Sunday, December 11, 2011

A mid-continent shallow snowpack

Went snow shoeing this weekend near the summit of Red Mountain Pass where we were a bit disappointed with the snow depth and quality of the snow. However, I admit that I am a snow snob and expect quite a bit when I am recreating on snow.

The snow is always dry and light here, but today it was dry, light and bottomless. Any step off of the packed trail would send our snow shoes to the ground (albeit that was not far away). The snow crystals looked very much like the image below, large facets not anything like their original snow flake self. This is what happens with a shallow snowpack.

Snow crystals on a pine tree after a little bit of metamorphism.
This winter saw a very nice snowfall of at least 50 cm, a few weeks ago, but since then hardly anything. The weather, clear and cold each day and it can get cold at 10,000 feet. In fact, when we started that morning the temperature at the car was 9 F.

The metamorphosis of snow crystals depends on a few factors, not the least of them is the temperature gradient through the snowpack. What we were seeing was a large gradient. The bottom of the snow pack at the ground is usually thought to be at 0 C while the top of the snowpack is equal to the air temperature. Lately we have seen temperatures approaching -25 F or -31 C. Making a gradient of 32 C over the depth of the snow which originally was 50 cm. It seems that if the gradient is smaller than 10 C/m then the metamorphosis of snow will lead to a more rounded grain of snow that can connect with its fellow snow grains easily making for a more stable snow pack while gradients greater than 10 C/m will result in a more faceted grain which will not connect all that well with its fellow snow grains. Our gradient of 32C/0.5 m is certainly within the this-snow-grain-does-not-play-well-with-others category.

A fantastic picture of a snow flake "growing" a large facet. Image courtesy of the Utah Avalanche Center
It seems that the newly fallen snowflake will sublimate small amounts of water vapor from the tips of the crystal arms (where the vapor pressure is greater) . If the temperature gradient is small enough (<1 C/cm) then the vapor will re-condense in the areas between the arms (where the vapor pressure is less) effectively making the snow flake into a round snow grain. If the temperature gradient is too large (as in the example) then the water vapor will sublimate from the tips of the crystal arms and re attach onto a cold region of a nearby snowflake creating the large faceted crystal. These crystals will not connect with its fellow snow grains well creating an unstable snow pack.

This is the problem with the early season snow followed by a week or so of good weather. The snow pack is now made of a bunch of grains that will make a poor base for the coming winter.

Stay tuned for any avalanche reports as the snow pack starts to pile up. But, perhaps the La Nina will keep the snow depths to a minimum and we can worry about drought instead of snowslides.

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