Snow - The Basics

Snow

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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.