Subnivean Temperatures

Nature Watch

By Susan Benson,
CNHM Director of Education

The subnivean space – perhaps it sounds like something out of a science fiction novel, something you might find on a journey to the center of the earth. However, it is not as mystifying as one might think. Quite simply, the subnivean space is the layer between the ground and snowpack. While the snow cover builds up, the ground gives off heat, creating water vapor that condenses and freezes under the bottom layer of snow, creating small spaces at ground level. Many small animals would never survive the winter cold without this space.

The minute a snowflake falls, it begins changing its shape. It can be affected by the internal snowpack characteristics and the external weather conditions. The first deterioration of a snowflake begins as a “destructive” change, in which the snow grains become more rounded. The unequal temperature distribution in the snowpack causes the water molecules to melt, changing the radiating arms of the snowflakes into a rough, spherical ice particle. These ice grains can then connect together, until they are roughly the same size. This metamorphosis occurs more quickly when air temperature is warmer, and is also influenced by wind or the weight of the snow above. It is this destructive metamorphosis that makes building snow shelters so effective.

The snowpack also is exposed to constructive metamorphosis, a process in which the temperature changes from the bottom to top of the snowpack is significantly different. The snow’s upper part is affected by the air temperature, and the lower part is warmed by the ground. Heat is conducted very slowly upward through the snow, affecting the water vapor distribution, and creating a 100% relative humidity in many of the air spaces in the snow. This process causes the ice crystals at the bottom of the snowpack to get continually smaller. Eventually, they form “depth hoar,” brittle, loosely arranged crystals that create easy movement for small mammals as they search for food under the winter snow.

The third factor which affects snow is the “melt metamorphosis”, in which happens when the snowpack is impacted by above freezing temperatures. Surface snow melt percolates down through the snowpack, encounters lower temperatures, and refreezes. As it freezes, it releases heat, bringing the entire snowpack to a more equal temperature. Rain and fog can also cause a similar effect.

Doesn’t the sun’s solar radiation warm the snowpack as well? We all know that spending a day in the snow in bright sunlight is shocking to our eyes, because snow is highly reflective of incoming solar radiation. Perhaps better than a mirror, snow reflects 75-95% of the sunlight hitting its surface. This means that a small amount of solar energy is available to raise the temperature of the snow. However, aging snow, as it accumulates dust, can decrease the sun’s reflection to as low as 45 percent.

It is easy to see that although complex processes occur, the snowpack provides amazing insulation for the winter survival of organisms. How much snow is enough? One theory is that 20 centimeters is a critical depth. The subnivean layer and its temperature and effectiveness for animal survival can also be affected by the type of snowpack metamorphosis that has occurred. By the time the snowpack reaches 40-50 centimeters, the subnivean environment temperature is almost constant.

Most importantly to our animal world, it is as if the snow peels away from the ground, leaving pockets where mice, voles, and shrews spend the winter. Some predators such as weasels live there too. Under that white snow in your yard there is an entire food chain still at work! Next time you are out for a ski, hike, or on a snowmobile trail, look at the edges of the trails to see if you can observe tunnels created by animals living in the snow.

Nature Watch is brought to you by the Cable Natural History Museum. For 40 years, the Museum has served as a guide and mentor to generations of visitors and residents interested in learning to better appreciate and care for the extraordinary natural resources of the region. The Museum invites you to visit its facility in Cable at 13470 County Highway M or on the web at www.cablemuseum.org to learn more about exhibits and programs.