Slope Aspect
with respect to the sun:
The direction a slope faces with respect to the sun (aspect)
has a profound influence on the snowpack. It often takes several
years of experience in avalanche terrain before most people
appreciate the importance of aspect.
If you don’t know your north from south, then you had
better learn, because someone who doesn't know the aspect has
missed one of the most important pieces of the avalanche puzzle.
Buy a compass. Use it often and work on developing an intuitive
feel for slope aspect. No excuses on this one.
The influence of aspect with respect to the sun is most important
at mid latitudes, say from about 30 degrees to around 55 degrees--from
about the southern U.S. border to about the northern British
Columbia border. At equatorial latitudes, the sun goes almost
straight overhead, which shines equally on all slopes. At arctic
latitudes, in the winter, the sun is too low on the horizon
to provide much heat and when it finally gets high enough in
the spring and summer, it just goes around in a big circle anyway,
shining on all the aspects with nearly the same intensity. Thus,
in the arctic spring, aspect has some influence but not nearly
as significantly as in mid latitudes. Therefore, the importance
of aspec is primarily at mid latitudes.
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At mid latitudes in
the northern hemisphere:
• North facing slopes receive very little heat
from the sun in mid winter. Conversely, south facing slopes
receive much more heat. Therefore, a north facing slopes
will usually develop a dramatically different snowpack
than a south facing slope.
• South facing
slopes tend to be warmer and often develop thin ice crusts.
Because these crusts tend to grow weak layers around them
from near-surface faceting, be careful not to assume southerly
aspects are safer.
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• How about east and west? East facing slopes
catch sun only in the morning when temperatures are colder
while west facing slopes catch the sun in the warm afternoon.
Consequently, east facing slopes are colder than west
facing slopes.
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• A cold snowpack tends to develop more
persistent weak-layers than a warm snowpack A cold snowpack
commonly develops notoriously fragile weak-layers such as facets
and surface hoar. Largely because of this, the lion's share
of avalanche accidents occurs on north and east facing slopes,
partly because that is where we find the best snow and people
tend to trigger more avalanches there, but mostly because they
exhibit more persistent weak layers.
• In wet
snow conditions due to strong sun, it's just the opposite of
a dry snowpack: south and west facing slopes will usually produce
more wet avalanches than the more shady slopes.
•
During prolonged cloudy or stormy conditions when the sun seldom
shines on the snow, there will be very little difference between
sunny and shady slopes.
• Remember that in the Southern Hemisphere it's just the
opposite. South facing slopes are colder than north facing ones.
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Human Factors:
Seemingly, subtle differences in slope aspect can have a huge
effect on the stability of the snow. I can't count the number
of accidents I have investigated in which people start snowmobiling,
skiing or snowmobiling in a bowl on a safe aspect, but as they
use up the snow, they not only gain confidence, but they tend
to slowly work their way around the bowl onto the progressively
more dangerous aspects, until someone finally triggers an avalanche.
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