Diffuse sky radiation

Diffuse sky radiation is solar radiation reaching the Earth's surface after having been scattered from the direct solar beam by molecules or suspensoids in the atmosphere. It is also called skylight, diffuse skylight, or sky radiation and is the reason for changes in the color of the sky. Of the total light removed from the direct solar beam by scattering in the atmosphere (approximately 25% of the incident radiation when the sun is high in the sky, depending on the amount of dust and haze in the atmosphere), about two-thirds ultimately reaches the earth as diffuse sky radiation. When the sun is at the zenith in a cloudless sky, with 1367 W/m² above the atmosphere, direct sunlight is about 1050 W/m², and total insolation about 1120 W/m². This implies that under these conditions the diffuse radiation is only about 70 W/m² out of the original 1367 W/m².

The dominant radiative scattering processes in the atmosphere (Rayleigh scattering and Mie scattering) are elastic in nature, by which light can be deviated from its path without being absorbed and with no change in wavelength.

The sunlit sky is blue because air scatters short-wavelength light more than longer wavelengths. Since blue light is at the short-wavelength end of the visible spectrum, it is more strongly scattered in the atmosphere than long-wavelength red light. The result is that when looking toward parts of the sky other than the sun, human eye perceives them to be blue. The color perceived is similar to that obtained by a monochromatic blue of a wavelength of 474–476 nm mixed with white light, i.e., an unsaturated blue light.

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