Avalanches are a rapid flow of snow down a slope and can be triggered by precipitation, wind, animals, and earthquakes.
Avalanches may be either slab or loose snow and can cause a powder snow avalanche if it moves fast enough.
Avalanches are a natural hazard in mountainous areas where they can happen at any time of year. They are described by their size, destructive potential, initiation mechanism, composition, and dynamics.
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Avalanche Facts for Kids
- Snow sliding down a slope in an avalanche
- Avalanches are caused by too much snow on a mountain.
- They can be dangerous to skiers.
- Experts can tell if there is a risk of an avalanche
- The majority of avalanches involve small amounts of ice and snow
Types of Avalanches
Slab avalanches
Slab avalanches form when snow deposited by wind is cut out from its surroundings. They have three fractures: a crown fracture, flank fracture, and a stauchwall.
Powder snow avalanches
Powder snow avalanches and mixed avalanches, the largest avalanches, can reach speeds of 300 km/h (190 mph).
Wet snow avalanches
Unlike powder snow avalanches, wet snow avalanches travel at a low speed due to the friction between the snow and the water-saturated flow. They are powerful and can scour boulders, earth, trees, and other vegetation, leaving exposed and often scored ground in the avalanche track.
Ice avalanche
Large pieces of ice can cause an ice avalanche when they fall. The resulting movement is similar to a rockfall or a landslide.
Avalanche pathway
Based on the slope’s steepness and the volume of snow/ice involved in the mass movement, avalanches have a specific track from origin to end.
What Causes an Avalanche?
Avalanches occur most often during the winter and spring months. Snowfall makes newly fallen snow unstable, and it slides off mountainsides. Earthquakes can also trigger an avalanche. Animals, including humans, can trigger avalanches too.
Experts are now using explosives to trigger small avalanches so that large ones can be detected early. This saves lives and protects homes and businesses from damage.
What Conditions Cause an Avalanche?
When it is cold outside, slopes can be dangerous. Slope shading helps keep slopes cool, so there are fewer avalanches.
This also means that the slope will melt faster in the spring. With warmer temperatures, slopes become much more likely to slide.
Snowmelt occurs when water seeps from the ground and melts the snow. This process happens slowly at first but increases rapidly after temperatures rise above freezing.
As the melt progresses, more water flows out of the ground and onto the surface. At some point, enough moisture reaches the surface to cause melting.
When the snow begins to melt, its weight decreases, and gravity pulls the melted water toward lower areas. Water continues flowing until there is no more snow left to melt.
What Triggers Wet Slab Avalanches
Wet Slab avalanches occur when a weak layer or interface is wet, moist, or saturated. Wet snow deteriorates, and avalanches result.
A wet slab fails due to a decrease in layer strength, as opposed to a dry avalanche that often fails due to an increase in load.
Why are Slab Avalanches so Dangerous
They reach a high speed in a very short period of time. Even small slab avalanches can be dangerous.
They have a distinct fracture line. If there is a bonded layer of snow (the slab) on top of a weak layer, they occur
How to Identify Avalanche Terrain
The slope’s shape is a giveaway and helps us understand if an avalanche could happen.
In avalanche terrain, avalanches can happen if the conditions are right. The terrain is steep enough to slide snow. Avalanches occur more frequently in certain terrain types due to certain features or a lack thereof.
An avalanche’s potential consequences are dictated by the slope shape, whether it’s a narrow couloir, multiple gullies, a broad slope, funnel-shaped, or hourglass-shaped.
Most dry snow avalanches occur on slopes of 30–45 degrees. A slab avalanche is less common on slopes steeper than 45 degrees since the snow doesn’t tend to build up into thick slabs.
Things to Know When Traveling in Avalanche Terrain
Whenever possible, travel with people who have more experience than you.
Take note of weak layers that emerge in the snowpack through the season.
Watch for signs of avalanches on the ski up-track, such as the snowpack collapsing
When in doubt, you should turn back
How Does Avalanche Control Work
Small buildups of snow will be loosened by explosives, preventing larger accumulations.
Avalanches can also be controlled by special fences, nets, or artificial walls that can influence how much snow will fall and in which direction.
