| Summary: | Visibility is a major factor affecting the safety and efficiency of aviation operations. As instrumental visibility measurements are the basis for calculating runway visual range, measurement accuracy is crucial. In this study, we collected instrumental visibility data (V<sub>I</sub>) from two sets of Vaisala transmissometers (referred to as LT<sub>05</sub> and LT<sub>23</sub>) and one set of forward-scatter meters (referred to as FD) installed at Xiamen Airport. We also considered manually observed visibility (V<sub>M</sub>) and relative humidity (RH) records of automatic weather stations for 2015 to 2020. Taking the V<sub>M</sub> data as the benchmark, we comprehensively evaluated the difference between the V<sub>I</sub> of each instrument and V<sub>M</sub> as well as the influence of RH on such deviations based on the deviation (ΔV), relative deviation (ΔV<sub>Mdi</sub>), root mean square error (RMSE), relative-RMSE, mean value, and correlation analysis method. Our results showed that: (1) among the three sets of visibility meters, the V<sub>I</sub> values of LT<sub>05</sub> were the closest to the V<sub>M</sub> values under different V<sub>M</sub> levels and at high RH. Deviations in the FD measurements were greater than those of the two LT sets under low-visibility conditions that significantly affect aviation operations (V<sub>M</sub> < 800 m). (2) In general, the V<sub>I</sub> values were lower than the V<sub>M</sub> values, and the larger the V<sub>M</sub> value, the greater the deviation. (3) Extremely large ΔV and ΔV<sub>Mdi</sub> values appeared in spring when the visibility level was rapidly increasing or decreasing. (4) The FD results were greatly affected by RH, with higher proportions of large ΔV and large ΔV<sub>Mdi</sub> data than the two LT sets under high-humidity conditions. Therefore, the Vaisala transmissometers outperformed the forward-scatter meter at the airports along the southeastern coast of China and those in high-humidity environments.
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