Altitude effect on the clearness index in the French alps

Simultaneous measurements of horizontal global solar irradiance were done over three years, at altitudes ranging from 200 m to 1,700 m in an industrial alpine valley, for studying the clearness index variations versus height. Estimated values of the Angstrom coefficient suggested that the main cause of depletion of the clear sky clearness index k(c), at the lower levels was the scattering by the aerosols. For irradiances, a depletion term D = 1 - k(c) was empirically assumed to be a function of the altitude z, as simple as D = (Mz + N)-1. A study carried out over 8,000 10-minute sequences of measurement, during clear sky periods, showed that the coefficient M was sensible to the season only, while N was mainly a function of the sun elevation h such as N = A(h - C)0.5 + B; from 240 m to 1,100 m, k(c) was found increasing by about 12% for h = 30-degrees and 7% for h = 60-degrees. The yearly global horizontal irradiation, although including many cloudy sequences, appeared linearly related to ln(z), its relative increase between 200 m and 1,700 m rising 10%; whereas, for shorter periods, the monotony of the vertical profiles of the clearness index for irradiation was often significantly affected by local stationary clouds or seasonal mists.