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Avalanche Safety Information

An avalanche is a mass of snow sliding down a mountainside. Avalanches occur when the stress (from gravity) trying to pull the snow downhill exceeds the strength of the snow cover.

Types of Avalanches

  • Loose Snow: Loose-snow avalanches occur when there is little to no cohesion in the snowpack. Loose-snow avalanches usually start at one point on or near the surface. They gather more snow and momentum as they progress down the slope, often forming a triangular-shaped path. Snow clumps falling onto the slope from a cliff above or melting within the snowpack can set off these slides. Loose-snow avalanches can occur in dry or wet snow and in winter or summer. During winter, loose-snow avalanches usually happen during or soon after a snowstorm. In warmer months, loose-snow avalanches are usually caused by meltwater or rain.
  • Slab: Even more dangerous are slab avalanches where a layer of snow breaks away from the layer beneath and slides downhill. Most backcountry travelers are caught in slab-type avalanches. Slab avalanches are the result of many snowstorms or strong winds depositing layers of snow that change over time. Some layers settle and become stronger, while other layers become weaker. The weak layer is frequently grainy or fluffy so that other layers can’t bond to them. When the top layer of snow is not well bonded to the layer beneath and is disturbed by some kind of trigger, often a skier or climber, a slab avalanche can happen. Rather than starting at a single point on the surface like a loose snow avalanche, slab avalanches start out bigger and deeper, usually at a fracture line running across the top of the slide.

Factors That Contribute to Avalanches

Terrain

  • Slope: Pay attention to slope angles when you’re skiing or climbing because avalanches occur most frequently on slopes between 30 and 45 degrees. Slopes under 25 degrees and over 60 degrees have a low avalanche risk. Snow does not accumulate significantly on steep slopes and does not easily flow on flat slopes. Distribution of avalanches by slope has a sharp peak between 35 to 45 degrees. That peak hazard lies at around 38 degrees. Unfortunately, it is the slopes with the most dangerous slope are favorites for skiing. Staying on open slopes below 30 degrees is a good idea.
  • Profile: Convex slopes are more dangerous than concave. This lies partly in behavior and the tensile strength of snow layers versus the compression strength.
  • Direction: During winter, a south-facing slope is more stable since it has sun exposure to melt and condense the snow. But during spring and early summer, a south-facing slope can mean more melting and this can result in dangerous wet-snow slides. The attractive north-facing slopes that have all the best powder are also most likely to have unstable layers of dry, icy snow that does not stick well to the adjacent layers. Since these slopes don’t have the benefit of sun to warm and compact the snow over the winter, they tend to be less stable than the south-facing slopes. It is enticing to explore onto those powdery, north-facing slopes, but you could set off a slide if you did.
  • Terrain Hazards: Features where the snowpack is more likely to be unstable are convex slopes, cliff bands, boulders or trees where the snowpack breaks, wind-loaded lee slopes or beneath cornices. Avalanches are more common on slopes covered with grass surfaces than slopes with dwarf pines Try to avoid bowls, cirques, and depressions where snow could settle after a slide. Steep, narrow couloirs over 45 degrees collect snow. They can be a trap for hikers or skiers and leave them without escape routes.

Weather
Weather determines the evolution of snowpack. The most important factors are heating by solar sun, cooling by radiation, temperature gradients in snow, and snowfall amounts and type. Avalanches usually happen during or soon after a storm.

  • Precipitation: Snow is least stable during or immediately after snow or rain. Large snowfall in a short time is a sign that there could be a slide. Heavy snowfall of wet snow over lighter powder snow can create layers of instability. Rain causes instability through additional load on the snowpack. Rains tend to percolate through the snowpack and warm the deeper snow and also lubricate layers, which make them more likely to slide.
  • Wind: Anything more than a gentle breeze can contribute to rapid build up of snow on sheltered slopes Often, high winds pick up surface snow on one slope and deposit it on the other side where it can slide as a wind slab. “Wind slab” is a particularly fragile brittle structure heavily loaded, poorly bonded. Pay attention to the intensity and direction of the wind throughout the day.
  • Temperature: Rapid changes in temperature can cause changes in snow crystal formation and can lead to weak layers. Surface hoar, which is known to be a “weak link,” forms during clear, still nights when the temperature drops. In spring, rapid warming of a slope can lead to wet-snow avalanches.

Snowpack
Snowstorms build up layers of snow one after the other, all winter long. The structure of the snowpack layers determines the avalanche danger. If the layers of snow remain constant, there is a lower risk of an avalanche. It’s when snowfall of different consistencies pile on top of one another that the snowpack becomes hazardous. Loose, cold snow does not adhere on top of old snow that has been crusted over by the sun. There are many different snow crystal forms and each of these forms tells something about the stability of the snowpack. A snow crystal identification card can be helpful for study in the field.

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