## 3 Results
### Prevailing Visibility--Extended Validation  
The extended validation experiment was targeted on the period of the cold half year from September 2018 to April 2019. This time span was selected because in Slovakia (or, in a broader sense, in Central Europe), it is the period of the year with higher probability of the occurrence of fogs and lowered visibility situations. It was a retrospective (post-facto) analysis, as it was carried out later, during the late spring and early summer of 2019, focusing predominantly on weather events with decreased visibility. All the situations were assessed by the local professional observers on one hand, and by three independent meteorologists as remote observers on the other. Due to the higher number of remote observers, one can compare the local and remote observations in different ways. The stricter approach (from now onward:'strict criterion of comparison') is to require meeting the ICAO accuracy conditions by all three remote observers at the same time. A more lenient approach ('loose criterion of comparison') is to declare that it is sufficient that at least one remote observer meets the accuracy conditions. When presenting results, we will prefer the first, stricter approach (if not emphasized otherwise). The extended validation experiment resulted in figures that are summarized in Table 9. The increase in the number of low visibility situations is clear in comparison to Table 8. \begin{table}
\begin{tabular}{c c} \hline
**Visibility Range [m]** & **Number of Situations** \\ \hline \(<\)600 & 1 \\
600–1500 & 2 \\
1500–5000 & 5 \\ \(>\)5000 & 237 \\ \hline \end{tabular}
\end{table} 
Table 8: Results of the basic validation experiment for assessing the accuracy of the visibility by the proposed system at LZTT. For each of the 34 weather events from the category with the lowest visibility ranges (i.e., 600 m or lower), i.e., the most adverse ones for the air navigation, a separate detailed analysis was carried out. It turned out that these situations could be classified into five, well-defined categories (Table 10). Category #1 of Table 10 represents the cases that all meet the strict ICAO accuracy condition, i.e., none of the three values of prevailing visibility assessed by the remote observers differ from the visibility estimated by the local observer by more than \(\pm\)50 m.  
Category #2 of Table 10 is directly related to the lack of the explicitly defined time to determine visibility conditions. Only the time period is defined when the observation should be made, and it is the period of 10 min just before the METAR validity time. Such a flexibility may lead to differences in the reported values of the local vs. remote observers, particularly in cases of rapid changes in meteorological conditions. On the basis of the data from the visibility sensors, we were able to identify situations such as that. For instance, on 9 October 2018, over the course of 20 min (from 06:10 to 06:30 UTC) the visibility increased by more than 1000 m. In case the local observer would evaluate the prevailing visibility a bit earlier, e.g., at 06:18, the final METAR record would be 200 m. If the observer decided to carry out the observation at 06:20, one would get a value of 300 m; and if at 06:22, the visibility would be 600 m.  
Category #3 of Table 10 indicates the number of cases, when the local observer reported the data directly from the visibility sensor, instead of his/her own observation. At the time of determining the visibility, each observer has got the current AWOS data (the latest averages of the visibility from a 1-min and a 10-min window) available.