Mountain Terrain, Weather, and Hazards

SNOW PITS
1-136. Snow pits can be used to determine slope stability.

Dig the snow pit on the suspect slope or a slope with the same sun and wind conditions. Snow
deposits may vary greatly within a few meters due to wind and sun variations. (On at least one
occasion, a snow pit dug across the fall line triggered the suspect slope). Dig a 2-meter by
2-meter pit across the fall line, through all the snow, to the ground. Once the pit is complete,
smooth the face with a shovel.

Conduct a shovel shear test.
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A shovel shear test puts pressure on a representative sample of the snowpack. The core of
this test is to isolate a column of the snowpack from three sides. The column should be of
similar size to the blade of the shovel. Dig out the sides of the column without pressing
against the column with the shovel (this affects the strength). To isolate the rear of the
column, use a rope or string to saw from side to side to the base of the column.
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If the column remained standing while cutting the rear, place the shovel face down on the
top of the column. Tap with varying degrees of strength on the shovel to see what force it
takes to create movement on the bed of the column. The surface that eventually slides will
be the layer to look at closer. This test provides a better understanding of the snowpack
strength. For greater results you will need to do this test in many areas and formulate a scale
for the varying methods of tapping the shovel.

Conduct a Rutschblock test. To conduct the test, isolate a column slightly longer than the length
of your snowshoes or skis (same method as for the shovel shear test). One person moves on their
skis or snowshoes above the block without disturbing the block. Once above, the person
carefully places one showshoe or ski onto the block with no body weight for the first stage of the
test. The next stage is adding weight to the first leg. Next, place the other foot on the block. If
the block is still holding up, squat once, then twice, and so on. The remaining stage is to jump up
and land on the block.

TYPES OF SNOW AVALANCHES
1-137. There are two types of snow avalanches: loose snow (point) and slab.

Loose snow avalanches start at one point on the snow cover and grow in the shape of an inverted
“V.” Although they happen most frequently during the winter snow season, they can occur at
any time of the year in the mountains. They often fall as many small sluffs during or shortly after
a storm. This process removes snow from steep upper slopes and either stabilizes lower slopes or
loads them with additional snow.

Wet loose snow avalanches occur in spring and summer in all mountain ranges. Large
avalanches of this type, lubricated and weighed down by meltwater or rain can travel long
distances and have tremendous destructive power. Coastal ranges that have high temperatures
and frequent rain are the most common areas for this type of avalanche.

Slab avalanches occur when cohesive snow begins to slide on a weak layer. The fracture line
where the moving snow breaks away from the snowpack makes this type of avalanche easy to
identify. Slab release is rapid. Although any avalanche can kill you, slab avalanches are
generally considered more dangerous than loose snow avalanches.
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Most slab avalanches occur during or shortly after a storm when slopes are loaded with new
snow at a critical rate. The old rule of never travel in avalanche terrain for a few days after a
storm still holds true.
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As slabs become harder, their behavior becomes more unpredictable; they may allow
several people to ski across before releasing. Many experts believe they are susceptible to
rapid temperature changes. Packed snow expands and contracts with temperature changes.
For normal density, settled snow, a drop in temperature of 10 degrees Celsius (18 degrees
Fahrenheit) would cause a snow slope 300 meters wide to contract 2 centimeters. Early ski
mountaineers in the Alps noticed that avalanches sometimes occurred when shadows struck
a previously sun-warmed slope.

26 July 2012

TC 3-97.61

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