## 2 Materials and Methods
### Data Sets and Methods  
The basic evaluation of the proposed system took place between 23 November 2018, and 18 December 2018, at the Poprad-Tatry Airport (LZTT), with the remote assistance of the professional meteorological observers on duty at Bratislava Airport (LZIB). It was a real-time experiment, i.e., the professional observers at Bratislava carried out parallel observations both at LZIB and LZTT as the weather was happening.  
\begin{table}
\begin{tabular}{c c c c c c c} \hline  & **Observer** & **0–600 m** & **600–1500 m** & **1500–5000 m** & **>5000 m** & **Sum** \\ \hline \multirow{3}{*}{Number of reference markers} & LO & 15 & 3 & 8 & 20 & 46 \\  & RO & 56 & 26 & 30 & 41 & 153 \\ \cline{1-1}  & LO & 8 & 2 & 4 & 8 & — \\ \cline{1-1}  & RO & 8 & 8 & 8 & 8 & — \\ \hline \end{tabular}
\end{table}
Table 1: The number and the coverage of the reference cardinal directions covered by markers used by local human observers (LO) and the proposed system for remote observation (RO), respectively.  
The data sets that appeared in the validation experiment can be shortly characterized as follows:  
* The data set termed as'standard Automatic Weather Observation System' (AWOS) consists of the data from common sensors for both of the examined meteorological variables. On one hand, it is the visibility sensor Vaisala FS11P. Measurements from this device are available four times a minute. From this data set, 1-min and 10-min averages were calculated. On the other hand, the automatic observations of the cloud heights are supplied by the ceilometer from Vaisala, from which data are available with a 30 s frequency.
* METAR messages are generated at LZTT every half an hour. The only problem of this data set is the missing information on the exact time of the local observation reported in the METAR messages. Based on the oral communication with the observers, this occurs normally 10 min before issuing METAR, i.e., approximately at HH:20 and HH:50. This data set is referred to as 'Local Observations' (LO) corresponding to the fact that it is created by the observers in charge that are present locally at the airport.
* The evaluation of the situations at LZTT that was carried out with the help of the staff of observers at Bratislava will be termed as 'Remote Observations' (RO). The proposed software was installed at the workplace of meteorological observers at LZIB. Observers received training how to validate the LO data set. Then, during the validation period, in addition to their standard work for the Bratislava Airport they also coded METAR messages remotely, for LZTT, using the HMI of the proposed software.  
The performance of the proposed system is evaluated by a comparison of the RO, the LO, and the AWOS data observed at the same time, mostly in terms of scatter plots, and also illustrated by a number of use cases.  
Figure 6: Adding markers to estimate the overall prevailing visibility—an example from LZTT, a view in the northern direction.  
Note that in the case of visibility observations, an additional experiment was carried out that specifically examined low visibility situations in the period of the cold half-year from September 2018 to April 2019. This additional experiment, unlike the basic evaluation, was carried out in a post-facto way, i.e., a bit longer after the weather happened. Section 3.5 below explains the reasons for this experiment.