Agricultural and Forest Meteorology 121 (2004) 135–140 Ratio of PAR to broadband solar radiation measured in Cyprus C.P. Jacovides a,∗ , F.S. Timvios a,b , G. Papaioannou a , D.N. Asimakopoulos a , C.M. Theofilou b a Laboratory of Meteorology, Department of Physics, Division of Applied Physics, University of Athens, Panepistimioupolis, Building PHYS-V, Athens 157-84, Greece b Department of Agrometeorology, Meteorological Service of Cyprus, Nicosia, Cyprus Received 6 June 2002; received in revised form 6 October 2003; accepted 9 October 2003 Abstract The relationship between the two radiant fluxes is studied from almost a 3-year data archive of hourly photosynthetically active photon flux (Q P ) and global solar irradiance (R S ) performed at Athalassa, Cyprus. These data are used to determine temporal variability of the ratio (Q P /R S ) 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 EJ -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. © 2003 Elsevier B.V. All rights reserved. Keywords: Photosynthetically PFD; Solar irradiance; Atmospheric parameters; Eastern Mediterranean 1. Introduction Photosynthetically active radiation (PAR) con- tributes significantly in comprehensive studies of radi- ation climate, remote sensing of vegetation, radiation regimes of plant canopy and photosynthesis. The PAR radiation covering both photon and energy terms lies between 400 and 700 nm or 380–700 nm in the solar spectrum (McCree, 1972). However, today’s more commonly accepted spectral interval 400–700 nm does not cause misunderstanding (Ross and Sulev, 2000). ∗ Corresponding author. Tel.: +30-210-727-6931; fax: +30-210-729-5281. E-mail address: kiakovid@cc.uoa.gr (C.P. Jacovides). Most published experimental results (Table 1) use measured values of global PAR (R P ) and global so- lar radiation (R S ) to determining the PAR fraction of the broadband solar radiation. In the literature, there exist three distinct measuring techniques for deter- mining PAR: (i) spectrally, by integrating spectral irradiance distribution measurements over the wave- band 400–700 nm; (ii) indirectly, through combined filtered data; and (iii) directly, via spectral PAR mea- surements (400–700 nm) by means of quantum sensor (Q P ). For further detail see the review article of Ross and Sulev (2000). Published values for the PAR frac- tion of global irradiance are around 0.45 or 2 EJ -1 for photon efficiency. Nevertheless, the range of the PAR fraction suggests the desirability for recalibra- tion accounting for local climatic differences. Thus, the present analysis aims to quantify temporal vari- 0168-1923/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.agrformet.2003.10.001