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 mol J�����1, presented statistically significant di erences 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.
