To visually illustrate ratios’ dependence on sky conditions, hourly correlation results between QP and RS versus the three sky conditions considered are shown in Fig. 4a, whereas respective results between QPd and Rd are given in Fig. 4b. The simple regression line Y = aX is applied, by omitting intercepts (Papaioannou et al., 1996; Udo and Aro, 1999). Herein, hourly ratio QP/RS values varying from 1.971 0.031 to 2.099 0.025 (mmol J1) indicate that the ratio on clear days is 6.5% lower than that on overcast days. Further, Fig. 4b suggests that, for Rd > 250 (W m2), both diffuse QPd and Rd components are larger under intermediate skies than under clear skies. This results from an additional contribution in the diffuse components from cloud edges (Min, 2005). Despite this, the enhancement in QPd is slightly lower than in the diffuse
Rd; this is verified through ratio a = QPd/Rd, which for intermediate skies is about 3.3% lower than on clear skies. The overall results obtained through linear fits are in line with earlier findings (Table 1), thus verifying Udo and Aro (1999), assertions.