Measurement of directional and spectral signatures of light reflectance by snow

The bidirectional reflection distribution functions (BRDF) of snow have been measured at high spectral resolution at various locations in Finland (Vuotso, Hyytiälä, Sodankylä, Kilpisjärvi, Rovaniemi, Sodankylä again). The measured snow types include fresh, new snow, both needle-like and hexagonal flakes, old, loose snow, and melting and refrozen snow. All snow types show strong forward scattering as previously reported, but there also appeared to be some enhancement in the backward directions that has not been reported in much detail. The grain size gives a clear signal at near-infrared, which was observed previously. A nontrivial dependence on grain shape was also observed, which has been ignored previously. Melting snow has a distinct forward feature not observable in dry snow: first a maximum in specular direction, a minimum after that, and then again brightening forward. There is a spectral signal at 1250/1350 nm that could be useful for wetness recovery in particular, even when the topography or BRDF model is not known. Density dependence was observed, partially contradicting earlier measurements. Microtopographic roughness slightly increases backscattering as expected. Much more detailed information about snow could be observed using hyperspectral, multidirectional remote sensing techniques than with current instruments. Measurements of more snow types need to be taken, especially dirty snow, snow/vegetation composites, and rough snow surfaces.