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firnification

Subnivean climate

Subnivean Climate

Subnivean Climate refers to the zone in and underneath the snow pack. This is the environment of many animals that remain active during the winter. This zone provides protection from predators and insulation from the elements. 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 snow pack; and melt metamorphosis, the melting/sublimation of snow to water vapor and it’s refreezing in the snow pack. These three types of metamorphosis transform individual snowflakes into ice crystals and create spaces under the snow where small animals can move.

Deconstructive Metamorphosis

Deconstructive 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 (Halfpenny, 1989). 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 snow pack by shrinking air spaces and causing the density and mechanical strength of the snow pack to increase. Freshly fallen snow with a density of 0.1/cm3 and has very good insulating properties; however as time goes on, due to destructive metamorphism the insulating property of the snow pack decreases because the air spaces between snowflakes disappear (Marchand, 1996). Snow that has been residing on the ground for a long period of time has an average density of 0.40g/cm3 and conducts heat well; however, once a base of 50 cm of snow with a density around 0.3g/cm3 has accumulated, temperatures under the snow remain relatively constant because the greater depth of snow compensates for its density (Marchand, 1996) . 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. As time goes on snow settles compacting air spaces, a process expedited by the packing force of the wind (Marchand, 1996). 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 snow pack decreases light availability under the snow. Only three percent of light can penetrate to a depth of 20 cm of snow when the density is 0.21g/cm3. At a depth of 40 cm less than two tenths of a percent of light is transmitted from the snow surface to ground below (Marchand, 1996). 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 snow pack (Marchand,1996).

Constructive Metamorphosis

Constructive metamorphosis is caused by the upward movement of water vapor within the snow pack. Warmer temperatures are found closer to the ground because it receives heat from the core of the earth. Snow has a low thermal conductivity so this heat is retained creating a temperature gradient between the air underneath the snow pack 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 snow pack surface) by traveling through the air spaces between ice crystals (Hindelang). When the water vapor reaches the top of the snow pack it is subjected to much colder air causing it to condense and refreeze, forming ice crystals at the top of the snow pack 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 temperatures, rain and fog. As snow melts water is formed and the force of gravity pulls these molecules downward. In 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 snow pack column to near equal temperature. The firnification of the snow strengthens the snow pack, 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 (e.g., pine needles, dirt, leaves etc…) accrue within the snow. These darkened objects absorb more short wave radiation causing them to rise in temperature, also reflecting more long wave radiation.

==Subnivean Environment== These these processes continue throughout the winter occurring one at a time or all at once. The outcome has important outcomes for organisms living in this environment as well as the dynamics of the snow. For the organisms, the subnivean zone becomes a blanket that provides insulation from the elements and protection from predators. For snow found on mountain slopes the development of the subnivean zone leads to a weaker bottom layer and a dense top layer creating ideal avalanche conditions.

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