Avalanches

Avalanches are the number-one hazard facing snow travelers. Avalanche hazard and the conditions that create it are fairly easy to recognize, though there's no way to predict an individual avalanche. Whenever the strength and cohesion of snow on a slope are no longer enough to support the accumulated weight of the snow, an avalanche is due.

Evaluating the stability of a specific snow pack is highly technical work, beyond the scope of this book. A little knowledge about digging snow pits or studying snow crystals sometimes does nothing but give climbers a false sense of security as they rely on an evaluation that may be little more than guesswork. However, climbers do need to leam to recognize generally hazardous terrain and weather.

Four out of five avalanches strike during and just after a storm. Danger increases rapidly with snowfall of an inch or more per hour, or accumulations of a foot or more. Storms starting with low temperature and dry snow, followed by rising temperatures, are even more likely to set up avalanche conditions. The dry snow forms a poor bond, without strength to support the heavier snow that falls later in the storm. Rainstorms, or spring weather with warm winds and cloudy nights, can result in water percolating into the snow and causing wet-snow avalanches. Rapid changes in temperature and wind increase avalanche danger.

Be prepared for two types of avalanches.

A slab avalanche has an obvious fracture line at the top (fig. 12-42). The avalanche is a large area of snow that begins to move suddenly at the same time, sliding on a weak layer within the snowpack or on the ground. The snow in a slab

Snow Avalanche
Fig. 12-42. Slab avalanche

avalanche can be from soft to hard, from wet to dry.

A loose-snow avalanche, on the other hand, starts at one point and grows in size as it slips

Snow Avalanche

Hazard Evaluation Summary

Note: Bold type indicates the most critical factors in the evalution.

1. COULD THE SLOPE PRODUCE AVALANCHES? Is the slope steep enough to slide?

□ Variations in incline — cliff bands What's the orientation of the slope to wind?

D Lee slopes What's the orientation of the slope to sun?

□ S and SW slopes exposed to strong radiation.

□ First major thaw of spring What's the nature of the slope?

□ Weak areas that might initiate failure

2. IS THE SNOW STABLE? COULD IT FAIL? How deep is the snowpack?

Are there any signs of avalanche activity? What layers are there in the snowpack?

□ Very hard or soft layers

□ Weak bond between layers

□ 30 cm or more of snow above a weak layer

How much snow has fallen?

□ New snow depth greater than 30 cm

□ Slow settlement of new snow

□ Very light or very heavy snow

□ Heavily rimed crystals, graupel

□ Heavy stiff layer above a light weak layer

□ Very easy or easy shovel test on new snow layer

Wind

□ Moderate or strong wind

LJ Cracking and settling of snow Air temperature

□ Rapid rise in temperature

□ Above freezing temperatures

□ Sun on slope under consideration

□ Temperature inversions Humidity

□ High relative humidity during snowfall or during periods of moderate and strong wind.

3. WHAT WILL HAPPEN TO ME IF THE SLOPE AVALANCHES?

Depth of avalanche

□ Deep weak layers in snowpack

□ Foot penetration greater than 60 cm Type of avalanche n Stiff slab above weak layers

□ Slope has not yet avalanched recently

Terrain

Long open slope above

Restricted deposition zone

Drop-off below

Trees to wrap around

4. WILL

CONDITIONS GET WORSE?

Continuing snowfall

Increasing temperature

Strong wind

Fig. ¡2-44. Hazard evaluation summary (reproduced by permission of the author from Avalanche Safety for Skiers & Climbers by Tony Daffern, Rocky Mountain Books, 1983)

downhill (fig. 12-43). The snow has little internal cohesion. There is no obvious fracture line and no clear layer where the sliding snow separates from snow beneath. A loose-snow avalanche can be wet and loose or soft and dry.

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