Environmental Engineering Reference
In-Depth Information
This determines the extraterrestrial irradiance experienced at Earth's orbital
distance from the sun. However, the distance between the sun and Earth is not
constant throughout the year. It varies between 1.47 10 8 km and 1.52 10 8
km. This causes a variation in the irradiance, E e , of between 1325 W/m 2 and
1420 W/m 2 . The average value, called the solar constant , E 0 , is:
E 0 = 1.367 ± 2 W/m 2
(2.9)
This value can be measured outside the Earth's atmosphere on a surface
perpendicular to the solar radiation. The symbol I 0 is also used for the solar
constant.
S OLAR I RRADIANCE ON THE S URFACE OF THE E ARTH
Values measured on the surface of Earth are usually lower than the solar
constant. Various influences of the atmosphere reduce the irradiance. They
are:
• reduction due to reflection by the atmosphere
• reduction due to absorption in the atmosphere (mainly O 3 , H 2 O, O 2 and
CO 2 )
• reduction due to Rayleigh scattering
reduction due to Mie scattering.
The absorption of light by different gases in the atmosphere, such as water
vapour, ozone and carbon dioxide, is highly selective and influences only some
parts of the spectrum. Figure 2.3 shows the spectrum outside the atmosphere
( AM 0) and at the surface of the Earth ( AM 1.5). The spectrum describes the
composition of the light and the contribution of the different wavelengths to
the total irradiance. Seven per cent of the extraterrestrial spectrum ( AM 0)
falls in the ultraviolet range, 47 per cent in the visible range and 46 per cent
in the infrared range. The terrestrial spectrum AM 1.5 shows significant
reductions at certain wavelengths caused by absorption by different
atmospheric gases.
Molecular air particles with diameters smaller than the wavelength of light
cause Rayleigh scattering . The influence of Rayleigh scattering rises with
decreasing light wavelength.
Dust particles and other air pollution cause Mie scattering . The diameter
of these particles is larger than the wavelength of the light. Mie scattering
depends significantly on location; in high mountain regions it is relatively low,
whereas in industrial regions it is usually high.
Table 2.4 shows the contributions of Mie and Rayleigh scattering and
absorption for different sun heights
S (see section on calculating the position
of the sun, p55). Climatic influences such as clouds, snow, rain or fog can
cause additional reductions.
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