Geology Reference
In-Depth Information
A well-defined fracture line occurs where the moving
snow breaks away from the stable snow. In slab
avalanches snow crystals tend to stick together. Angular
blocks or chunks of snow may be part of the slide.
According to Perla and Martinelli (1978), the slab may
range from 100 to 10,000 square meters in area and from
0.1 to about 10 m thick. Slab avalanches are the more
dangerous of the two types.
Avalanches are the result of both the underlying
terrain and the conditions of the snowpack. This exer-
cise concentrates on terrain factors, including slope
steepness, slope profile, slope aspect, and ground cover.
The relevant terrain factors, as described by the
U.S. Forest Service, are:
Slope steepness.
Avalanches most commonly release from
slopes of 30 to 45 degrees (60 to 100 percent), but may
begin on slopes ranging from 25 to 65 degrees (45 to 215
percent). LaChapelle (1985) notes that many avalanches
begin on slopes of 35 to 40 degrees (70 to 85 percent).
Slope profile.
Dangerous slab avalanches are more likely
to begin on convex slopes but may also begin on concave
slopes. Short slopes may be as dangerous as long slopes;
42 percent of all avalanche fatalities result from slides
with a slope distance of less than 300 ft (100 m).
Slope aspect.
Snow on north-facing slopes may be
slower to stabilize than snow on slopes that face other
directions. South-facing slopes are especially dangerous
in the spring due to solar heating. Leeward (downwind,
or the direction the wind is blowing toward) slopes are
dangerous because they accumulate wind-deposited
snows that add depth and may create unstable slabs of
snow. Windward (upwind, or the direction the wind is
coming from) slopes generally have less snow; the snow
is more compacted and usually more stable than snow
deposits on leeward slopes.
Ground cover.
Large rocks, trees, and heavy brush help
anchor the snow. Smooth, open slopes are more
dangerous, but avalanches can start even among trees.
Old slide paths and recent avalanche activity.
Generally,
avalanches may repeatedly occur in the same areas.
Watch for avalanche paths. Look for pushed-over small
trees, trees with limbs broken off. Avoid steep, open
gullies and slopes. If you see new avalanches, suspect
dangerous conditions. Beware when snowballs or
"cartwheels" roll down the slope.
If you are planning to travel in avalanche coun-
try, you should also be aware of the impact of different
weather conditions on the occurrence of avalanches.
Some of these factors, as described by the U.S. Forest
Service, are:
Old snow.
When the old snow depth is sufficient to cover
natural anchors, such as rocks and brush, additional snow
layers will slide more readily. The nature of the old snow
surface is important. For example, cold snow falling on
hard, refrozen snow surfaces, such as sun or rain crusts,
may form a weak bond. Also a loose under-lying snow
layer is more dangerous than a compacted one. Check
the underlying snow layer with a ski pole, ski, or rod.
Wind.
Sustained winds of 15 mi/hr and over may
cause avalanche danger to increase rapidly even
during clear weather, if loose surface snow is available
for the wind to transport. Snow plumes from ridges
and peaks indicate that snow is being moved onto
leeward slopes. This can create dangerous conditions.
Storms.
A high percentage of all avalanches occur
shortly before, during, and shortly after storms. Be
extra cautious during these periods.
Rate of snowfall.
Snow falling at the rate of 1 in/hr or
more increases avalanche danger rapidly.
Crystal types.
Observe general snow-crystal types by
letting them fall on a dark ski mitt or parka sleeve.
Small crystals—needles and pellets—often result in
more dangerous conditions than the classic, star-
shaped crystals.
New snow.
Be alert to dangerous conditions with a foot
or more of new snow. Remember that new snow depth
may vary considerably with slope elevation, steepness,
and direction.
Temperature.
Cold temperatures will maintain an
unstable snowpack while warm temperatures (near
freezing) allow for snow settlement and increasing
stability. Storms starting with low temperatures and
dry snow, followed by rising temperatures and
wetter snow, are more likely to cause avalanches. The
dry snow at the start forms a poor bond to the old
snow surface and has insufficient strength to support
the heavier snow deposited late in the storm. Rapid
changes in weather conditions (wind, temperature,
snowfall) cause snowpack changes. Therefore, be
alert to weather changes. Snowpack changes may
affect snow stability and cause an avalanche.
Temperature inversion.
Higher temperatures with higher
elevations can occur when warm air moves over cold
air, which is trapped near the ground and in valleys.
This weather situation may produce dramatic variations
in local snow stability.
Wet snow.
Rainstorms or spring weather with sunny
days, warm winds, and cloudy nights can warm the
snow cover. The resulting free and percolating water
may cause wet snow avalanches. Wet snow avalanches
are more likely on south slopes and slopes under
exposed rock.
The U.S. Forest Service suggests that "the safest
routes for travel in avalanche terrain are on ridgetops
and slightly on the windward side of ridgetops, away
from cornices (accumulations of drift snow on the lee-
ward side of the crest of a ridge). Windward slopes are
usually safer than leeward slopes. If you cannot travel
on ridges, the next safest route is out in the valley, far
from the bottom of slopes. Avoid disturbing cornices
from below or above. Gain access to ridgetops by
detouring around cornice areas ....
If you must cross dangerous slopes, stay high
and near the top. If you see avalanche fracture lines in
the snow, avoid them and similar snow areas. If the
snow sounds hollow, particularly on a leeward slope,
