-->The subnivean climate (From

Latin Latin ( or ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally spoken by the Latins (Italic tribe), Latins in Latium (now known as Lazio), the lower Tiber area aroun ...

for "under" ('' sub-'') and "of snow" ('' niveus'') and English -an.) is the environment between fallen snow and terrain. This is the environment of many hibernal animals, as it provides insulation and protection from predators. 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 Water vapor, water vapour, or aqueous vapor is the gaseous phase of Properties of water, water. It is one Phase (matter), state of water within the hydrosphere. Water vapor can be produced from the evaporation or boiling of liquid water or from th ...

to the surface of the snowpack; and melt metamorphosis, the

melting Melting, or fusion, is a physical process that results in the phase transition of a substance from a solid to a liquid. This occurs when the internal energy of the solid increases, typically by the application of heat or pressure, which inc ...

/ sublimation of snow to water vapor and its refreezing in the snowpack. These three types of metamorphosis transform individual

snowflake A snowflake is a single ice crystal that is large enough to fall through the Earth's atmosphere as snow.Knight, C.; Knight, N. (1973). Snow crystals. Scientific American, vol. 228, no. 1, pp. 100–107.Hobbs, P.V. 1974. Ice Physics. Oxford: C ...

s into ice crystals and create spaces under the snow where small animals can move.

Subnivean fauna

Subnivean

fauna Fauna (: faunae or faunas) is all of the animal life present in a particular region or time. The corresponding terms for plants and fungi are ''flora'' and '' funga'', respectively. Flora, fauna, funga and other forms of life are collectively ...

includes small mammals such as

mice A mouse (: mice) is a small rodent. Characteristically, mice are known to have a pointed snout, small rounded ears, a body-length scaly tail, and a high breeding rate. The best known mouse species is the common house mouse (''Mus musculus' ...

vole Voles are small rodents that are relatives of lemmings and hamsters, but with a stouter body; a longer, hairy tail; a slightly rounder head; smaller eyes and ears; and differently formed molars (high-crowned with angular cusps instead of lo ...

shrew Shrews ( family Soricidae) are small mole-like mammals classified in the order Eulipotyphla. True shrews are not to be confused with treeshrews, otter shrews, elephant shrews, West Indies shrews, or marsupial shrews, which belong to dif ...

s, and

lemming A lemming is a small rodent, usually found in or near the Arctic in tundra biomes. Lemmings form the subfamily Arvicolinae (also known as Microtinae) together with voles and muskrats, which form part of the superfamily Muroidea, which also inclu ...

s 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 Permafrost () is soil or underwater sediment which continuously remains below for two years or more; the oldest permafrost has been continuously frozen for around 700,000 years. Whilst the shallowest permafrost has a vertical extent of below ...

, 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

owl Owls are birds from the order Strigiformes (), which includes over 200 species of mostly solitary and nocturnal birds of prey typified by an upright stance, a large, broad head, binocular vision, binaural hearing, sharp talons, and feathers a ...

s, can hear their prey through the snow and pounce from above. Ermine (stoats) can enter and hunt below the snowpack.

Snowmobile A snowmobile, also known as a snowmachine (chiefly Alaskan), motor sled (chiefly Canadian), motor sledge, skimobile, snow scooter, or simply a sled is a motorized vehicle designed for winter travel and recreation on snow. Their engines normally ...

s 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 use 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 bear The polar bear (''Ursus maritimus'') is a large bear native to the Arctic and nearby areas. It is closely related to the brown bear, and the two species can Hybrid (biology), interbreed. The polar bear is the largest extant species of bear ...

s 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.

Subnivean climate formation

Destructive metamorphosis

Destructive metamorphosis begins as the snow makes its way to the ground, often melting, refreezing, and settling. Water molecules become reordered, causing the snowflakes to become more spherical in appearance. These melting snowflakes fuse with others around them, becoming larger until all are uniform in size. While the snow is on the ground, the melting and joining of snow flakes reduces the height of snowpack by shrinking air spaces, causing the density and mechanical strength of the snowpack to increase. Freshly fallen snow with a

density Density (volumetric mass density or specific mass) is the ratio of a substance's mass to its volume. The symbol most often used for density is ''ρ'' (the lower case Greek letter rho), although the Latin letter ''D'' (or ''d'') can also be u ...

of 0.1 g/cm³ has very good insulating properties; however as time goes on, due to destructive metamorphism, the insulating property of the snowpack decreases, because the air spaces between snowflakes disappear. Snow that has been on the ground for a long period of time has an average density of 0.40 g/cm³ and conducts heat well; however, once a base of 50 cm of snow with a density around 0.3 g/cm³ has accumulated, temperatures under the snow remain relatively constant because the greater depth of snow compensates for its density. Destructive metamorphosis is a function of time, location, and weather. It occurs at a faster rate with higher temperatures, in the presence of water, under larger temperature gradients (e.g., warm days followed by cold nights), at lower elevations, and on slopes that receive large amounts of

solar radiation Sunlight is the portion of the electromagnetic radiation which is emitted by the Sun (i.e. solar radiation) and received by the Earth, in particular the visible light perceptible to the human eye as well as invisible infrared (typically p ...

. As time goes on, snow settles, compacting air spaces, a process expedited by the packing force of the wind. Compaction of snow reduces the penetration of long- and short-wave radiation by reflecting more radiation off the snow. This limitation of light transmission through the snowpack decreases light availability under the snow. Only 3% of light can penetrate to a depth of 20 cm of snow when the density is 0.21 g/cm³. At a depth of 40 cm, less than 0.2% of light is transmitted from the snow surface to ground below. This decrease in light transmission occurs up to the point at which critical compaction is reached. This occurs because the surface area of the ice crystal decreases and it causes less refraction and scattering of light. Once densities reach 0.5 g/cm³, total surface area is reduced, which in turn reduces internal refraction and allows light to penetrate deeper into the snowpack.

Constructive metamorphosis

Constructive metamorphosis is caused by the upward movement of water vapor within the snowpack. Warmer temperatures are found closer to the ground because it receives heat from the core of the earth. Snow has a low

thermal conductivity The thermal conductivity of a material is a measure of its ability to heat conduction, conduct heat. It is commonly denoted by k, \lambda, or \kappa and is measured in W·m−1·K−1. Heat transfer occurs at a lower rate in materials of low ...

, so this heat is retained, creating a temperature gradient between the air underneath the snowpack and the air above it. Warmer air holds more water vapor. Through the process of sublimation, the newly formed water vapor travels vertically by way of diffusion from a higher concentration (next to the ground) to a lower concentration (near the snowpack surface) by traveling through the air spaces between ice crystals. When the water vapor reaches the top of the snowpack, it is subjected to much colder air, causing it to condense and refreeze, forming ice crystals at the top of the snowpack that can be seen as the layer of crust on top of the snow.

Melt metamorphism

Melt metamorphism is the deterioration of snow by melting. Melting can be stimulated by warmer

ambient temperature Room temperature, colloquially, denotes the range of air temperatures most people find comfortable indoors while dressed in typical clothing. Comfortable temperatures can be extended beyond this range depending on humidity, air circulation, and ...

s, rain, and fog. As snow melts, water is formed and the force of gravity pulls these molecules downward. En route to the ground, they refreeze, thickening in the middle stratum. During this refreezing process, energy is released in the form of latent heat. As more water comes down from the surface, it creates more heat and brings the entire snowpack column to near equal temperature. The

firn __NOTOC__ Firn (; from Swiss German "last year's", cognate with ''before'') is partially compacted névé, a type of snow that has been left over from past seasons and has been recrystallized into a substance denser than névé. It is ice that ...

ification of the snow strengthens the snowpack, due to the bonding of grains of snow. Snow around trees and under canopies melts faster due to the reradiation of long-wave radiation. As snow gets older, particles of impurities (pine needles, soil, and leaves, for example) accrue within the snow. These darkened objects absorb more short-wave radiation, causing them to rise in temperature, also reflecting more long-wave radiation.

References

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