students learning snow science in the field

Welcome to the Subnivean Zone!

Bring the wonders of winter into your teaching with our upcoming virtual snow science workshop. Dig into the foundations of snow science using materials we’ll send you ahead of the workshop and learn ways it can be used to explore and introduce concepts in elementary and secondary science.

Register for the TSS Snow Science Workshop before February 18, 2021

students learning snow science in the field


In the chillier portions of the Northern Hemisphere, we are deep into winter. And even though the days are lengthening, January is typically the coldest month of the year in the regions of Idaho, Montana, and Wyoming that comprise the Greater Yellowstone Ecosystem. January also tends to be the highest snowfall month around here. Humans cope by wearing extra layers, stocking up at the grocery store, and hunkering down in warm houses, but how do animals get through this lengthy season of cold? Some opt-out by migrating to warmer regions, curling up for a long winter’s nap, or adding their own insulating layers, but a number of small mammals survive winter tunneled beneath the snow in a space called the subnivean zone. 

The subnivean zone is a hollow space between the ground and the base of the snowpack. As snow begins to accumulate in early winter, two factors help create air pockets under the snow. First, rocks and sturdy plants catch those first flakes and prevent them from building up directly below. Second, warmth radiating from the ground transforms the lowest lying layer of snow into water vapor, skipping the melting stage entirely. That water vapor freezes again when it contacts the snow overhead, crystallizing into a sturdy roof. The fancy word for this is sublimation, and the result is a zone of open space that forms directly above the ground. And all of this can take place with only six inches of snow on the ground! As snow continues to accumulate, it truly “blankets” the earth, creating a cozy habitat for wintering animals. Air temperatures may plummet well below freezing, while the zone below the snow holds steady right around 32 degrees Fahrenheit.

So, who is down there, anyway? And are they stuck in a tunnel for the entire winter? Good questions! Mice, voles, and shrews make up most of the subnivean population. And they are not trapped there. Their tunnel system is elaborate and contains multiple entry and exit points. It also features stockpiles of snacks (these critters know to plan ahead), access to other food sources like insect eggs, seeds, and bark, and vertical holes running to the surface which provide essential ventilation.

mouse tracks leading to entry hole
PC: Jon Hayes

This world under the snow remains mostly invisible to our eyes, but you can spot clues if you look closely. You might notice a tiny set of tracks leading to a small entrance hole, or glimpse the architecture of ice-roofed tunnels as the snow begins to melt in spring. But if you were a fox or an owl, subnivean activity would be at the forefront of your mind. Indeed, your very life would depend on the ability to sense movement under the snow. Have you ever been lucky enough to see a fox gaze intently at a spot in a field, then plunge headlong into the snow after a meal? Their keen sense of smell helps them detect the presence of potential prey below and if their aim and timing are true, they’ll return to their den with a full belly. Owls can hear mice and voles scurrying around under the snow from 30 yards away! Clenching their feet, they’ll punch through the crust and layers of snow and claim their prize. Fortunately for the subnivean dwellers, these hunters don’t succeed every time. The tunnels offer enough protection that a large part of the population lives to see the arrival of spring.

red fox pouncing in snow
PC: Michael Furtman

The life cycles of these creatures, great and small, depend entirely on the snow. The smaller ones couldn’t survive harsh winters without the warmth and protection afforded by the subnivean zone, and the predators who depend on them as a food source wouldn’t find sufficient calories anywhere else. And this is just the tip of the iceberg! Snow affects our world in many fascinating ways. 

We hope you’ll consider how a deeper knowledge of snow science opens new avenues of interest for learners of all ages. So much goes on beneath the surface in the quiet world of winter and we’d love to help you unlock the secrets in our upcoming virtual snow science workshop for educators.

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