The significance of the daily clearness index (the ratio of K↓ to Kex) was taken into consideration because it accounts for the attenuation of solar radiation by all atmospheric constituents. Fig. 1 shows the seasonal trend in the daily clearness index. The substantial contributor to the variability present was cloudiness. Fig. 2 shows the relationship between the daily PAR/K↓ ratio and the clearness index. A weak inverse relationship between them exists, with significant scatter present about the trend line. Some of the scatter can be attributed to the influence of pyrogenic aerosols from biomass burning; decreases in the daily clearness index associated with high turbidities due to biomass burning show a concomitant decrease in the daily PAR/K↓ ratio.