The solar constant I 0 is the beam solar radiation outside the Earth's atmosphere when the sun is at its mean distance from the Earth. Variations in the distance of the sun from the Earth due to the ellipticity of the Earth's orbit cause the actual intensity of solar radiation outside the atmosphere to depart from I 0 by a few percent. All the values of solar radiation intensity given below, which are for the sun at its mean distance from the Earth, must be multiplied by F to obtain the actual values on day n d . The processes affecting the intensity of solar radiation that are important in solar energy work are scattering, absorption, and reflection.
Reflection occurs in the atmosphere and on the Earth's surface.
The scattering of solar radiation is mainly by molecules of air and water vapor, by water droplets, and by dust particles. This wavelength effect can be seen in the blue color of the clear sky and the red color of the setting sun. The sky appears blue because the shorter wavelength blue light is scattered more strongly than the longer wavelength red light. The setting sun appears red because much of the blue light has been scattered out of the direct beam. Scattering from large particles with radius greater than 25 lambda is independent of the wavelength.