## 2 Materials and Methods  
### Study Area and Data Sources  
The datasets used in this study include the hourly meteorological optical range (V\({}_{\text{I}}\)) and RH collected by the AWOS at Xiamen Airport from January 2015 to December 2020 and the hourly data of manually observed visibility (V\({}_{\text{M}}\)) and RH from METAR for the same period. The AWOS is equipped with two sets of the Vaisala double-ended transmissometers of model LT31 (referred to as LT) with a baseline length of 30 m as well as one set of forward-scatter visibility meters of model FD12P (referred to as FD) with a scattering angle of 33\({}^{\circ}\). LT was used to measure the atmospheric transmissivity between the transmitter and receiver to obtain the mean extinction coefficient to evaluate visibility value V\({}_{\text{I}}\). Meanwhile, FD was used to evaluate the energy of near-infrared light with a forward-scatter angle of 33\({}^{\circ}\) caused by various suspended particles in the volume of air sampled, which was compared to the total light scattering to determine the total scattering coefficient. Thereafter, the extinction coefficient was derived, and visibility value V\({}_{\text{I}}\) was inverted. According to civil aviation regulations [28; 29], these instruments were installed on the northern side of the runway, 80-100 m from the runway centreline. The two sets of LT (referred to as LT\({}_{\text{05}}\) and LT\({}_{\text{23}}\)) were deployed at the two ends of the runway approximately 500 m inward, and FD was installed at the midpoint of the runway (see Figure 1). LT and FD are continuously monitored and regularly maintained. The field equipment is maintained weekly, and visibility calibration is performed monthly. The data processing software has an automatic alerting function so that engineers can be immediately notified to check data effectiveness in case of anomalies. Thus, the output data are of high quality. After eliminating abnormal data during brief equipment shutdowns required for maintenance, sensor coverage by foreign objects, or instrument damage due to typhoons, a valid V\({}_{\text{I}}\) dataset consisting of 52,368 hourly time points was obtained. \(\text{V}_{\text{M}}\) is the maximum horizontal distance visible throughout at least half of the horizon, not necessarily continuously [30]. It was determined by an observer at the manual observation platform using visual reference targets (target lamps) (see Figure 2). V\({}_{\text{M}}\) provided services in the form of METAR messages. There were six to eight meteorological observation personnel issuing V\({}_{\text{M}}\) messages at Xiamen Airport, and two to three people were arranged in rotation daily. The V\({}_{\text{M}}\) value was generally published once daily. The \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Instrument** & **Measurement Range** & **Accuracy** & **Temporal Resolution** & **Baseline Length or Scattering Angle** & **Operating Environment** \\ \hline LT & 10–10,000 m & \(\pm\)10\% & 1 min & 30 m & \(-40\pm 60^{\circ}\)C \\  & & & & RH: 0–100\% \\ FD & 10–15,000 m & \(\pm\)10\% & 1 min & 33\({}^{\circ}\) & \(-40\pm 50^{\circ}\)C \\  & & & & RH: 0–100\% \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Performance parameters of LT transmissometer and FD forward scatter meter. Figure 1: Locations of the three sets of visibility meters in relation to the manual observation platform. The performance parameters of the three sets of visibility meters are as described in Table 1.  
manual observation platform was on the northern side of the end of runway 05, with LT\({}_{05}\) located approximately 120 m to its east, LT\({}_{23}\)approximately 2500 m to its northeast, and FD approximately 1400 m to its northeast (see Figure 1). LT\({}_{05}\) was the closest to the manual observation platform, followed by FD and then LT\({}_{23}\).