“Snow is a changing, fragile substance, which accumulates in layers: a deep puffy storm, followed by an inch of rain. Wind crust followed by cold light flakes. Avalanches are a combination of three factors: a sliding surface, a slope steep enough to slide, and a trigger. Here in Utah—and in other high, dry parts of the Rockies—more often than not, there’s a deep unbonded layer in that snowpack that could always slide, given a trigger. It seems to happen the same way almost every season. The first thin snowfall covers the mountains in a crystalized layer of sugar and anticipation. Then it stops, like climatic clockwork, for a few weeks. That layer of unbonded snow is exposed to the air, which sucks out moisture, creating slippery, faceted snow crystals called depth hoar. It forms a perfect sliding surface. When the snow starts in earnest, that surface, which avalanche forecasters call a persistent weak layer, is at the very bottom, slick and unbonded, ready to slide. That’s one of the constant hazards of skiing, you always know it’s down there. Just how big it could break is a question of what comes in on top of it.”