Subnivean Climate - Deconstructive Metamorphosis

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. 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 g/cm3 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. Snow that has been residing 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. 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 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 0.21 g/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. 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.