Radiative Transfer In Snow And Bubbly Ice

Snow appears white under most conditions of illumination. Yet holes in snow are blue. Both are consequences of the absorption properties of pure ice and multiple scattering by ice grains. The albedo of pure snow is high and does not vary greatly over the visible spectrum because incident photons that are scattered many times before re-emerging from the snow are unlikely to be absorbed before doing so. It is only by observing transmitted light that colors are seen. Beautiful blue light is often seen in quite shallow holes in snow. Although it takes many meters of ice to selectively absorb visible light, a few tens of centimeters of snow are sufficient because multiple scattering increases the effective path length a photon travels before reaching a given depth. In contrast with snow, frozen waterfalls (bubbly ice) are sometimes observed to be slightly green or bluish-green. Moreover, they are not as highly reflecting as surrounding snow. A first approximation to radiative transfer in bubbly ice is to consider it to be snow with a very large grain size. For snow with grain sizes a few millimeters or less the albedo is high and does not vary appreciably over the visible spectrum. But if the grain size is increased to 10 millimeters or more the albedo drops markedly and differences over the visible spectrum become more pronounced.