- The albedo of a land surface is the non-dimensional ratio of the radiation flux reflected by a (typically horizontal) surface in all directions and the incoming irradiance, which is the radiation flux from the upper hemisphere. This is technically known as the bihemispherical reflectance factor, and both fluxes must be relative to the same spectral range. For bare soils and other solid, convex objects, the material interface between the ground and the atmosphere constitutes the reference surface. In the case of vegetation, a reference surface is typically defined at or near the top of the canopy and must be specified explicitly. This “generic” albedo is highly variable in space and time as a result of changes in surface properties (snow deposition and melting, changes in soil moisture and vegetation cover and so on), as a function of fluctuations in the illumination conditions (solar angular position, atmospheric effects, cloud properties and so on) and with human activities (for example, clearing and planting forests, sowing and harvesting crops, burning rangeland and so on). Albedo is thus not an intrinsic surface property, but a joint property of the surface and the overlying atmosphere, since the latter’s composition (gases, clouds and aerosols) significantly affects the spectral and directional distribution of the irradiance.
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Created 5/3/17, last modified 6/14/17