A mass of snow sliding, tumbling, or flowing down an inclined surface.
Types of Avalanches
If you’re looking for the killer then this 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 the avalanche deaths in North America.
A “slab” is a cohesive plate of snow that slides as a unit on the snow underneath. Picture tipping the living room table up on edge and a magazine slides off the table. Now picture you standing in the middle of the magazine. The crack forms up above you and there you are, there’s usually no escape and you’re off for the ride of your life.
The bonds holding a slab in place typically fractures at 350 kilometers per hour (220 miles per hour) and it appears to shatter like a pane of glass. It’s typically about the size of half a football field, usually about 30-80 centimeters (1-3 feet) deep and it typically reaches speeds of 30 km/hr (20 mph) within the first 3 seconds and quickly accelerates to around 130 km/hr (80 mph) after the first, say, 6 seconds. Dry slab avalanches can lie patiently, teetering on the verge of catastrophe, sometimes for days to even months. The weak-layers beneath slabs are also extremely sensitive to the rate at which they are stressed. In other words, the rapid addition of the weight of a person can easily initiate the fracture on a slope that would not have avalanched otherwise. A slope can lay in waiting like a giant boobie trap--just waiting for the right person to come along. The crack often forms well above the victim leaving little room for escape. Does any of this sound dangerous to you?
Loose Snow Avalanche:
Loose snow sliding down a mountainside is called a loose snow avalanche. Small Loose snow avalanches are called "sluffs".
Loose snow avalanches usually start from a point and fan outward as they descend, and because of this they are also called “point releases.” Very few people are killed by sluffs because they tend to be small and they tend to fracture beneath you as you cross a slope instead of above you as slab avalanches often do. The avalanche culture tends to minimize the danger of sluffs, sometimes calling them "harmless sluffs." But, of course, this is not always the case. Houses have been completely destroyed by "harmless sluffs," and if caught in one, it can easily take the victim over cliffs, into crevasses or bury them deeply in a terrain trap such as a gully. Most of the people killed in sluffs are climbers who are caught in naturally-triggered sluffs that descend from above--especially in wet or springtime conditions.
Sluffs can actually be a sign of stability within the deeper snow when new snow sluffs down without triggering deeper slabs.
When glaciers flow over a cliff they form the ice equivalent of a waterfall—an icefall. Falling blocks of ice create an avalanche of ice, which often entrain snow below it or triggers slabs. Especially in big mountains, icefall avalanches can be large and travel long distances. Despite this, icefall avalanches kill few people compared to dry slabs that people trigger themselves. Most of the deaths from icefall avalanches occur to climbers in big mountains who just happen to be in the wrong place at the wrong time.
Icefall avalanches occur more or less randomly in time. However, in warmer climates, more ice tends to come down in the heat of the day than at night. Also, on a longer time scale, glaciers tend to surge, meaning that they actually have very slow waves that travel through them that produce a surge of movement for a few days to a month, followed by less movement for several more days or even months. For instance, sometimes an icefall seems very dormant for several months, then suddenly, it produces lots of activity for several days to a month.
But besides these exceptions, icefalls are fairly random--pretty much a roll of the dice when traveling under an icefall. The best way to deal with icefall avalanches, of course, is to avoid traveling on them or beneath them. And when you choose to travel beneath them, do so quickly. At the risk of being too obvious--never camp under icefalls. But sometimes bad weather prevents climbers from seeing icefall hazard when they set up camp, or bad weather forces them to camp in the wrong spot. Many accidents with icefall avalanches happen this way.
Cornice Fall Avalanches:
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 onto the 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. Be aware that cornice fragments often "fan out" as they travel downhill, traveling more than 30 degrees off of the fall line. Cornices tend to become unstable during storms, especially with wind, or during times of rapid warming or prolonged melting. Each time the wind blows, it extends the cornice outward, thus, the fresh, tender and easily-triggered part of the cornice usually rests precariously near the edge while the hard, more stable section usually forms the root.
Similar to icefall avalanches, cornice fall avalanches don’t kill very many people. And similar to slab avalanches, the ones who get into trouble almost always trigger the avalanche, in this case, by traveling too close to the edge of the cornice. Cornices have a very nasty habit of breaking farther back than you expect. NEVER walk up to the edge of a drop off without first checking it out from a safe place. Many people get killed this way. It's kind of like standing on the roof of a tall, rickety building and walking out to the edge for a better view. Sometimes the edge is made of concrete but sometimes the edge is made of plywood cantilevered out over nothing but air. It feels solid until, zoom, down you go. Check it out first.
But cornices aren't all bad. You can use cornices to your advantage by intentionally triggering a cornice to test the stability of the slope below or to intentionally create an avalanche to provide an escape route off of a ridge.
Most avalanche professionals make a hard separation between wet snow avalanches and dry snow avalanches, because wet and dry avalanches are so different. You forecast for wet and dry avalanches very differently, much of the mechanics are different, they move differently, and it's only natural for us to think of them as two separate beasts altogether. But really, there’s a continuum between wet and dry avalanches. For instance, there are damp avalanches, and often, large, dry avalanches start out dry and end up wet by the time they get to the bottom because either the energy of the descent heats the snow up or they travel into a region of warmer snow. 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 much more slowly than dry snow avalanches--like a thousand concrete trucks dumping their load at once instead of the hovercraft-like movement of a dry avalanche. A typical wet avalanche travels around 15 to 30 km/hr (10 or 20 mph) while a typical dry snow avalanche travels 100 to130 km/hr (60 or 80 mph)--big difference. Wet slides are also harder for a person to trigger than a dry slide. Because of these two facts, wet avalanches don’t account for nearly as many avalanche fatalities as dry snow avalanches. But they’re certainly not insignificant. They still account for a sizeable percentage of avalanche fatalities in maritime climates, especially to climbers. Wet slides can also do quite a bit of damage to property or to forests and often cause significant hazards on highways.
Glide occurs when the entire snowpack slowly slides as a unit on the ground, similar to a glacier. Don't mistake glide for the catastrophic release of a slab avalanche that breaks to the ground. Glide is a slow process, that usually occurs over several days. Glide occurs because melt water lubricates the ground and allows the overlying snowpack to slowly "glide" downhill. Usually, they don't every produce an avalanche but occasionally they release catastrophically as a glide avalanche. So the presence of glide cracks in the snow do not necessarily mean danger. It's often difficult for a person to trigger a glide avalanche but at the same time it's not smart to be mucking around on top of them and especially not smart to camp under them.
We tend to find them in wet climates and when they occur in dry climates they do so in spring when water percolated through the snow or sometimes during mid winter thaws.
When do they come down? Like an icefall, they come down randomly in time--when they're good and ready--not before. You would think that they would come down during the heat of the day or when melt water running along the ground reaches its maximum. But oddly enough, they tend to release just as often with the arrival of cold temperatures following melting as during melting itself. It's hard to play a trend with glide avalanches. They come down when they're good and ready and it's impossible to tell when that is. Just don't spend much time underneath them.
An oddity in most of the avalanche world, slush avalanches usually occur in very northern latitudes such as the Brooks Range of Alaska or in northern Norway. They're unusual because they occur on very gentle slopes compared with other avalanches, typically 5-20 degrees and they rarely occur on slopes steeper than 25 degrees. A typical slush avalanche occurs in impermeable permafrost soil, which allows water to pool up, and occurs during rapid saturation of a thin, weak snowpack. When water saturates the snowpack, it catastrophically looses its strength and the resulting slush often runs long distances on very gentle terrain. Once again, very few people are killed by slush avalanches possibly because so few people live in high latitude permafrost mountains. But they can certainly be dangerous to people camped in the wrong spot or structures built in the wrong locations.