WORDS: Doug Chabot
Doug Chabot is the director of the Gallatin National Forest Avalanche Center based in Bozeman, Montana.
There are two ways to trigger avalanches: stress the snowpack or weaken it. Dry slab avalanches, which are common in the colder winter months, occur when too much weight (or stress) is added to the snowpack. Layers can collapse under a heavy load of new snow, windblown snow, or skiers. In the spring, though, wet slab avalanches happen when the snowpack loses strength. Melting water breaks down snow crystals and provides lubrication between layers, which causes them to avalanche. Some types of layers are more vulnerable to this type of weakening, and this season the western U.S. is especially prone to one of the worst layers for wet slabs--depth hoar.
Depth hoar are large grains of sugary snow found near the ground that do not bond strongly to each other. A snowball cannot be made with a fistful of these grains because they just fall out of your palm like sand. Although much of the West had historically low snowfall this winter, depth hoar still formed at the ground and was capped with seasonal snow, albeit a meager amount in some areas. These crystals form very quickly in thin snowpacks, but we are seeing depth hoar crystals even in the mountain ranges with higher than average snowfall. In those places, the depth hoar grew during a fierce December cold snap when the snowpack was still young and not deep.
This spring, these faceted grains are the primary weak layer that can result in destructive wet slab avalanches. Sunny days are "solar storms" that when coupled with above freezing nighttime temperatures will raise the avalanche danger quickly. Typically, 48 to 72 hours of warm conditions will ripen the snowpack for wet slabs. Once water begins to flow, it weakens the internal structure of the snowpack, and when the liquid reaches the depth hoar crystals, they will crumble and avalanche rapidly.
Water flowing for a week or more will find paths of least resistance and create channels, which will act as pipes and flush the water out of the snowpack with only a minimal reduction in strength. The days before these channels form are the most dangerous. That's when the entire snowpack is moist, weak, and primed to avalanche.
Stepping out of our skis and sinking to the ground is a sign the wet snow does not have enough strength to support us. Sinking deeply is a warning to stay out of avalanche terrain. If temperatures plummet and the snow refreezes, then the opposite is true: It is solid and stable as a rock. We need to be mindful of air temperatures, especially at night, and be cautious on the first days of round-the-clock above freezing temperatures. Depth hoar was a problem all winter and it will remain a big one throughout spring.
This is a video from March 2012, when a wet slab pulled out from an explosive. When the slide ran, it triggered another fracture line, this one ripping down to the depth hoar found at the bottom of the snowpack.