Contrary to above, the daily QPd/Rd ratio turns out to follow the kt patterns (Fig. 3b); a weak positive correlation between the two exists, which is well fitted by an exponential equation with coefficient of determination of R2 = 0.276, determined at the confidence level of 97.5%. The present findings agree well with Alados and Alados-Arboledas (1999) and Min
(2005) results according to which under cloudless skies the ratio QPd/Rd reaches its highest values. It is known
that under clear sky the regulating factor in the solar radiation modification is the scattering processes that are expected to influence the diffuse spectral distribution. Spectral investigations support the premise that under clear skies with aerosol loads, short wavelengths are preferentially scattered, thus increasing more diffuse PAR than diffuse broadband component
(Jacovides et al., 2000; Dye, 2004). Further, the degree of dispersion in the daily QPd/Rd ratio around the mean value is higher than that found in the daily QP/RS ratio. Also, the behavior of QPd/Rd ratio as a function of the kt parameter, is different to that encountered for the daily QP/RS ratio. For lower kt values usually related with cloudy conditions, weak changes in the QPd/Rd ratio occur; by contrast, for larger kt values, likely associated with clear conditions and/or translucent clouds, greater variability can be seen. Viewing Fig. 3b in conjunction with Fig. 2, it is clear that during wet seasons (kt < 0.3) the ratio QPd/Rd decreases due to the enhancement of the extinction process mainly at shorter wavelengths; in contrast, large daily kt values associated with increases in the daily QPd/Rd values are likely due to the dominant scattering processes.