Life Under the Snow

Snow is the most transforming element. It changes everything. Winter’s first covered landscape seems a different place than the world just a few hours earlier—softer, quieter, held more still. Its whiteness glints clean, sparkling in bright sunlight like a million tiny diamonds brought to a boil. Later, blue shadows pooling in subtle hollows enforce the impression of coolness.

But snow affects more than just the way the world looks. It alters the lives of creatures living in and beneath it. And on a level that is at once minute and massive, its arrival triggers a myriad of lifecycles that, until the past decade, nobody knew existed. The role of microbial life beneath the snow, it turns out, may have serious ramifications for how we measure and gauge the effects of global warming.

Illustration by Marco Cibola

For many animals that don’t migrate or hibernate, snowpack provides shelter and food throughout the winter. The snow world, or nivean environment, is divided into three regions: supranivean (above the snow), intranivean (within the snowpack), and subnivean (beneath the snow). Birds such as grouse may cover themselves in powder near the surface to stay warm, while deeper snow shields mice and voles from birds of prey and foxes, coyotes, and bobcats. Tunnels that form along tree trunks and shrubs allow weasels and other small mammals to move throughout the layers. In the subnivean space, near the warmer earth, rodents such as mice and shrews graze on grass or insect eggs. And at the ground surface, fungi and bacteria communities thrive, a source of carbon dioxide that’s been recognized only in the past decade.

Although we think of snow as cold, it is in fact an excellent insulator, says Jim Halfpenny, as he shovels out a roughly four-foot-by-four-foot snow pit, one of many he digs every winter, on a slope in Yellowstone National Park one early April day. Author of several books, Halfpenny is a renowned naturalist and winter ecology expert who teaches classes in winter ecology, tracking, and the natural history of wolves, cougars, and bears at his Gardiner, Montana, facility on the park’s northern boundary.

Early April in Yellowstone is not spring by more civilized standards. The land sloping toward the Lamar River lies beneath a frozen sea of white, though a bright sun softens the snow, rounds the drifts. But a quick look at the layers exposed by his shovel tells Halfpenny that spring has definitely arrived here. The snowpack, which has accumulated over several months, is now only 26½ inches thick and has decayed into four layers—in midwinter it might be twice as deep, with many distinct layers. Standing thigh-deep in his snow pit, Halfpenny measures the depth and temperature of each. He sprinkles crystals—mostly granules now—onto his tongue to gauge their size, pushes his hand against the snow to assess each layer’s hardness, then removes samples and weighs them to determine the snow’s water content.

Water content, or density, is critical to determining snow’s insulating properties. Denser snow—like this pack—is wetter and provides less insulation. Cold, dry snow consists of billions of crystals separated by tiny air pockets. Six to eight inches at a relatively dry density of, say, 10 percent water content is enough to protect ground-level life from bitter nighttime temperatures. (In the high Rocky Mountains, seven percent density is common for new snow; closer to the coasts, 20 percent is typical.)

Fresh-fallen snow is a near mirror for short-wave radiation—sunlight and starlight—something skiers and mountaineers who get snowburn around the edges of their goggles and sunglasses can attest to. Older, dirtier snow may reflect only 40 percent to 50 percent of short-wave radiation.

Snow itself is almost black to long-wave radiation; it absorbs the heat emanating from basically everything that’s not a sun. Where snow lies against any object—a rock, a tree trunk—long-wave radiation melts it, creating soft spots or spaces that small mammals can move through. Once an insulating layer covers the land, temperatures in the interstice between snow and the ground stabilize near 32 degrees Fahrenheit. Gaps open. Warmed by radiant heat from the earth, ground soil thaws, providing an abundant supply of that most important element of life, water.