## 3 Results
### Prevailing Visibility--Basic Validation  
In the basic validation experiment, the daytime visibility for all the METAR schedule times were parallel assessed (i) by the local observers (LO) at LZTT, and (ii) by the meteorologists of the Slovak Hydrometeorological Institute remotely, by means of the proposed system (i.e., RO). Both data sets are comprised of 245 individual observations.  
First, the performance of the standard automated sensors (AWOS) and the novel RO approach were inter-compared, in the light of the LO data set, which is considered as the ground truth. The results are presented in the form of scatter plots (Figures 13 and 14). Note that due to the natural dominance of situations with good visibility, i.e., multiple  
\begin{table}
\begin{tabular}{c c c c} \hline \hline  & **AWOS** & **RO** & **LO** \\ \hline Maximum cloud cover & FEW & BKN & BKN \\ Minimum cloud base [ft] & 6300 & 6300 & 6300 \\ \hline \hline \end{tabular}
\end{table}
Table 6: The same as Table 4, but for 21 September 2018, at 09:20 UTC. BKN—Broken.  
Figure 12: The same as Figure 9, but for 27 August 2018, at 07:20 UTC.  
occurrence of pairs of [9999; 9999] (211 times in both figures, indicated by yellow color), only a reduced set of pairs of observations are discernible in the scatter plots.  
Figure 14: The same as Figure 13, but with the prevailing visibility determined by the local observers (LO) and the remote observers (RO).  
Figure 13: Scatter plot of the values of the prevailing visibility determined by the local observers (LO) and the standard automated sensors (AWOS) at LZTT.  
The scatter plots in Figures 13 and 14 indicate an improvement in the observation accuracy. The comparison of the prevailing visibility is in favor to the RO data set with a correlation coefficient of 0.73, whereas the reference AWOS scenario reached only 0.59.  
Secondly, the LO and RO data sets were compared and evaluated directly, according to the criteria of the ICAO [1] 'Operationally desirable accuracy of measurement or observation'. They require the following accuracy for the prevailing visibility:  
* \(\pm\)50 m up to 600 m;
* \(\pm\)10% in the range of 600 to 1500 m; and
* \(\pm\)20% above 1500 m.  
Note that to obtain a finer stratification, we decided to split the last visibility interval into two parts: 1500 m to 5000 m, and 5000 m and more, respectively. In the light of these categories, the results of the basic validation experiment are stratified as summarized in Table 8.  
The most important finding of the summary in Table 8 is the fact that the categories with medium and low visibility ranges are considerably underrepresented--the prevailing visibility dropped below 5000 m only in eight cases (3.3%). Consequently, no generally valid conclusions can be drawn on the basis of such spare data. This is particularly a pity, as the situations with medium and low visibility ranges are the most crucial for the air traffic navigators.  
The basic validation experiment was conducted on data set with natural distribution of low visibility situations, and there lies its relevance. However, it also indicated the necessity of carrying out a further, more extensive one, with the focus set mostly on the weather events accompanied by low to medium visibility ranges.