a state of repose,” Atwater wrote. “It is continually being pushed, pulled, pressed, bent, warmed, chilled, ventilated, churned.” The topmost layer might be evaporating into the night air; at the same time, radiant heat from the ground, or from nearby trees, could be melting the lowest layer. When the temperature differences between the layers are small, snow tends to sinter, or coalesce: the crystals knock off one another’s arms, becoming rounded grains that fuse into a strong, dense snowpack. When the differences are larger—say, between the pack and the ground—snow vaporizes upward and refreezes, creating hollow, cup-shaped crystals. The result is brittle, spiky snow, called depth hoar. (In ice cream, a similar process creates freezer burn.)

Neither settled snow nor weak hoar is dangerous in itself. The problem arises when a dense layer lies atop a weak layer to which it is poorly bonded. Depth hoar is “the eeriest stuff on any mountain,” Atwater wrote; it grows unseen, rotting the snow until it is weak and potted. It is strong in compression but weak in shear. Like a row of champagne glasses slowly loaded with bricks, it can hold a surprising amount of weight until, with the slightest shove, the structure falls apart, creating a slab avalanche.

The word “avalanche” is too graceful for the phenomenon it describes. On slopes shallow enough to accumulate snow but steep enough for it to be unstable—the sweet spot is said to be thirty-nine degrees—the layers will separate, and the slab will crack and slide. Churning violently, the snow reaches eighty miles per hour within a few seconds. A skier who avoids colliding with trees and rocks is likely to be pulled under, then pinned in place by thousands of pounds of snow that harden like concrete. Very few people can dig themselves out; most can’t even move their fingers. Within minutes, an ice mask forms around your face. You asphyxiate on your own exhaled carbon dioxide."