E6 Snow

Topic

Snow is a porous, heterogeneous medium whose physical properties—including density, thermal conductivity, permeability, and albedo—change continuously through a process of metamorphism: snow grains recrystallize and slowly bond together, driven by temperature and vapor pressure gradients within the snowpack itself. Freshly deposited snow has a density of between 50 and 150 kilograms per cubic meter and reflects 80 to 90 percent of incoming solar radiation; however, after weeks or months of metamorphism, that same snow increases in density to between 300 and 500 kilograms per cubic meter, while its reflectivity drops to just 40 to 70 percent.

Under the weight of accumulating snow layers year after year, the metamorphosed snow compacts further and transforms into firn—with a density of 400 to 800 kilograms per cubic meter—and this firn, when compressed for a sufficient period, eventually turns into true glacial ice once its density exceeds 830 kilograms per cubic meter.

Heat is transferred through the snowpack primarily via direct conduction between snow grains, though it is also transferred through water vapor convection—driven by internal temperature gradients—and through the latent heat released by metamorphism itself as snow grains recrystallize. The thermal conductivity of the snowpack depends heavily on its density, varying nearly a hundredfold between freshly fallen snow (which is very light and a poor conductor) and compacted snow (which is much denser and a much better conductor). The very low thermal conductivity of light snow makes the snowpack an excellent thermal insulator, capable of protecting the underlying ground from cold winter temperatures—much like the effect produced by an igloo built from snow blocks.

The snowpack's energy balance determines whether it undergoes surface melting or continues to freeze further: at the surface, the snow exchanges energy with the atmosphere through both shortwave solar radiation—a large fraction of which is reflected due to the snow's albedo—and longwave radiation, which the snow emits into the atmosphere with an efficiency of nearly ninety-eight percent. Added to this radiation exchange are turbulent sensible and latent heat fluxes, which depend on the temperature and humidity gradients immediately above the snowpack surface.

When the snowpack's energy balance favors melting, the resulting liquid water percolates into the snowpack itself; this percolating water may either drain out of the snowpack as runoff or refreeze upon encountering a deeper, colder layer within the snowpack, thereby forming an internal ice layer within the structure of snow, firn, and glacial ice described earlier.

ID:2637