What is an avalanche?

Simply put, it is a mass of snow sliding down a mountainside

All that is necessary for an avalanche is a mass of snow and a slope for it to slide down. For example, after a heavy snowfall a car might look like this.

What causes an avalanche?

While the temperature is cold, the snow sticks to the surface and doesn’t slide off.

What causes an avalanche?

After temperatures warm up a little, however, the snow will “sluff,” or slide, down the front of the windshield, often in small slabs. This is an avalanche on a miniature scale.

Of course, mountain avalanches are much larger and the conditions that cause them are more complex. A large avalanche in North America might release 300,000 cubic metres of snow. That’s the equivalent of 20 football fields filled 10 feet deep with snow. However, such large avalanches are often naturally released. Skiers and walkers are usually caught in smaller, but often more deadly avalanches.

Anatomy of an avalanche

Slab avalanches are the most common and most deadly avalanches, where layers of a snowpack fail and slide down the slope. Since 1950, 235 people in the U.S. have been killed in slab avalanches. Hard slab avalanches involve large blocks of snow and debris sliding down a slope. In soft slab avalanches, the snow breaks up in smaller blocks as it falls.

An avalanche has three main parts. The starting zone is the most volatile area of a slope, where unstable snow can fracture from the surrounding snowcover and begin to slide. Typical starting zones are higher up on slopes, including the areas beneath cornices and “bowls” on mountainsides. However, given the right conditions, snow can fracture at any point on the slope.

The avalanche track is the path or channel that an avalanche follows as it goes downhill. Large vertical swaths of trees missing from a slope or chute-like clearings are often signs that large avalanches run frequently there, creating their own tracks. There may also be a large pile-up of snow and debris at the bottom of the slope, indicating that avalanches have run.

The runout zone is where the snow and debris finally come to a stop. Similarly, this is also the location of the deposition zone, where the snow and debris pile the highest. Although underlying terrain variations, such as gullies or small boulders, can create conditions that will bury a person further up the slope during an avalanche, the deposition zone is where a victim is most likely to be buried.

What causes an avalanche?

Put simply, an avalanche occurs when the stress from gravity trying to pull the snow downhill exceeds the strength of the snow holding it together

There are five main ingredients to an avalanche

• a steep slope – Not quite as simple as it seems. Most avalanches occur on slopes between 30 and 45 degrees, but can occur on any slope angles given the right conditions. Very wet snow will be well lubricated with water, meaning it might avalanche on a slope of only 10 to 25 degrees. Compacted, well-bonded layers create a snowpack that can cling to steeper slopes until a weak layer is created.

•  a snow cover - Avalanches are most likely to run either during or immediately after a storm where there has been significant snowfall. The 24 hours following a heavy snowstorm are the most critical.

• a weak layer in the snow cover - Snowpack conditions are extremely important because many layers of snow build up over the winter season. Each layer is built up under different weather conditions and will bond differently to the subsequent layers. Snowflakes, or snow crystals, within the snowpack eventually become more rounded due to melting/re-freezing and settlement. This metamorphism allows them to compress and (generally) form stronger bonds.

In between snows, the temperature may rise and melt the exposed surface layers, which when they re-freeze create a smoother, less stable surface for the next snowfall. Failure is much more likely to occur during or after the next few snowfalls. Rain between snows creates a slicker surface as well, and can weaken the bonds between snow layers. On the other hand, light snowfalls and consistently cold temperatures help strengthen the snowpack and make it more resistant to avalanche. Weak layers deep in the snowpack can cause avalanches even if the surface layers are strong or well bonded.

• the weather Because snow is a good insulator, small temperature changes do not have as much effect on snowpack as larger or longer changes do. For instance, shadows from the sun crossing the snow surface throughout the day will not significantly change snowpack stability. Changes that last several hours or days, such as a warm front moving through, can gradually increase temperatures that cause melting within the snowpack. This can seriously weaken some of the upper layers of snow, creating increased avalanche potential, particularly in combination with other factors.

• a ‘trigger’ – often the weight of a single skier

When are avalanches most likely to occur?

After major snowstorms

Especially January to April – most snow, spring thaw

Where do avalanches happen?

• 30-45 degree slopes – as mentioned previously

• Slopes in shadow - Although avalanches will run on slopes facing any direction, most avalanches run on slopes facing north, east, and northeast (also the slope directions that most ski areas are located on). Because the sun is at such a low angle, particularly during the winter, a colder and deeper snowpack develops. Slopes that are under shadow throughout most of the day are suspect because the snowpack remains cooler, without much of the melting and bonding that can make the snow layers stronger.

• Slopes with little vegetation cover – generally vegetation cover tends to make the snowpack more secure and blocks the path of an avalanche, however this is not always the case

Lone trees, bushes, or large rocks on a mountainside can sometimes weaken the stability of the snowpack. A fracture line (the break-off point for an avalanche) may run from a lone tree to a rock to another tree. Also, during avalanches, trees and rocks catch debris and cause excessive snow pile-up, as well as provide lethal obstacles for anyone caught in an avalanche.

The tree line, above which conditions become too harsh for trees to grow, also plays a significant role in avalanche areas. Many avalanches start above the tree line, making high-elevation mountains especially risky. Although forests help stabilize the snowpack, if an avalanche starts above tree line, it can cut its own path, or chute, through the trees below.

A man-made avalanche

Not all avalanches are natural. Some are triggered by humans to prevent the occurrence of an even bigger avalanche in the future.

Here, a man-made avalanche roars down a slope at the Jackson Hole Ski Resort. Every morning during ski season, before the slopes open, members of the Jackson Hole Ski Patrol take to the mountainsides, using howitzers and hand charges to trigger avalanches that otherwise might endanger skiers.

A survivor’s story

Kent Swanson was climbing on Mount Rainier in Washington state, USA when an avalanche was triggered above him