Reflectance and bidirectional reflectance

The main feature of snow in the shortwave spectral range is its very high reflectance, especially over the visible spectrum. This explains its white color. Its high reflectance comes from the combination of its microstructure with the optical properties of ice. Over the visible spectrum, pure ice is quasi-transparent (absorption between 0.02 and 0.05 cm-1), while in the near-infrared spectrum absorption increases until ice becomes quasi-opaque above 1.5 pm (Hobbs, 1974). The refractive index of ice is close to 1.30. A photon incident on the snow surface is either refracted or reflected at each encounter with an ice/air interface until it is either absorbed or ejected from the layer. At visible wavelengths, and if snow has a very low impurities content, the photon may travel large distances through ice without being absorbed, and therefore may be refracted and reflected many times within the snow cover. Snow therefore behaves as a diffuser. Multiple refraction and reflection drastically change the propagation direction of an individual photon, so that the probability of its ejection from the snow layer is very high. This explains the high reflectance of clean snow at visible wavelengths (Wiscombe and Warren, 1980). If snow contains many impurities, the probability that photons are absorbed before being ejected is much higher and the reflectance is therefore smaller. At visible wavelengths, snow reflectance depends slightly on the grain size and shape, but is mainly affected by carbon soot particles present inside the snow layer and by the surface deposition of dust (Warren and Wiscombe, 1980). Naturally, high reflectance requires a deep enough snow layer to ensure that photons be diffused in depth before being ejected. Many radiative models consider snow as a semi-infinite medium. In practice, this assumption is valid for snow layers deeper than about 10 cm.

Ice absorption increases at longer wavelengths, which decreases the probability for photon ejection and consequently decreases snow reflectance. This probability primarily depends on the ice distance traversed by the photon during multiple refractions and reflections. If snow grains are large and granular rather than small and planar, this distance is longer. This explains why snow reflectance in the near-infrared spectrum generally decreases with increasing grain size, except when facets are growing during temperature gradient metamorphism. Snow reflectance has been investigated in the field as well as in the laboratory and by means of specific models. In most models, the reflectance of pure snow is described only as a function of grain size and impurities content (see Figs. 2.18 and 2.19, Wiscombe and Warren, 1980), while assuming that snow consists of spherical grains. Sergent et al. (1998) emphasize that grain shape must also be considered.

According to the relative isotropy of the orientation of ice/air interfaces in snow, shortwave radiation reflected by snow is diffuse and relatively isotropic. Nevertheless, bi-directional reflectance must be considered. Bi-directional effects are sensitive only at high angles of incidence and are more pronounced in the near-infrared spectrum than in the visible spectrum. Bi-directional effects on reflectance are more sensitive for hexagonal particles than for spherical ones (Dozier and Warren, 1982; Leroux, 1996).

Specular reflection can also be observed at a very high angle of incidence when the snow surface presents special features such as a sun crust or firnspiegel (class 9c).

Renewable Energy 101

Renewable Energy 101

Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable. The usage of renewable energy sources is very important when considering the sustainability of the existing energy usage of the world. While there is currently an abundance of non-renewable energy sources, such as nuclear fuels, these energy sources are depleting. In addition to being a non-renewable supply, the non-renewable energy sources release emissions into the air, which has an adverse effect on the environment.

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