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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. The important processes in the atmosphere (Rayleigh scattering and Mie scattering) are elastic processes, by which light can be deviated from its path without being absorbed and with no change in wavelength. ==Color== 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 the human eye perceives blue when looking toward parts of the sky other than the sun.〔"(Rayleigh scattering )." ''Encyclopædia Britannica''. 2007. Encyclopædia Britannica Online. 16 Nov. 2007.〕 The color perceived is similar to that obtained by a monochromatic blue of wavelength 474–476 nm mixed with white light, ''i.e.'', an unsaturated blue light.〔"(Human color vision and the unsaturated blue color of the daytime sky )", Glenn S. Smith, ''American Journal of Physics'', Volume 73, Issue 7, pp. 590-597 (2005).〕 Near sunrise and sunset, most of the light we see comes in nearly tangentially to the Earth's surface; thus, the light's path through the atmosphere is so long that much of the blue and even green light is scattered out, leaving the sun rays and the clouds it illuminates red. Therefore, when looking at the sunset and sunrise, we see the color red more than the other colors. Scattering and absorption are major causes of the attenuation of radiation by the atmosphere. Scattering varies as a function of the ratio of the particle diameter to the wavelength of the radiation. When this ratio is less than about one-tenth, Rayleigh scattering occurs in which the scattering coefficient varies inversely as the fourth power of the wavelength. At larger values of the ratio of particle diameter to wavelength, the scattering varies in a complex fashion described, for spherical particles, by the Mie theory; at a ratio of the order of 10, the laws of geometric optics begin to apply. In the example of the sky at the zenith, it is blue during broad daylight due to Rayleigh scattering involving diatomic gases (, ). Near sunset and especially during twilight, ozone () absorption significantly contributes to maintaining the blue sky color. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Diffuse sky radiation」の詳細全文を読む スポンサード リンク
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