Theory – about snow

Physical properties

From the mechanical point of view, snow is a complex material. Its behavior depends on a number of parameters. The various types of snow are characterized by their mechanical properties. For example, the resistance to pressure of newly fallen snow is lower than that of mechanically processed (groomed) snow. Snow deforms under its own weight, depending on its temperature and density. Its viscosity increases at lower temperature and with greater density.

From an optical point of view, the snow absorbs only part of the short-wave radiation received from the sun. For example, up to 95% of the sun’s radiation is reflected by new snow. The albedo (the reflectivity of an object) depends on the condition of the snow surface (grain shape and size, contamination and water content). Dirty snow has low albedo and will melt faster.

The snow’s thermal properties appears clearly when the temperature difference between the various layers of snow strongly influences the properties and the metamorphism (change of form) of the snow layers. Temperature changes are greatest at the surface and smallest close to the ground.

The properties of machine-made snow are distinctly different from those of natural snow. The basic difference is that natural snow freezes from water vapor while machine-made snow freezes from water droplets. In the latter case, water droplets freeze on the outside before the core does. The thermal differences contribute to the fact that compacted natural snow on the surface stays colder than equivalent artificial snow (which melts easier and thereafter freezes)

  • Compacted natural snow stays ca. – 3 to -5 °C colder than artificial snow during equal condition in the the winter.

Meteorological factors

Solar radiation varies throughout the year. It reaches it’s maximum level in the summer and it’s minimum level in the winter. It depends on the time of the day, the slope of the terrain and the altitude of the location. In December, a 30 degree gradient slope facing south will receive almost two and a half times as much radiation as a horizontal surface.

Wind causes a rapid thermal exchange between the surface of the snow and the surrounding air. The stronger the wind, the faster the exchange. A warm wind accelerates the melting process on the surface rapidly and a cold wind accelerates its cooling.

When the surrounding air is warmer than the snow surface, the temperature of the snow rises. With strong emission or pollution, a stable layer of air can form at the bottom of valleys, since cold air is heavier than warm air. Air exchange progresses at a much slower rate in this case.

When air humidity is high, water vapor condenses on the snow surface. Water then accumulates on the surface. With very low air humidity, air can absorb more water vapor from the snow surface. Snow cools down during evaporation. Low air humidity is therefore beneficial for the freezing of the snow surface. In dry air, ice/snow also sublimates (evaporates without turning to water first) and the snow surface cools down.

Rain or snow transfers energy to the snow surface, depending on the temperature of the precipitation. Due to its higher temperature and thermal exchange effect, free water (wet snow or rain) raises the temperature of the surface of the snow and causes it to partially melt. Rain has however not as much melting effect as wind. If it rains 10 mm and this rain is cooled to 5 °C in the snow, it will only cause 0,6 mm of snow to melt (from Wikipedia).

As energy is absorbed when the snow is at 0℃, the snow crystals/grains start to melt at the edges and corners. They become rounder and a thin layer of water forms around the grains. As melting progresses, pores keep filling with water. When water content is high, the bond between the grains will also dissolve causing the snow to soften. Heat intake and melting may frequently be interrupted by colder night temperatures, and water freezes again. Consequently, the water present between snow grains freezes, forming strong bonds again.