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It is calculated as the fraction of the actual total solar radiation on the surface of the Earth during a certain period over the theoretical maximum (i.e., clear sky) radiation during the same period. [1] [2] The clearness index is a dimensionless quantity and can vary from 0 (sky is completely covered) to 1 (perfectly sunny). [3]
A clear daytime sky, looking toward the zenith. Earth's atmosphere scatters short-wavelength light more efficiently than that of longer wavelengths. Because its wavelengths are shorter, blue light is more strongly scattered than the longer-wavelength lights, red or green.
Infrared radiation can pass through dry, clear air in the wavelength range of 8–13 μm. Materials that can absorb energy and radiate it in those wavelengths exhibit a strong cooling effect. Materials that can also reflect 95% or more of sunlight in the 200 nanometres to 2.5 μm range can exhibit cooling even in direct sunlight. [9]
Losing the dark. Light pollution, the term for the brightening of the night sky by unnatural lights, is increasing worldwide. On average, skies are getting 10% brighter each year globally, with ...
World map showing the areas of Earth receiving daylight around 13:00 UTC in April. Daylight is the combination of all direct and indirect sunlight during the daytime.This includes direct sunlight, diffuse sky radiation, and (often) both of these reflected by Earth and terrestrial objects, like landforms and buildings.
Even light cirrus clouds reduce this to 50%, stronger cirrus clouds to 40%. Thus the solar energy arriving at the surface with the sun directly overhead can vary from 550 W/m 2 with cirrus clouds to 1025 W/m 2 with a clear sky.
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