## 6 Conclusions  
The analysis of photosynthetic photon flux density to broadband solar radiation ratios registered between April 2019 and January 2020 in Burgos, Spain, at ten-minute intervals, has shown a representative dependency on the sky type conditions classified by CIE taxonomy. The higher values of the \(Q_{p}/R_{s}\) ratio were for overcast sky types, while the values were lower and more dispersed under clear sky conditions.  
Statistically significant differences have been found in the \(Q_{p}/R_{s}\) ratios for each CIE standard sky type. The overcast sky types presented the highest values of the ratio, with the clear sky categories presenting the lowest and most dispersed values. During the experimental campaign, there were only two exceptions to the expected behavior: sky types 5 and 9. They belonged to covered and partial covered sky type categories, respectively, presenting values closer to the clear sky categories according to the CIE standard. The main characteristic of both categories was a high uniformity in terms of illumination.  
The higher dispersion of data corresponding to clear skies categories could be explained by the presence of aerosols or atmospheric turbidity, characteristics of clear sky types 13, 14 and 15, with FOC's 15%, 8% and 7% (Figure 3).  
No seasonal dependency of \(Q_{p}/R_{s}\) can be established, as highlighted in this and the other studies that were reviewed, due mainly to the different sky conditions recorded during the experimental campaigns. As shown in Figure 5, between April 2019 to January 2020 in Burgos, Spain, clear skies predominated in summer, while in winter the overcast conditions presented the highest frequency of occurrence.  
The analysis of the hourly values also revealed a daily pattern with higher and more stable values of \(Q_{p}/R_{s}\) in the first hours of the day that tended to stabilize around noon and to decrease around sunset. The study of the daily pattern of the sky types (clear, overcast and partial) show that, although clear skies are predominant at all hours of the day, the differences of the frequency of occurrence with respect to overcast and partially overcast skies decreases towards noon, as Figure 6 showed.  
As has been demonstrated in this study and in others, the most influential factor in the \(Q_{p}/R_{s}\) value was the presence of overcast sky conditions. Although other authors used different climatological parameters for sky classification, the CIE Standard sky classification has proven itself to offer a good overall framework that can represent the sky conditions, covering the complete spectrum of sky categories.  
Conceptualization, M.D.-M. and C.A.-T.; methodology, A.G.-R. and S.G.-R.; software, A.G.-R.; validation, A.G.-R., M.D.-M.; formal analysis, A.G.-R. and S.G.-R.; investigation, A.G.-R. and S.G.-R.; original draft preparation, M.D.-M.; writing--review and editing, C.A.-T.; visualization, A.G.R. and S.G.-R., supervision, M.D.-M. and C.A.-T.; project administration, M.D.-M.; funding acquisition, C.A.-T. All authors have read and agreed to the published version of the manuscript.  
This research was funded by Spanish Ministry of Science and Innovation, grant number RTI2018-098900-B-I00 and Consejeria de Educacion, Junta de Castilla y Leon, grant number BU021G19.  
The authors declare no conflict of interest.