Global photosynthetically active radiation and its relationship with global solar radiation in the Eastern Mediterranean basin

The relationship between global photosynthetically active radiation (PAR) and global solar radiation is studied with a 2-year data archive of hourly values H-PAR and H-SW of these irradiances obtained at Athalassa, Cyprus. These data are used to determine the temporal variability of H-PAR and its dependence on sky conditions. The seasonal variation of the ratio H-PAR/H-SW obtained from daily data ranges from 0.456 (during summer) to 0.451 (during winter) with an annual mean value of 0.454. The ratio increases from 0.454 to 0.467 (daily values) or from 0.460 to 0.501 (hourly values), as sky conditions change from clear to overcast. Finally, the effect of aerosol content on the H-PAR/H-SW ratio is found to incur substantial changes in the PAP, fraction.