ABSTRACT
A surface reects and scatters a fraction of the incident solar irradiance that falls on it, including the direct normal irradiance (DNI) and the scattered sunlight that arrives from all parts of the sky, that is, the diffuse horizontal irradiance (DHI). However, a surface reects the DNI and DHI components very differently, as will be discussed in this chapter on albedo. In general, albedo is dened as the total reected irradiance divided by the total incident irradiance. Snowelds and deep oceans represent extremes in albedo. For newly fallen snow, 90% or more of the incident radiation may be reected at visible wavelengths, but as snow ages, melts, and refreezes the reectivity decreases. For the deep ocean the albedo is around 4% for calm seas at all solar wavelengths. However, if the seas have breaking waves, the albedo increases. If one looks toward the sun at the angle of incidence, a glint is detectable from the ocean surface. This glint obeys Fresnel’s equations. Albedos of surfaces can be very different spectrally with some surfaces showing higher reectivity in the visible than in the near-infrared while other surfaces can show the opposite behavior. The albedos of vegetated areas have considerable spectral structure and change appreciably as the plants go through stages of growth, owering, and senescence. Measurements by Bowker, Davis, Myrick, Stacy, and Jones (1985) resulted in cataloged reectances for many common surface types, and this remains a primary reference for surface albedo for real surfaces.
