The ratio between photosynthetic photon flux density and broadband solar irradiance and its dependency on sky cloudiness conditions, defined with the CIE Standard sky classification, are established. A single pattern, only dependent on sky conditions and that is independent on the temporal basis used for the study, are demonstrated.

Plant growth is directly related to levels of photosynthetic photon flux density, Qp. The improvement of plant-growth models therefore requires accurate estimations of the Qp parameter that is often indirectly calculated on the basis of its relationship with solar irradiation, RS, due to the scarcity of ground measurements of photosynthetic photon flux density. In this experimental campaign in Burgos, Spain, between April 2019 and January 2020, an average value of the Qp/Rs ratio is determined on the basis of measurements at ten-minute intervals. The most influential factor in the Qp/Rs ratio, over and above any daily or seasonal pattern, is the existence of overcast sky conditions. The CIE standard sky classification can be used to establish an unequivocal characterization of the cloudiness conditions of homogeneous skies. In this study, the relation between the CIE standard sky type and Qp/Rs is investigated. Its conclusions were that the Qp/Rs values, the average of which was 1.93±0.15 μmol·J⁻¹, presented statistically significant differences for each CIE standard sky type. The overcast sky types presented the highest values of the ratio, while the clear sky categories presented the lowest and most dispersed values. During the experimental campaign, only two exceptions were noted for covered and partial covered sky-type categories, respectively, sky types 5 and 9. Their values were closer to those of categories classified as clear sky according to the CIE standard. Both categories presented high uniformity in terms of illumination.