Ratio of PAR to broadband solar radiation measured in Cyprus

The relationship between the two radiant fluxes is studied from almost a 3-year data archive of hourly photosynthetically active photon flux (QP) and global solar irradiance (RS) performed at Athalassa, Cyprus. These data are used to determine temporal variability of the ratio (QP/RS) and its dependence on sky conditions. The seasonal variation of the ratio obtained from daily data ranges from 1.942 E MJ−1 (summer) to 1.892 E MJ−1 (winter) with an annual mean value of 1.919 E MJ−1. The ratio increased from 1.865 to 2.01 E MJ−1 (daily values) or from 1.878 to 2.197 μE J−1 (hourly values), as sky conditions changed from clear to overcast. Effective atmospheric parameters such as sky clearness, brightness and path length were found to cause substantial changes to the PAR fraction.