How we define cold depends a good deal on where we live. In the northeast US, we complain when the temperature gets down into single digits (Fahrenheit.) In Canada, they don't complain much until it gets into the negative numbers. But Key West Florida thinks an arctic front has descended upon them if it's below 60 F. Most people though agree that a warm fire is a great place to spend a cold evening. Many birds and some insects simply migrate south to escape the cold. Many animals, living below the permafrost, go to ground for warmth. How well, and why, does it work?

     While some animals such as bears hibernate, many of them stay active throughout the winter. Smaller animals lose heat more rapidly than larger animals and it can be quite a challenge for them to keep warm. Underground burrows are quite warm keeping the animal comfortable. Burrows are a great shelter because snow and soil have insulating properties. While snow is colder than freezing, the air spaces between the crystals can trap warmer air and acts as insulation. In deep snowfall, the temperature of the snow at the top may be well below freezing while the snow closest to the ground is barely below freezing.

     Soil acts much the same way. The upper layers may freeze solid but it does not penetrate very deeply. The factors which determine how deep the soil freezes are surface air temperature, length of cold season, amount of snow cover, soil moisture content and type of soil. The lighter the soil (in density, not colour) the better an insulator it is. Rocky soils are not good insulators. Heavy foliage or leaf content on the surface will also add to it's insulating properties.

     In some areas, where the soil is deep (as opposed to a shallow rock layer) if you were to burrow down three to six feet, you might well find that the range in temperature throughout the entire year is only a few degrees, whereas the surface temperature might vary by 100 degrees! This situation allows an animal to live in relative comfort.

     For animals which hibernate, temperature is far more critical and a few degrees can cross the line from life to death. While in hibernation, an animal reduces it's energy usage to a fraction of what it would expend while awake. As an example, there is a species of ground squirrel which reduces it's energy consumption to only two percent of normal while hibernating. Since the animal does not take in food while hibernating, it must rely on burning fat reserves to keep that spark of life going. If the temperature gets too low for too long, the animals will run "out of fuel" and they could well freeze to death. Cold robs the body of heat and makes the body burn more fuel to keep the temperature of the body up. By hibernating in an area where there is little air movement and temperature variation, the animal increases it's chances of surviving the winter.

     Some creatures depend on snow for warmth. Grouse will burrow into a snow bank for warmth when it gets extremely cold. Other animals will utilize the snow to enhance their burrow's insulation. Many mice and other small animals will create a tunnel under the snow between their burrows and food storage sites which provides them a nicely protected pathway out of the surface cold and wind. Such tunnels will also provide ventilation for the burrow without letting in too much cold air.

     It may appear that a forest is empty and unmoving in the winter, while under the snow and soil another world remains alive waiting for spring.