Snow is one of the most amazing insulation materials in nature. It limits heat exchange. It stabilizes temperature. It even muffles sound!
The average insulating R-value of snow is about R-1. That’s in the same ballpark as wood chips (R-1) and straw bales (R-1.5) and a lot better than plain wood (0.75) or brick (R-0.2). Inch for inch, snow’s about half as good as those rigid fibreglass panels you see everywhere (R-2.5), but the snow’s usually thick enough to make up the difference. Of course, that amount of snow on a flat roof that’s not built for it is going to cause all kinds of other headaches.
Where you really see the amazing insulation properties of snow in action is in the igloo. Without the insulation properties of snow, the Inuit could never have survived in the harsh Arctic wilderness. Even in howling winds at 40 below zero, a properly built igloo with at least one person inside is never colder than -7 degrees Celsius and can be as warm in places as 16 degrees Celsius, and that’s from body heat alone. That’s the equivalent of a balmy spring day!
The secret of snow is its molecular structure. When water freezes into ice crystals, a lot of air is trapped in its lattice structure. Trapped air is always good for insulation, especially when it’s in the form of tiny pockets of trapped air.
The same air pockets also make snow a great sound insulator. Sound vibrations are dampened by those air pockets, which is what makes everything sound so amazingly silent in the softly falling snow. That doesn’t last long. Once the snow stops falling and the sun comes out, the snow forms a hard crust which echoes as well as any concrete.
All snow is not created equal. Snow pellets or snow that’s been accumulating on the ground a long time has had much of the air squeezed out of it. A lot of air’s still there so it’s still decently insulating, but its insulation qualities are nothing like powder snow or freshly fallen snow. The kind of snow that goes into igloos is also pretty special. It’s denser than most snow, but it’s been packed so that its ice crystals interlock. That makes it strong as well as having good insulation properties.
On top of the direct insulating qualities, snow is also good at regulating its own temperature. Whenever the air temperature goes above freezing, the ice crystals really start pulling in heat from the surrounding environment. They can store a lot of heat this way before they break down and start to melt. That’s part of why water has such a great moderating effect on temperature.
All this means that anything under a layer of snow is warmer than the typical winter temperatures outside. In Goose Bay, Labrador, Canada, the mean January air temperature is -16.4 degrees, but it’s only -2.1 degrees at soil level, under the snow. That’s great for hibernating creatures and wintering plants. Perennials often die off in a hard frost, but they do just fine if they’re covered in snow first.
However, in regions with heavy snow cover, the insulation properties of snow work both ways. Spring is really slow to come where the snow is deep, especially in shady places. The same stuff that kept the ground close to freezing while the winter winds howled keeps right on insulating the ground and keeping it close to freezing while everything outside the snow is warming nicely. Sunlight has a hard time getting through, both because the snow insulates the ground from its warmth and because the white snow reflects most of the light. It really doesn’t help that the snow also keeps air temperatures up to 10 degrees colder just by being there.
The insulating properties of snow can even be extremely dangerous. Ice which has been covered with snow is a lot more likely to be rotten underneath than ice which is exposed to the open air. That’s a real problem for places that depend on ice roads.