## 4 Discussion  
Generally, the validation experiments confirmed our preliminary hypotheses: the remote observer (RO) approach clearly demonstrated better performance than the standard AWOS, and, at the same time, the performance of the RO was comparable with the local observations (LO), both for the estimation of the cloud base height (Figures 7 and 8) and the prevailing visibility (Figures 13 and 14). Nevertheless, the obtained results are far from an ideal relationship between the RO and the LO approaches with R\({}^{2}\) close to 1.00, probably due to the following reasons:  
1. The conclusions are drawn from a small sample, and
2. The whole experiment is based on the hypothesis that the LO represent the ground truth. Here, the second point is rather questionable. What if we admit that mistakes may also appear on the side of local observers? As the observations of the prevailing visibility and cloud coverage are carried out by human observers, using the tool named 'human eye', and with no other objective measurement devices, the result is inherently biased by subjectivity. However, as far as we know, there is no study that would evaluate this bias in the light of diverse perceptions of individual observers and/or complex conditions for observation and other factors. This is a problem, as ignorance of the error rate of the reference observation reduces the accuracy of the validation of any new system targeted on the determination of these meteorological variables. The extended validation experiment with the three independent remote observers proved that in spite of the availability of significant technological support (photos with associated reference points that can be checked repeatedly), different professional observers may evaluate a given low visibility situation in a different way, even with a difference of \(\pm 50\) m in their estimates. Local observers are also human, and they have to cope with the dynamic and unrepeatable character of nature in the absence of the technological support  
Figure 16: The same as Figure 15, but for 28 October 2018, at 06:50 UTC. \begin{table}
\begin{tabular}{c c c c} \hline  & **AWOS** & **RO** & **LO** \\ \hline Visibility [m] & 3000 & 700 & 700 \\ Phenomenon & BR & FG & FG \\ \hline \end{tabular}
\end{table} 
Table 12: The same as Table 11, but for 28 October 2018, at 06:50 UTC. BR—mist, FG—fog. of the database of camera images. Therefore, we suppose that it is reasonable to question the unmistakable character of the observations carried out by the local professionals. But how could be this hypothesis verified? Obviously, one could design a long-term experiment with three independent local observers parallel in charge. Nevertheless, it is not a straightforward task to carry out such an experiment, for different serious reasons. Generally, there is the lack of human resources. The minimum requirement is to have five experts to cover the weather service duties in 24/7. To accomplish the task with three local observers operating at the same time, one would need an at least three times larger staff of observers, further extra budget to cover their salaries, and not less importantly, to have sufficient infrastructural background; for instance, to ensure the limitation/blockage of communication of the observers among themselves. Otherwise, it is nearly impossible to evaluate the degree of subjectivity of the professional observers. It is in the future plans of the authors of this article to design and carry out such an experiment. The extended validation experiment shed light on the fact that the correct character of the observations may also be influenced by the attitude of the local observers, e.g., when they simply copy the data supplied by the standard AWOS or when a minor detail in the process of the evaluation of a weather event results in more significant differences in the overall assessment of the given situation.