## 1 Introduction
Some studies have proposed experimental models of PAR that include different parameters to take into account the atmospheric water vapor content, such as vapor pressure [16; 24] and/or dewpoint temperature [4]. The definition of sky types (clear, overcast, and partly-cloudy) for this task take into account different combinations of meteorological variables, mainly the clearness index, \(k_{t}\) (ratio of global solar radiation to extraterrestrial solar radiation) [14; 25; 26; 27; 28]; \(k_{t}\) and relative sunshine, \(S\), [29; 30]; [PERSON]'s clearness index, \(\varepsilon\), and sky brightness, \(\Delta\), [4; 31], and types and extent of cloud cover [32]. However, the conclusions of a previous work [33] suggested that the use of meteorological variables or meteorological indices, showed limited results for sky classification. The use of meteorological indices for sky classification depends more on their availability than on their accuracy and various authors have used such indices (or combinations thereof) in different ways. Sky classification is a complex problem, due in part to such abstract conceptual definitions as clear, partial cloudy, and overcast, as well as other intermediate ranges. The study of the dependence of any magnitude with respect to the type of sky firstly requires a standardized classification of the skies, to specify the atmospheric characteristics and illumination levels of each of the established types. In 2003, the International Commission on Illumination (Commission Internationale de L'Eclairage or CIE) defined 15 standard sky types, five categorized as clear, five as partial cloudy, and five as overcast skies. In several works, it was concluded that the CIE standard sky classification adequately represented empirical sky conditions [34; 35; 36; 37; 38; 39; 40]. Sky types of the same category have the same well-defined sky luminance patterns that easily yield the solar irradiance and daylight illuminance on the surfaces of interest through simple mathematical expressions [41]. Therefore, the CIE standard classification characterizes each type of sky in terms of energy and daylight. Figure 1 shows the main characteristics of each CIE standard sky type. Taking into account that illuminance (400-780 nm) and PAR (400-700 nm) share part of the spectrum of visible radiation, the use of the CIE Standard Classification is proposed in this work as the main parameter for characterizing the dependency of the \(Q_{p}/R_{s}\) ratio on atmospheric conditions. The study is focused on the determination of the \(Q_{p}/R_{s}\) ratio in Burgos, Spain and its dependency on sky conditions. Experimental data on horizontal solar global irradiation, \(R_{s}\); photon photosynthetic flux density, \(Q_{p}\); and, the CIE standard classification for homogeneous skies; collected through experimental sky scanner measurements, were available for this work. A complete statistical analysis of the results over different temporal (ten-minute, hourly, daily, and monthly) phases was completed. The results collected under different sky conditions were tested in a 10-month experimental test campaign. Figure 1: CIE Standard sky conditions. Images of different sky types recorded with a SONA201D All-Sky Camera—Day in Burgos, Spain. The paper will be structured as follows. The experimental facility and the measurement campaign as well the quality filters applied to the experimental data will be described in Section 2. The CIE Standard sky classification in Burgos during the experimental campaign will be introduced in Section 3. Temporal variability of the \(Q_{p}/R_{s}\) ratio over the different temporal intervals will be explained in Section 4. A variability analysis of the \(Q_{p}/R_{s}\) ratio in accordance with the CIE Standard Sky types will be analyzed in detail in Section 5. Finally, the main results and the conclusions of the study will be summarized in Section 6.