The White Book – How Does an Avalanche Happen?

WHAT IS AN AVALANCHE?
An avalanche is a mass of snow sliding down a mountainside. Avalanches come in all sizes.

  • Small avalanches can fracture less than 30 centimeters deep, be 20–30 meters wide and travel at 50 km/hour.
  • Medium avalanches may fracture 1–2 meters deep, have a width of 100–500 meters and travel at 100 km/hour.
  • Large avalanches can be 2–3 meters deep, or deeper, have a width of 1000 meters or more, and travel at speeds of over 150 km/hour.
  • An avalanche has four ingredients: a steep slope, a slab, a weak layer in the snow cover and a trigger.

TYPES OF AVALANCHES
There are two basic types of avalanches: loose snow and slab. Loose snow and slab avalanches can both release in dry snow, damp snow, or wet snow.

Loose snow avalanches occur in snow that lacks cohesion, such as fresh new snow that has not bonded, or wet snow that has lost its cohesion because of thaw. Loose snow avalanches are sometimes called point releases because the avalanche starts at a point and spreads out into a triangle as it slides down the mountainside. Loose snow avalanches tend to be small in size and seldom cause injury. However, the consequences of even a small loose snow avalanche can be very serious or even fatal if you are swept into a terrain trap.

Slab avalanches occur when one or more layers of snow have bonded into a cohesive slab. The slab can stay in place until the downhill stress exceeds the snow’s strength. The downhill stress is caused by gravity steadily pulling on the snow, and this stress can be increased by the added weight of new or wind-blown snow, or the weight of a person. When the slab fractures, cracks shoot out through the slab, and the snow breaks loose from its anchors at the top, sides and bottom. The snow-slab material can vary greatly. The slab can be very soft (excellent powder skiing) or very hard, like a wooden floor. Slab avalanches are the most dangerous for people and property because they can move large amounts of snow at high speed.

WHEN ARE AVALANCHES MOST LIKELY TO OCCUR?
The risk of an avalanche increases during major snowstorms and periods of thaw. More than three quarters of avalanches release during or just after large snowstorms. There is no rule of thumb on how much new snow will cause conditions to become dangerous, but the greater the snowfall, the greater the stress, the greater the danger. While most avalanches occur during a storm, many avalanche accidents occur shortly after a storm when the weather is nice but the snow remains unstable.

The faster the snow falls the faster the danger increases. For example, 30 cm of snow falling in six hours is far more dangerous than 30 cm falling in 24 hours. Someone once said, “The building blocks of an avalanche are made of snow, but wind is the builder.” This is especially true at higher elevations. Here, the wind can transport tremendous quantities of snow onto steep leeward slopes. The wind can strip snow off windward slopes and redeposit it onto leeward slopes at a much faster rate than snow falling from the clouds. When winds cause blowing snow the avalanche danger can quickly increase on leeward slopes.

Changes in snow temperature can create strong or weak layers of snow, significantly affecting snow stability.

Snow temperatures affect the rate at which snow gains or loses strength. Snow temperatures are strongly influenced by air temperature and solar radiation. Temperature trends are much more important than a single temperature reading. In a cold snowpack, unstable snow conditions persist because the strengthening process is slowed down. Even strong snow can loose strength making conditions unstable and more dangerous, especially when the snow is cold and shallow. Thus, dangerous avalanche conditions can persist for days or even weeks after a storm.

When snow is warm it typically settles quickly and becomes stronger and more stable. However, melt caused by intense warming, especially in the springtime, turns the snow wet and melts bonds between snow grains and snow layers. This can rapidly increase the threat of avalanches. A slope that is stable in the morning can become unstable by afternoon.

WHERE DO AVALANCHES OCCUR?
Avalanches occur on steep slopes that make for great skiing and snowboarding. Slope angle is the most important factor leading to avalanches. As the slope angle increases, so too does the stress exerted on the snow. Most avalanches (about 90%) release on slopes of 30-45 degrees. When the snow is very unstable it is possible to trigger an avalanche from shallow or even flat slopes below steep slopes. The fractures can start at the bottom of the slope and then propagate upslope, releasing the slab. It is akin to pulling out the bottom log from a woodpile.

Avalanches release most often on leeward slopes (facing away from the wind). Winds strip away snow from windward slopes and redeposit the snow as a slab layer on leeward slopes. The way a slope faces the sun is also important. In the Northern Hemisphere slopes facing away from the sun (northerly aspects) may have a colder and typically weaker snowpack, especially in early and mid-winter that is more apt to avalanche than southerly-facing slopes. However, south aspects (facing the sun) can be more prone to wet avalanches during spring and periods of thaw.

If the slope is steep enough and the snow weak enough, avalanches can run on any slope no matter how short or how long. Trees, bushes, and rocks tend to anchor the snow but do not necessarily prevent avalanches. Once an avalanche releases it can flow through trees and rocks. Most avalanches occur in the backcountry, outside developed ski areas. Within ski areas ski patrols use a combined program of monitoring, compaction, and explosives to provide safe and enjoyable conditions. However, off piste or out-of-bounds, there is no such program. By simply passing under a 6-millimeter strand of rope one goes from a safe and managed environment to the uncertainty of the wilderness.

“Avalanches occur on steep slopes that make for great skiing and snowboarding.”

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