## 1 Introduction  
Visibility manifests a visual obstruction phenomenon and is a basic meteorological parameter. It is widely applied in transportation, meteorology, ecological conservation, and environmental protection. Runway visual range (RVR) is an essential meteorological indicator in the minimum standards set for airport operations [1], directly affecting the safety and efficiency of aviation operations [2]. Article 37 of the Technical Specification for Automatic Meteorological Observation System for Civil Aviation (AP-117-TM-2018-03R1, Civic Aviation Administration of China, Beijing, China) issued by the Civil Aviation Administration of China sets strict requirements for the accuracy of visibility meters. When the meteorological optical range (MOR) measured using a visibility meter is less than or equal to 600 m, the maximum permissible error is 50 m. When 600 m < MOR \(\leq\) 1500 m,the maximum permissible error is 10% of the MOR. When MOR > 1500 m, the maximum permissible error is 20% of the MOR. Hence, accurate visibility observations are crucial for aviation safety. Before the 1990s, visibility data from airports in China were obtained entirely by manual observation. With the deployment and operation of the automatic weather observation system (AWOS) at Beijing Capital International Airport in 1985 [3], visibility measurements started to be performed both automatically and manually at China's airports, and real-time, objective, and high-temporal-resolution monitoring of visibility at airport runways has already been realised. However, due to differences in the measurement principles and detection ranges between visibility meters and manual observation methods [4], the visibility values in the meteorological aerodrome reports (METAR) provided by airports are still those obtained through manual observation. The available visibility meters can be divided into transmissometers and scatter meters according to their measurement principles [5]. They are all designed based on Koschmeier and Allard's law, but the detection methods differ [6; 7; 8; 9], with different sources of error [10; 11; 12]. Researchers have conducted many comparison experiments, assessments, and calibrations to reduce the errors of these two types of visibility meters [13; 14; 15; 16; 17; 18; 19; 20]. Meanwhile, many scholars have compared the instrument-observed visibility values with those obtained through manual observation to apply the instrumental measurements rationally [21; 22; 23; 24; 25]. Some studies have focused on the differences between the records of several visibility meters of the same type and the manual observations within specific regions, and others have analysed the differences between the visibility values obtained using multiple sets of instruments of the same type and the manual observations obtained at the same site. However, most of these studies performed qualitative statistical analyses of correlation results and deviation trends. Data on manual observations available for comparative analysis are limited, and studies comparing instrumental and manual visibility observations at airports are scarce. [PERSON] et al. calculated the average absolute deviation between visibility values obtained from an FD12P forward-scatter(VAISALA, Vanta, Finland) meter at Guangzhou New Airport and those obtained through manual observation and comparatively analysed their differences under different visibility levels and low cloud cover [26]. [PERSON] et al. used visibility data obtained using a DNQ1/V35 forward-scatter(Huayun Shengda (Beijing) Meteorological Technology Co., Ltd., Beijing, China) meter during the site selection stage for Ezhou international logistics core hub airport as well as visibility data obtained through manual observation during the daytime to analyse the differences between the two by calculating the mean and standard deviation of the difference between them [27].