## 3 Results
### Quantitative Evaluation of Fog Detection
#### 3.1.2 Fog Detection Performance by Algorithm
[31], which highlighted the need to refine the daytime fog detection algorithm to address issues such as satellite navigation errors, cloud shadow contamination, and uncertainty in the clear-sky reflectance composite. The findings indicate that further algorithmic sophistication and improved post-processing are necessary to enhance the performance of the GK2A daytime algorithm. Figure 4 presents the GK2A_FDA's monthly and seasonal fog detection performance, categorized by region and time. The quantitative detection statistics are summarized in Appendix A, Tables A1-A4. The monthly average POD in inland areas ranges from 0.33 (all day, August) to 0.74 (all day, January), representing the lowest and highest values observed during summer and winter, respectively (Figure 4d). Notably, during daytime, POD maintains relatively stable and high values between 0.65 and 0.74, outperforming other periods (Figure 4a). In contrast, both FAR and bias remain high throughout most of the year, except autumn, as indicated by their monthly averages ranging from 0.70 to 0.95 and 2.08 to 9.68, respectively (Figure 4d). During nighttime, the seasonal POD reaches its maximum in winter, remains moderate in spring and autumn, and declines substantially in summer (Figure 4b), a pattern consistent with the seasonal variation in nighttime fog and low cloud detection accuracy using the Day/Night Band and infrared channels of S-NPP/VIIRS (Suomi National Polar-orbiting  
Figure 4: Monthly and seasonal average detection performance (POD, FAR, and bias) of the GK2A_FDA over three years (2021–2023), categorized by time (daytime, nighttime, twilight) and geographic region (inland, coastal, overall). (**a**) Inland—Daytime, (**b**) Inland—Nighttime, (**c**) Inland—Twilight, (**d**) Inland—All Day, (**e**) Coastal—Daytime, (**f**) Coastal—Nighttime, (**g**) Coastal—Twilight, (**h**) Coastal—All Day, (**i**) Overall—Daytime, (**j**) Overall—Nighttime, (**k**) Overall—Twilight, (**l**) Overall—All Day. Partnership/Visible Infrared Imaging Radiometer Suite), with missed detections and false alarms predominantly occurring at cloud edges due to parallax effects and temporal mismatches between satellite and ground observations [41]. The monthly bias values are more stable than those in coastal regions but show peaks in January and June. Although twilight yields the lowest POD among all-time categories, they are associated with relatively lower FAR and bias values, suggesting more conservative detection behavior in that period (Figure 4c). Although the monthly daytime POD in coastal regions is less stable than in inland areas, it generally shows superior performance, except in August (Figure 4e). However, FAR and bias are also higher, indicating a clear tendency toward over-detection. During nighttime, the POD gradually decreases from winter to spring, summer, and autumn. The FAR remains high during the night and reaches its maximum in August, while the bias attains its highest value in winter (Figure 4f). Similar to inland conditions, twilight periods exhibit relatively lower FAR and bias values (Figure 4g). [PERSON] et al. [42] reported that higher air temperatures can reduce the efficiency of radiative cooling near the surface, thereby making fog formation and maintenance more difficult or leading to its rapid dissipation. August on the Korean Peninsula is influenced by the East Asian monsoon and is characterized by the highest monthly average air temperatures [43]. During this month, the daily mean sea surface temperatures (SSTs) are often comparable to, or even exceed, the air temperatures [33]. Moreover, coastal regions are frequently affected by convective and multilayered cloud development. As a result, the quality of the 30-day minimum value composition of visible channel reflectance is often degraded, making it more challenging to distinguish between fog and low-level clouds.