# Variability of Diffuse Daylight Due to the Diversity of Cloud Arrays
## 4 Discussion and Conclusions  
The presented results clearly show that diverse cloud configurations cause considerable variability in diffuse sky light. A different altitude and size of the low-level clouds affect the diffuse illuminances at the ground in various ways depending on their distribution in the sky and cloud fraction. For example, the horizontal illuminance or illuminance on a vertical wall can differ by several klx for clouds of the same size but 500 m apart in altitude. However, there is no universal rule how to predict such results. They are determined by particular circumstances.  
Considering clouds with the same parameters, a variation in their distribution in the sky at the same cloud fraction changes the sky luminance pattern significantly, resulting in noticeably different diffuse illuminances on the horizontal plane and facades. The differences between individual values can be by more than 10 klx. In principle, this could lead to ambiguous classification of the standard sky type [18; 19], although we still have the same cloud cover (only a different cloud layout).  
However, the opposite situation is also possible. Many empirical models of sky radiance/luminance distribution [15; 16; 17] use various diffuse and direct (or global) irradiance ratios to classify the sky quality and to determine corresponding coefficients for calculation of the relative sky radiance/luminance. Therefore, if some two cloud configurations result in the same direct and diffuse horizontal irradiance, these models are not able to distinguish them, although the actual luminance distributions may be different.  
All the preceding statements imply that if we want more realistic predictions of diffuse light availability, we need to use more realistic models that account for various actual sky conditions. We should simulate different cloud configurations to gain a sense of how the luminance might be distributed in the sky. The empirical models only provide averaged sky types that do not correspond to actual conditions, which can result in noticeable deviations in predicted diffuse light amount.  
Mass calculations using more realistic models simulating actual sky conditions could serve as an alternative or supplement to climate-based modeling, which obviously uses the averaged sky luminance models applied to some typical illuminance/irradiance conditions  
Figure 6: Diffuse illuminance on vertical walls calculated for the sky luminance patterns depicted in Figure 4.  
at given time during a year. Knowing the typical properties of a cloud coverage (cloud fraction, cloud altitudes... ) and properties of atmospheric aerosol during a year, one could be able to obtain an estimation of possible diffuse light variations due to random cloud configurations in a particular period.  
Conceptualization, J.P. and M.K.; Investigation, J.P.; Formal analysis, J.P.; Writing--original draft, J.P. and M.K. All authors have read and agreed to the published version of the manuscript.  
This research was funded by Slovak National Grant Agency, grant number VEGA 2/0010/20 and Slovak Research and Development Agency, grant number APVV-18-0014.  
Not applicable.  
The numerical solver used to model diffuse illuminance on the horizontal surface and vertical planes is publicly available on [[http://unisky.savk./?lang=en&page=aplikacia&subpage=sim](http://unisky.savk./?lang=en&page=aplikacia&subpage=sim)]([http://unisky.savk./?lang=en&page=aplikacia&subpage=sim](http://unisky.savk./?lang=en&page=aplikacia&subpage=sim)) (accessed on 2 August 2021). The computational data are available from the corresponding author upon reasonable request.  
We are grateful to [PERSON] for parallelization of UniSky Simulator and development of graphical user interface which both made the mass numerical modeling possible.  
The authors declare no conflict of interest.