by Stan Tekiela
February 7, 2003
Photo by Stan Tekiela©
I am often asked about how the harsh winter weather is affecting the local birds and mammals,
and this winter is no exception. So here is the answer.
Harsh winter weather is good for some animals and bad for others. For small animals like mice, voles and moles, a
harsh winter means a good life—under the snow. Living under the snow is called subnivean. And it’s a surprisingly
comfortable for these small critters. Consider this, the temperature under the snow rarely fluctuates more than a
few degrees and the cold biting winds never blow.
Some plants stay green all winter under the snow providing a source of food for plant eating mice. In addition
there are many seeds and nuts left over from last autumn that are easily accessible under the snow. In addition
to comfortable living conditions and a good food supply, living under the snow pack also offers protection from
predators like hawks, foxes and owls who have a hard time diving through the deep snow to catch them. What’s
good for one animal is usually bad for another. It’s the balance of nature.
If you dig down through the snow to the ground you will see several different types or layers of snow. At the
top will be the light and fluffy snow. Midway down the snow appears like small clusters of ice—kind of like crushed
ice. At the bottom near the earth, the snow turns into larger ice clusters—appearing much like the ice cubes in
your freezer. The animals have no troubles tunneling through the jumble of ice cubes. It’s certainly easier than
tunneling through frozen soil.
The different layers of ice form due to the accumulated weigh of the snow above. As the snow piles up the weight of the
snow compacts the snow beneath. When snow compresses, the individual ice crystals push against each other and melt slightly.
The ice crystals stick together to form clumps of ice.
Now, if you could measure the temperature in a cross section through the snow pack you would find that the coldest
temperature would be just above the snow. Due to a very complicated process, much to lengthily to describe here, the
snow looses radiant energy creating the coldest temperatures just a few inches above the snow. Above this layer of
extra cold air the temperature warms to the ambient air temperature. That is why we don’t measure the air temperature
next to the snow.
Going down from the top of the snow pack the story is very different. Depending upon the depth of the snow (the deeper
the snow the better the insulation and the warmer the ground temperature) the temperature increases, sometimes dramatically
if the air temperature is very cold. At ground level the temperature is usually right around 32 degrees F. On a very cold
night there can be a 30 to 40 degree temperature gradient from ground level to the air temperature above the snow pack.
(None of this holds true if there is no snow or only a few inches.) Are you starting to see that a subnivean life under
the snow isn’t so bad?
Here is another interesting snow phenomena. Have you ever noticed how the snow becomes hollowed-out around the base of a
tree trunk? Many people mistakenly believe that this is caused by the wind. Actually the depression is cause by melting
of the snow, but not the melting your probably envisioning right now.
During the day the sun emits a strong short-wave, non-heat energy that is absorbed by the mass of the trees. After the sun
goes down the trees radiate back a long-wave heat energy that melts the snow directly around the base of the tree. But again,
not a typical melting. (Right about now your probably wishing you paid more attention in your physics class). Anyway, the
reason you don’t see water puddles in the bottom of the depression is the snow “melts” directly into water vapor in a
process called sublimation. During sublimation, the snow “melts” or changes from a solid directly into water vapor
and is carried away on the air. The air temperature never gets warm enough to allow the snow to change into water.
The next time you are out shoveling the driveway think about all the little critters that are running around under all that
snow and look for the depressions around the bases of your trees. There is a lot more to winter than meets the frozen eye.
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