Spectral variation of albedo

Although a single figure for the albedo of a surface is useful in estimating radiation fluxes, the reflectivity is actually strongly dependent on the wavelength of the electromagnetic radiation. Within the photosynthetically active region (0.4 -0.7 microns), plant leaves normally have a low reflectivity and high absorptivity, so that the visible light energy can be used effectively. The reflectivity is only about 0.05 for wavelengths shorter than 0.60 microns for maize and lucerne, but 0.32 for maize and 0.40 for lucerne at the wavelength of green light (0.60-0.63 microns) (1). The reflectivity is lower again in orange and red (0.64-0.71 microns), but at 0.75 microns it exceeds 0.50 for most plants. A large fraction of the sunrays' energy is in the near infrared and to prevent overheating it is important that plants absorb little of this photosynthetically useless radiation.

On the other hand, the reflectivity is about 0.98 for new snow with waves shorter than 0.7 microns, falling to 0.80 at 1, 0.5 at 1.3, 0.2 at 1.4, and less than 0.1 above 1.45 microns. So vegetation captures the more powerful shortest wavelengths, whilst snow reflects it away. The reason why even fresh, white snow will melt under the sun when the air temperature is just below freezing, is because about half of the sun rays' energy is in the infrared, especially the near-IR, which is largely absorbed by the snow.

It is the albedo value around 0.5 microns that matters most, since that is the dominant wavelength of sunlight (Fig. 2.2). The albedo of a surface is calculated as the integral of the spectral reflectivity times the radiation (W m^-2 m), over all wavelengths in the visible spectrum. Or it is measured with two pyranometers, one pointing up and one down (towards the surface).

(1) McGuffie, K. and A. Henderson-Sellers 1997. A Climate Modelling Primer (John Wiley & Sons) 253pp. (They refer to from Henderson-Sellers & Wilson 1983. Rev. Geoophys. Space Phys., 21, 1743-78.)