Subnivean

Subnivean climate (From Latin for "under" () and "snow" () refers to the zone in and underneath snowpack. This is the environment of many hibernal animals, as it provides protection from predators and insulation. The subnivean climate is formed by three different types of snow metamorphosis: destructive metamorphosis, which begins when snow falls; constructive metamorphosis, the movement of water vapor to the surface of the snowpack; and melt metamorphosis, the melting/sublimation of snow to water vapor and its refreezing in the snowpack. These three types of metamorphosis transform individual snowflakes into ice crystals and create spaces under the snow where small animals can move.

Subnivean fauna include small mammals such as mice, voles, shrews, and lemmings that must rely on winter snow cover for survival. These mammals move under the snow for protection from heat loss and some predators. In winter regions that do not have permafrost, the subnivean zone maintains a temperature of close to 32 °F (0 °C) regardless of the temperature above the snow cover, once the snow cover has reached a depth of six inches (15 cm) or more. The sinuous tunnels left by these small mammals can be seen from above when the snow melts to the final inch or so.

Some winter predators such as foxes and large owls can hear their prey through the snow and pounce from above. Ermine (stoats) can enter and hunt below the snowpack. Snowmobiles and ATVs can collapse the subnivean space. Skis and snow shoes are less likely to collapse subnivean space if the snowpack is deep enough.

Larger animals also utilize subnivean space. In the Arctic, ringed seals have closed spaces under the snow and above openings in the ice. In addition to resting and sleeping there, the female seals give birth to their pups on the ice. Female polar bears also den in snow caves to give birth to their young. Both types of dens are protected from exterior temperatures. Formation of these large spaces is from the animals' activity, not ground heat.

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