If you’re looking for the killer, then the slab avalanche is your man. This is the White Death, the Snowy Torrent, the Big Guy in the White Suit. Dry slab avalanches account for nearly all of the avalanche deaths in North America.
Wet avalanches are triggered by warm air temperatures, sun or rain, causing water to percolate through the snowpack and decreasing its strength.
Most avalanche professionals make a hard separation between wet snow avalanches and dry snow avalanches, because they are so different. Much of their mechanics are different, they move differently, and it's only natural for us to think of them as two altogether separate beasts. But really there's a continuum between wet and dry avalanches.
Like dry snow avalanches, wet avalanches can occur as both sluffs and slabs. Wet avalanches usually occur when warm air temperatures, sun or rain cause water to percolate through the snowpack and decrease the strength of the snow, or in some cases, change the mechanical properties of the snow. Once initiated, wet snow tends to travel more slowly than dry snow-like a thousand concrete-carrying trucks dumping their loads at once, rather than the hovercraft-like movement of a dry avalanche.
A typical wet avalanche travels at 10 to 20 mph (15 to 30 km/h), but on steeper terrain they can go nearly as fast as a dry avalanche. Probably because not as many recreationists are out on wet snowy days, wet avalanches don't account for nearly as many avalanche fatalities as dry snow avalanches. However, they still account for a sizeable percentage of avalanche fatalities in maritime climates (mountains bordering oceans), especially to climbers.
Dry (Powder) Avalanches
Dry avalanches, though beautiful, are extremely violent and seize up like concrete the instant they come to a halt.
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-travelling 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.
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.
Dry Slab Avalanches
Nearly all avalanche deaths in North America are caused by slab avalanches, caused when a cohesive plate of snow suddenly slides away.
A "slab" is a cohesive plate of snow that slides as a unit on the snow underneath. A slab doesn't have to be hard it just has to be relatively stronger than the snow underneath. A typical slab is about half the size of a football field, about one to two feet (30-60 cm) deep, and usually reaches speeds of 20 mph (32 km/h) within the first three seconds, quickly accelerating to around 80 mph (128 km/h) after the first, say, six seconds. The bonds holding a slab in place fracture at about 220 mph (352 km/h) and the slab appears to shatter.
Dry slab avalanches can lie teetering on the verge of catastrophe, sometimes for days or even months. The weak layers beneath the slabs are extremely sensitive and the rapid addition of the weight of just one person can easily initiate a fracture on a slope that would not have avalanched otherwise. A slope can sometimes be a giant booby trap-seemingly waiting for just the right person to come along. The crack often forms well above the victim, leaving little room for escape.
Loose Snow Avalanches
Loose snow sliding down a mountain is called a loose snow avalanche. Small loose snow avalanches are called "sluffs." Few people are killed by loose snow avalanches because they tend to be smaller, and they tend to fracture beneath you as you cross a slope, rather than above you, as slab avalanches often do. Most of the people killed in loose snow avalanches are climbers, or extreme skiers and boarders in very steep terrain.
Ice & Cornice Fall Avalanches
Ice and cornice fall avalanches occur when overhanging ice or snow breaks off.
When glaciers flow over a cliff they form the ice equivalent of a waterfall-an icefall. Falling blocks of ice can trigger an avalanche of ice, which often entrains snow below it, or triggers slabs. Especially in big mountains, ice avalanches can be large and travel long distances.
Despite this, ice avalanches kill few people compared to dry slabs that people trigger themselves. Most of the deaths from ice avalanches occur to climbers in big mountains who happen to be in the wrong place at the wrong time.
Cornices are the fatal attraction of the mountains, their beauty matched only by their danger. Cornices are elegant, cantilevered snow structures formed by wind drifting snow on to the lee (downwind) side of an obstacle, such as a ridgeline. Similar to icefall avalanches, the weight of a falling cornice often triggers an avalanche on the slope below, or the cornice breaks into hundreds of pieces and forms its own avalanche-or both.
Again like ice avalanches, statistically cornice fall avalanches don't kill very many people. And similar to slab avalanches, the skiers who get into trouble almost always trigger the avalanche, in this case by travelling too close to the edge of the cornice. Cornices have a nasty habit of breaking farther back than you expect. Never walk up to the edge of a drop-off without wearing a rope or first checking out the drop-off from a safe place.
Bruce Tremper and Mountaineers Books
Extracts and quotes reproduced from 'Staying Alive in Avalanche Terrain' by Bruce Tremper and reproduced with the kind permission of the publisher, The Mountaineers Books. The author Bruce Tremper Is the Director of the Forest Service Utah Avalanche Center and one of North America's foremost experts on avalanches. He has appeared in news reports and documentaries produced by National Geographic, PBS, and Discovery Channel, among others.
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