Dry avalanches

Dry avalanches are the stunningly beautiful ones that roar down the mountain, light and fluffy, like clouds of powder, but beneath the misty powder cloud is a rushing mass of snow—the “core” of the avalanche—that is a fluidized mix of air (70 percent) and ice particles (30 percent). As the snow rushes through the air it kicks up an envelope of powder, appropriately enough called a “powder cloud,” which comprises only about one percent snow and 99 percent air, and this is the part of the avalanche that gives it its beauty.

In front of the powder cloud is the invisible “air blast” that pushes out in front of the moving snow. The air blast carries only about one tenth of the impact of the core, but it can travel fast enough to explode your lungs if you are caught by the full impact of the blast.

The avalanche is slowed down mainly by friction with the rocks, vegetation, and the snow surface it runs on. The snow nearest the bed travels more slowly than the snow above. Often, if you look close enough, you can see avalanches come down in waves. One wave shoots out, is slowed by friction with the ground and air, and then the next wave—traveling on the back of the first wave—shoots out ahead of the first wave, and so on. It looks like pulses of snow being spat out the front of the avalanche about once every few seconds.

Why avalanches create “concrete”
The extreme violence inside the flowing debris grinds up all the snow into finer and finer particles, and even if the snow started out light and fluffy, it can become very dense by the time it finally comes to a stop. A large avalanche that starts out with a density of 5 to 10 percent (volume of snow versus air) can often end up as 30 to 40 percent at the bottom. This means that when everything comes to a stop, the dense snow packs very tightly.

Also, small grains sinter (coalesce) much more quickly than large grains, and the tiny grains making up avalanche debris can sinter as much as ten thousand times faster than the larger grains of the initial slab.

Finally, all of the kinetic energy liberated on the way down heats up the snow a little and creates small drops of liquid water on the surface of the ice grains. Combining all these factors, it’s easy to see why avalanche debris seizes up like concrete the instant it comes to a stop. The avalanche victim is frozen in place.