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Figure 5. Increase of spectral global irradiance due to an increase of albedo by 0.1 for albedo values of 0.2, 0.4 and 0.6.

The results for albedo effects showed so far assume that the ground has an unlimited extension and that the albedo is homogeneous over the terrain. However, in practise the terrain is inhomogeneous and in that case the more complex three-dimensional radiative transfer calculations are necessary [5]. In comparison with the one-dimensional radiative transfer calculations an 'effective' albedo can be defined, which describes the reflection of a given inhomogeneous terrain by an area-averaged single albedo value [6]. Model calculations have shown that the radius of significance, where the albedo has still an influence on global irradiance, extends up to 30 km [7]. Measurements in high mountain areas with partly snow covered terrain have shown values for effective albedo between 0.7 and 0.4 as a consequence of complex distribution of snow covered and snow free surfaces.

1.5. Effect of Altitude

The increase of global irradiance with altitude is called 'altitude effect', and it is expressed as percentage increase for an increase in altitude by 1000 m. This increase of irradiance is mainly a consequence of the smaller irradiated air mass at higher altitudes. Therefore the altitude effect depends on wavelength, with higher values at shorter wavelengths due to stronger scattering at shorter wavelengths. Further important influencing parameters are the optical characteristics of the air layer between the two altitudes, mainly the amount of aerosols and the amount of tropospheric ozone in this layer. The higher these two quantities are, the more reduced is the irradiance at the lower station and therefore the higher is the altitude effect. Additionally, the altitude effect can be increased, if at higher altitudes the terrain is snow covered. From all these influencing parameters it is clear that for the altitude effect not one single number can be given, but that it is necessary to describe it with a certain range of values in dependence on wavelength.

Examples for measurements of the altitude effect show this large range of variability. In the Chilean Andes, Piazena [8] found about 8% in the UVA and 9% in the UVB, Zaratti [9] found about 7% for erythemally weighted irradiance in Bolivia. In the Alps usually higher values were measured, as a consequence of higher amounts of aerosols and tropospheric ozone in the lowest layers of the atmosphere. An example is shown in Fig. 6 for measurements near Garmisch-Partenkirchen, Germany, where the spectral dependence was derived from simultaneous spectroradiometric measurements at different altitudes [10]. The given standard deviation shows the variability during this measurement campaign. Average values for the altitude effect in the UVA range are about 10% and for erythemally weighted irradiance about 15-20%. If in addition the surrounding of the mountain station is covered by snow and the station at low altitude is snow free, then the altitude effect might by further increased by 5-10%.

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