Longwave radiation is much less reflected at the snow surface than shortwave radiation.

The BRDF is the "Bidirectional Reflectance Distribution Function".

It gives the reflectance of a target as a function of illumination geometry and viewing geometry. The BRDF depends on wavelength and is determined by the structural and optical properties of the surface, such as shadow-casting, mutiple scattering, mutual shadowing, transmission, reflection, absorption and emission by surface elements, facet orientation distribution and facet density.

Sounds technical? Well, it is: The BRDF is needed in remote sensing for the correction of view and illumination angle effects (for example in image standardisation and mosaicking), for deriving albedo, for land cover classification, for cloud detection, for atmospheric correction and other applications. It gives the lower radiometric boundary condition for any radiative transfer problem in the atmosphere and is hence of relevance for climate modeling and energy budget investigations.

However, it should not be overlooked that the BRDF simply describes what we all observe every day: that objects look differently when viewed from different angles, and when illuminated from different directions. For that reasons painters and photographers have for centuries explored the appearance of trees and urban areas under a variety of conditions, accumulating knowledge about "how things look", knowledge that today we'd call BRDF-related knowledge. As modern painters, programers of virtual reality in computers also need to be concerned about the BRDFs of the surfaces they use.

Bidirectional reflectance distribution functions: causes