from Doug Latimer, Rocky Mountain Hiking
Snow
Snow can fall in many different forms, but in general it is usually some variation
of the classic six arm flake. It usually snows in fairly mild temperatures which
helps promote settlement. Snow changes over time through a process called sublimation.
Sublimation is the change of a solid directly into a gas without first becoming
a liquid. With mild temperatures water vapor travels from the arms of the crystal
to refreeze at the center. This produces a rounding of the snowflake. These rounds
will often begin to form necks and bond to each other. The process is sometimes
called destructive metamorphosis. This will generally produce a good cohesive
layer of snow. The colder the temperature, the longer this process takes.
In a cold shallow snowpack, like early season in the Rockies, another process
is a work changing the crystals. When it's cold and the snowpack is thin ( a
change of 10 degrees Celsius or more per metre of snow) a different form of
recrystalization occurs. Water vapour recrystalizes at the bottom of the snowpack
where it is warmest, into sharp angular faceted crystals. The longer and more
severe the temperature gradient, the more extensive the faceting. Facets will
further recrystalize into grooved (or striated) crystals of depth hoar. Facets
and depth hoar tend to be less cohesive and more unsupportive within a snowpack.
In short facets and depth hoar tend to weaken the strength of the snowpack from
the bottom up.
Crusts
Sun and light rain can produce thin crusts on the surface of the snow. Successive
snowfalls may not bond well to the buried crusts, and form potential sliding layers
for an avalanche.
Rain
Rain will initially weaken the strength of the snow and increase its mass. This
will greatly increase the risk of an avalanche. When the water has frozen, it
may strengthen the snow, especially if it has percolated deep into the snowpack.
This can produce a very dangerous snowpack when wet, and a very safe snowpack
when it has frozen solid.
Surface Hoar
Surface hoar is the formation of crystals on the snow surface from condensation
during cold periods. These crystals look much like ice feathers. Surface hoar
usually bonds very poorly to snow, causing a sliding layer in the snowpack as
it is buried by successive storms. This is a very stable crystal and can remain
unchanged for long periods of time.
Melt Freeze Cycle
Late in the season as the snow melts, it often refreezes overnight. Wet corn snow
with no support is unstable, and dangerous. If it freezes solid, it becomes very
stable. This cycle can occur daily in the spring.
Wind
Wind can transport snow during and after a storm. This can reduce the amount of
snow on windward slopes and accumulate it on lee slopes. Wind is a major factor
in the development of unstable slabs on lee slopes.
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