Solar Engineering of Thermal Processes, 4th Edition

2.6 Atmospheric Attenuation of Solar Radiation

Solar radiation at normal incidence received at the surface of the earth is subject to variations due to change in the extraterrestrial radiation as noted in Chapter 1 and to two additional and more significant phenomena: (1) atmospheric scattering by air molecules, water, and dust and (2) atmospheric absorption by , , and . Iqbal (1983) reviews these matters in considerable detail.

Scattering of radiation as it passes through the atmosphere is caused by interaction of the radiation with air molecules, water (vapor and droplets), and dust. The degree to which scattering occurs is a function of the number of particles through which the radiation must pass and the size of the particles relative to , the wavelength of the radiation. The pathlength of the radiation through air molecules is described by the air mass. The particles of water and dust encountered by the radiation depend on air mass and on the time- and location-dependent quantities of dust and moisture present in the atmosphere.

Air molecules are very small relative to the wavelength ...