Chapter 1

SLOPE STABILITY
1-125. Slope stability is the key factor in determining the avalanche danger.

Slope Angle
1-126. Slopes as gentle as 15 degrees have avalanched. Most avalanches occur on slopes between 30 and
45 degrees. Slopes above 60 degrees often do not build up significant quantities of snow because they are
too steep.

Slope Profile
1-127. Dangerous slab avalanches are more likely to occur on convex slopes, but may occur on concave
slopes.

Slope Aspect
1-128. Snow on north facing slopes is more likely to slide in midwinter. South facing slopes are most
dangerous in the spring and on sunny, warm days. Slopes on the windward side are generally more stable
than leeward slopes.

Ground Cover
1-129. Rough terrain is more stable than smooth terrain. On grassy slopes or scree, the snow pack has
little to anchor to.

TRIGGERS
1-130. Various factors trigger avalanches.

Temperature
1-131. When the temperature is extremely low, settlement and adhesion occur slowly. Avalanches that
occur during extreme cold weather usually occur during or immediately following a storm. At a
temperature just below freezing, the snowpack stabilizes quickly. At temperatures above freezing,
especially if temperatures rise quickly, the potential for avalanche is high. Storms with a rise in temperature
can deposit dry snow early, which bonds poorly with the heavier snow deposited later. Most avalanches
occur during the warmer midday.

Precipitation
1-132. About 90 percent of avalanches occur during or within twenty-four hours after a snowstorm. The
rate at which snow falls is important. High rates of snowfall (2.5 centimeters per hour or greater),
especially when accompanied by wind, are usually responsible for major periods of avalanche activity.
Rain falling on snow will increase its weight and weakens the snowpack.

Wind
1-133. Sustained winds of 15 miles per hour and over transport snow and form wind slabs on the lee side
of slopes.

Weight
1-134. Most victims trigger the avalanches that kill them.

Vibration
1-135. Passing helicopters, heavy equipment, explosions, and earth tremors have triggered avalanches.

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TC 3-97.61

26 July 2012