Contrary to above, the ratio QPd/Rd decreases with sky conditions changing from clear to overcast as Table 1 indicates; hourly ratio values varying from 2.556 0.27 to 2.422 0.24 (mmol J1) reveal that the ratio on clear days is 5.5% higher than that on overcast days. It is known from microphysics that clouds exhibit relatively constant extinction across the PAR spectrum, while aerosols commonly exhibit extinction that significantly decreases with increasing wavelength (Min, 2005). Also, cloud’s droplets are much larger than aerosol particles, enhancing thus diffuse Rd irradiance at longer wavelengths more effectively than aerosols; therefore, under cloudy skies the ratio QPd/Rd decreases. In contrast, under clear skies aerosol scattering processes enhance diffuse QPd more than diffuse broadband Rd; thus increasing ratios QPd/Rd (Alados and Alados- Arboledas, 1999). Table 1 entries also reveal an annual mean hourly QPd/Rd value of 2.434 0.26, a cold period mean value of 2.298 0.27, and a warm period mean of 2.538 0.22 (mmol J1), respectively. The overall variation in QPd/Rd found here is in line with that reported in Williams (1976) and Olseth and Skartveit (1997).