3.3 Radiation

Radiation is primarily responsible for inducing the melt/freeze process, which results in crusting and other types of crystal evolution. Two types of radiation are important for snowpack evolution.

Incoming shortwave (solar) radiation
Solar energy reaching the Earth's surface is reflected, absorbed, or scattered, depending on the type of surface. Snowpack is particularly reflective, especially when the top of the pack contains newly fallen snow. The degree of a surface's reflectivity is referred to as its albedo or ratio of reflected solar energy to incoming solar energy. The albedo of snow is relatively large—typically in the range of 0.3 to 0.9. This means that 30% to 90% of the energy is reflected back to the atmosphere.

The albedo depends largely upon the age of the snow at the surface, with old snow having lower albedos than new snow: 0.3 to 0.5 as compared to 0.6 to 0.9. That's largely due to the presence of foreign matter such as dirt and dust, which are less reflective and have lower albedos. Albedo is also dependent on the size of the crystals in the top of the snowpack, with smaller crystals having larger albedos.

Basic depiction of incoming shortwave and outgoing longwave radiation for snowpack

Outgoing infrared radiation
All surfaces on Earth, be they bare ground or snowpack, constantly emit infrared radiation. The amount is primarily controlled by factors such as the temperature of the surface and the presence of nearby or overhanging vegetation. For snowpack, the warmer and more vegetation-free the surface, the greater the rate of radiative loss. This cools the snowpack at the very top few mm of the surface. Contrast this with the warming effect of incoming solar radiation, which heats up the top 15 to 30 cm (6 to 12 in) of snowpack.

On a calm, clear night, a snow-covered surface will cool much more quickly than a bare one given the same surface temperatures. That's due to several factors. Even though both surfaces have the same temperature at sunset, the snow radiates heat very efficiently, increasing the rate of heat loss at the surface. Snow is also a very good insulator. This prevents heat from rising through the snowpack, which allows the surface to cool quickly. In contrast, bare soil conducts much more heat upward from below, which helps slow down cooling at the surface.

Schematic of 2 scenarios at night: one with snowpack, one without.