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WAVELENGTH [nm]

Figure 6. Increase of spectral global irradiance with altitude, average (solid line) and range of variation (dashed lines). From [10].

Figure 6. Increase of spectral global irradiance with altitude, average (solid line) and range of variation (dashed lines). From [10].

2. Effect of Clouds

In general, clouds attenuate the level of global irradiance at the Earth's surface, but this reduction can vary within a broad range. It depends on the optical depths of the cloud and if the clouds occult the sun itself. In some special cases it happens that global irradiance at the ground is enhanced by cloudiness [11], if bright clouds are close to the sun but do not occult the sun itself. Due to reflections from the sides of the clouds an enhancement of 10-20% may occur for short time periods.

Due to the large variability of clouds a great number of measurements under different situations of cloudiness are necessary to determine the influence of clouds on global irradiance. Such an analysis could be carried out from long-term measurements at Jungfraujoch, Switzerland [12]. The different wavelength ranges were measured simultaneously and the data were normalised to the respective values under cloudless conditions. Cloudiness was estimated in tenths of the sky, covered by clouds (Fig. 7, left). The reduction for complete cloudiness (10/10 of the sky covered by clouds) reaches values up to 50% for this high altitude station, whereas at sea level average values of reduction up to 75% were measured [13]. Although the scattering process by clouds is only slightly dependent on wavelength, the effect of clouds on global irradiance is significantly dependent on clouds (Fig. 7, right). The ratio between erythemally weighted irradiance and UVA irradiance is almost constant with cloudiness, whereas the ratios of both ranges of UV irradiance over total global irradiance depend strongly on cloudiness. This means that under complete cloud cover erythemally weighted irradiance is about 40% less attenuated than total irradiance. The reason for this is the higher diffuse component of UV irradiance compared with total irradiance, and that diffuse irradiance is much less attenuated by clouds than direct irradiance.

0123456789 10 CLOUDINESS (TENTH)

LU 12

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