## 4 Conclusions  
We obtained the following conclusions by comparing the V\({}_{\rm I}\) results of two sets of Vaisala transmissometers (LT\({}_{05}\) and LT\({}_{23}\)) and one set of forward-scatter meters (FD) with manually obtained V\({}_{\rm M}\) values and analysing the variation of the differences between V\({}_{\rm I}\) and V\({}_{\rm M}\) with RH.  
The \(\Delta\)V values of the three visibility meters were predominantly negative. In particular, the proportion of negative \(\Delta\)V values accounted for more than 80% at V\({}_{\rm M800}\), which is conducive to ensuring the safety of aviation operations.  
The V\({}_{\rm I}\) values of LT\({}_{05}\) were the closest to those of V\({}_{\rm M}\). At V\({}_{\rm M800}\), the RMSE and RRMSE of FD were higher than those of the two sets of LT, the number of zero-\(\Delta\)V values for FD was significantly lower than that for LT, and the large \(\Delta\)V and large \(\Delta\)V\({}_{\rm Mdi}\) values of FD were much higher than those of LT. Thus, a transmissometer is the optimum visibility detection equipment among Vaisala instruments for airports along the south-eastern coast of China.  
RH substantially impacted FD. When RH < 75%, the RMSE and RRMSE of FD were lower than those of the two sets of LT; as RH increased, the RMSE and RRMSE of FD increased rapidly, with a particularly significant increase in high humidity, and the large \(\Delta\)V and \(\Delta\)V\({}_{\rm Mdi}\) values of FD were significantly higher than those of LT. Therefore, transmissometers are more suitable visibility detection instruments than FD in high-humidity environments. The performance of FD at Xiamen Airport was restricted by the factory-calibrated physiochemical properties of aerosols, but the physiochemical properties of aerosols vary regionally, and their optical properties change significantly under the influence of environmental humidity. Thus, the detection performance of FD in high-humidity environments requires further assessment based on measurements obtained for inland and plateau areas.  
Anthropogenic causes increased the difference between V\({}_{\rm I}\) and V\({}_{\rm M}\) at night. Adding target lamps can effectively reduce this difference.  
**Author Contributions:** Conceptualization, H.J. and F.L.; Data curation, X.R. and K.H.; Formal analysis, Z.L.; Investigation, Z.C.; Methodology, H.J., F.L., Z.L. and P.W.C.; Resources, Z.C.; Software, X.R. and K.H.; Supervision, P.W.C.; Validation, H.J. and Z.L.; Writing--original draft, H.J.; Writing--review and editing, F.L. and P.W.C. All authors have read and agreed to the published version of the manuscript.  
**Funding:** This study was supported by the Natural Science Foundation of Fujian Province, China (2021)01463 and 2022J01446) and the Guided Foundation of Xiamen Science and Technology Bureau (3502Z20214 ZD4004).  
**Institutional Review Board Statement:** Not applicable.  
**Informed Consent Statement:** Not applicable.  
**Data Availability Statement:** The data used to support the findings of this study are available from the corresponding author upon request.  
**Acknowledgments:** The researchers express special gratitude to [PERSON] from Meteorological Observation Center of CMA, who approved and supported this research.  
**Conflicts of Interest:** The authors declare that there are no conflict of interest.