Backyard Biology

Nature stories from my backyard and beyond

Nature Stories

Happy It Snowed

Saturday, February 9, 2013

Today, I am happy.

I am happy because it snowed last night.

I am happy because I am not inconvenienced by the snow. I do not need to drive in it. I do not need to shovel it. Today, I can sit by the fire, look out my window and just enjoy it.
The birds at my feeder appear to be happy too. They seem to dance as they flit back and forth from the feeders to the trees.
These birds that bring me so much pleasure as I watch them, get me to thinking. Are they in fact as happy as they appear? Do they really enjoy the snow as I do? Or is the snow just another one of winter’s hardships that they must endure if they are to survive?

For some animals, a blanket of snow presents real challenges. Food is harder to come by, since the seeds and ground plants that many animals eat are no longer available, being buried under the snow. Hunting becomes more difficult as well, since chasing prey in deep of snow is no easy task. As anyone who has gone for a walk in snow knows, even just moving around takes more energy. More energy used means more food needed, food that is already hard to come by.

For other animals, a deep snow bank is a blessing, helping to insure survival through the cold winter. A thick blanket of snow is a good insulator, and many insects, amphibians and reptiles are able to keep just warm enough under the snow on a bitterly cold winter day to prevent their freezing while they hibernate.

One animal in my back yard must be very delighted with the snow. That animal is the meadow vole. Meadow voles are also known as field mice, but they are quite different in appearance. Voles have small ears that are usually hidden by their thick fur, short slightly furry tails and short legs. However, one does not often get a good look at a vole, because to be seen is usually to be eaten. Meadow voles are preyed upon by just about every predator large enough to catch one. Therefore, hiding is the name of the game for the voles. And underneath a snow bank is a great place to hide.

Meadow voles are active all year, day and night They make their nests in clumps of grass or in underground burroughs. Connecting their nests are runways, or paths of close-cropped grass, often with a grassy overhang. Running along these paths while moving from one nest to another or while in search of food is a risky business for the vole. So a bank of snow covering the runway offers a good deal of security.

One might imagine that an animal as small as a vole would have a difficult time tunneling through the snow. However, judging by the number of tunnels I find in my back yard in the winter, they seem to do just fine. One thing that may help them is the changes that begin to occur to the snow as soon as it has fallen.

*The first of these changes is called “destructive metamorphism”, a reordering of the watersnowflake molecules, and thus the overall shape of the snowflake. The water that makes up the fine needle-like arms of the snowflake migrates towards its center, creating a more spherical ice crystal. This causes the snow to become more densly packed. It is easy to observe this process. Just catch a snowflake and watch it for a short time. You should be able to see the spikes of the snowflake shrink and the center of the snowflake grow. This happens much faster when two snowflakes come in contact with each other.**

The next step in the metamorphosis of the snow, and the one that may help the voles and other small creatures that move under the snow, is called “constructive metamorphism”. Constructive metamorphism occurs because of the temperature difference between the bottom of the snowbank at ground level and the upper levels of the snowbank. As mentioned above, snow is a good insulator. In the graph below, the green dotted line represents the temperature at ground level under a foot of snow, and the solid blue line represents the air temperature above the snow, for a seven week period in the winter of 2008. As you can see, the air temperature is, on average, about ten degrees colder than the temperature under the snow.


To understand what happens during constructive metamorphism, it is helpful to review a little physics.

  1. Water comes in three forms; solid (ice or snow), liquid (water) or gas (water vapor).

  2. Heat ice and it turns to water. Heat water and it turns to water vapor. But ice can skip the water phase and go directly from ice to water vapor in a process called sublimation.

  3. Air can only hold so much water vapor. The amount of water vapor that is actually in air compared to how much the air can possibly hold is called the relative humidity. Air that is holding the maximum amount of water vapor is at 100% humidity. Add any more water vapor to the air and some of it will condense, - turning back to water or ice - depending on the temperature.

  4. Warm air can hold more water vapor than cold air. If both the warm air and cold air are at 100% humidity, holding as much water vapor as possible, the warm air holds more water vapor.

  5. Just as water seeks its own level, so too does water vapor. If there is more water vapor in one area than in another, it will flow from the area of high concentration to low concentration until the amount of water vapor is equal throughout.

Now that we have reviewed the physics, let’s see how this affects the snow, and, the vole.

Some of the water that makes up the snow grains are continuously sublimating - turning into water vapor ( see 2 above). The air spaces between the snow grains very quickly reach 100% humidity (3). But since the snow near the ground is warmer than the snow at the surface, the air spaces near the ground have more water vapor than the air in the snow near the surface, even though both air spaces are at 100% humidity (4).
Since there is more water vapor in the air close to the ground than higher up, some of the water vapor migrates upwards to equalize the concentration (5). Once this happens, the relative humidity of the air spaces at the top of the snow bank is greater than 100% so some of the water vapor will condense, turning back to ice (3), keeping the relative humidity at 100%. The relative humidity at the bottom of the snow bank, on the other hand, is now less than 100%, since some of the water vapor migrated to the top of the snow bank. This allows more of the snow grains to sublimate, bringing the relative humidity back up to 100%.

As this process continues, the air spaces at the bottom of the snow bank become larger, making it easier for voles (and other small animals) to move around. They can now go about the business of making their runways, feeding, breeding and doing all the things that voles do, hidden from the predators above. This must make for a very happy vole. However, while enjoying life under the snow, the voles must still beware of predators. These predators may not be able to see the voles, but they may still hear them!

*For a more complete description of the changes in snow, see Life in the Cold by Peter Marchand

** see the nature activity, Catching Snowflakes



Watch this video of a fox hunting a vole under the snow.