Curious Nature: The dangers of layers in the snow
January 27, 2013
As you look at the lovely blankets of undisturbed snow on our local mountains and outside the ski area boundaries, you may not know that our snow pack is made up of a variety of different layers, created by different weather conditions.
If you have been reading the Vail Daily lately or staying in tune to other local media sources, you have no doubt heard about the lingering avalanche danger caused by persistent weak layers in the snow pack.
Just recently, the second unfortunate avalanche fatality of the season occurred in Colorado; our state statistically experiences the highest number of annual fatalities in the United States. These persistently weak layers in the snow pack are often the culprit, and if you recreate in snow, it is to your benefit to have some understanding of what causes them.
It is typical for the Colorado snowpack to have a weak layer of snow near the ground, commonly called sugar snow.
The early winter season in Colorado is conducive to the formation of sugar snow, due to prolonged cold periods and a shallow snowpack. As the early season snowpack builds, there is typically a temperature gradient between the ground, which remains at a fairly constant temperature of 0 degrees Celsius, and the air temperature above the snowpack, often well below freezing.
This gradient causes water vapor to migrate upwards in the snowpack. Water has the ability to sublimate, meaning it can pass from a solid state, such as a snow grain, directly to a vapor state, omitting the liquid phase.
Water vapor travels from warmer areas near the ground, where the pressure is greater, to colder areas higher in the snowpack, where the pressure is less. This same process is visible when you see steam rising above boiling water.
When a temperature gradient larger than 1 degree Celsius per 10cm of snow exists, faceted snow will form in the snowpack. Faceted snow grains, or sugar snow, are square and angular-shaped grains that do not bond well together.
These square crystals are easy to identify in an area where the snow has been undisturbed. Simply dig down to the ground and you will find a handful of it. This process does not reverse itself during the winter months, and this layer of snow near the ground lingers.
This lingering layer continues to be a hazard throughout the season. Even as the snowpack gets deeper, such as what we experienced in December, the weak layers that formed in November remain. With prolonged dry and cold periods, such as what we are now experiencing, many locations, especially on northern aspects that receive less solar radiation, have metamorphosed completely into faceted snow.
You can still venture safely into the backcountry, provided that you stay on low angle slopes.
Generally, slopes that are 30 to 45 degrees are most prone to sliding. Gravitational force on slopes less than 30 degrees are typically not enough for them to slide, and slopes over 45 degrees tend to slide more often, which can lessen their hazard. Keep in mind that these are rules of thumb and that exceptions do exist, such as wet and dense snow that can slide on slopes less than 20 degrees.
At this point, you may be thinking that you need a better understanding of snow physics to be able to judge the snowpack. Simple resources do exist that can help you assess the weak layers in the snow and the subsequent avalanche hazard.
You can take an avalanche course, offered through Colorado Mountain College and local guide services, or you can utilize the Colorado Avalanche Information Center (CAIC) website (avalanche.state.co.us/index.php),
which provides a daily weather and avalanche hazard forecast.
It can be a lot of fun to read it on a daily basis and to keep track of the weather and snowpack history. CAIC’s website divides the state into various zones, and the Summit-Vail zone report emphasizes the persistent weak layers that exist in the snowpack. Remember, these layers will be buried during the next storm cycle, but will continue to contribute to instability.
So, layers in the snow can be a friend or foe.
Weak layers are clearly foes for humans wanting to venture onto steep slopes; however, they allow for animals to move freely in the subnivean (under the snow) zone so they can survive the harsh winter. It is really just a matter of perspective.
Markian Feduschak is the executive director of Walking Mountains Science Center, and can be often found in the backcountry analyzing layers in the snow using the base of his skis.