## 3 Results
### Distributions of the Deviation and Relative Deviation of \(V_{l}\) Relative to \(V_{m}\)
#### 3.4.2 Distributions of Deviation and Relative Deviation at V\({}_{\rm M800}\)  
Figure 7a shows that, at V\({}_{\rm M800}\), the \(\Delta\)V values of the three visibility meters were predominantly negatively deviated, accounting for more than 80% of all \(\Delta\)V values, with an overall negatively skewed distribution pattern. The proportion of negatively deviated \(\Delta\)V was the highest for LT\({}_{23}\), reaching 90.6%. The \(\Delta\)V values were mainly in the range of [\(-\)200 m, 0 m], accounting for 57.8-62.5%, and this proportion was the highest for LT\({}_{23}\) and the lowest for FD. The \(\Delta\)V of LT\({}_{05}\) was mostly distributed between [\(-\)200 m, 200 m], accounting for 71.9%, and the \(\Delta\)V of LT\({}_{23}\) and FD mainly lay in [\(-\)400 m, 0 m], accounting for 75.0% and 73.4%, respectively. FD had significantly fewer zero \(\Delta\)V values than the two sets of LT but significantly more absolute \(\Delta\)V values exceeding 1000 m, indicating that the distribution of \(\Delta\)V was negatively skewed and that the V\({}_{\rm I}\) of LT\({}_{23}\) was generally smaller than V\({}_{\rm M}\) at V\({}_{\rm M800}\). The V\({}_{\rm I}\) of LT\({}_{05}\) was the closest to V\({}_{\rm M}\), while the deviation of V\({}_{\rm I}\) from V\({}_{\rm M}\) was the greatest for FD. At V\({}_{\rm M800}\), the \(\Delta\)V\({}_{\rm M}\)\({}_{\rm M}\)\({}_{\rm I}\) of the three visibility meters mainly lay in the range of [\(-\)100%, 0%], accounting for 81.3-90.6% of all \(\Delta\)V\({}_{\rm M}\)\({}_{\rm M}\)\({}_{\rm I}\) data, and this proportion was the highest for LT\({}_{23}\) and the lowest for LT\({}_{05}\). In general, the \(\Delta\)V\({}_{\rm M}\)\({}_{\rm M}\)\({}_{\rm I}\) values were concentrated between[\(-\)80%, \(-\)40%], accounting for 51.4-54.7%, and the overall distribution pattern was negatively skewed by 40% (see Figure 7b). Among the three sets of visibility meters, the \(\Delta\)V\({}_{\text{Mdi}}\) of LT\({}_{\text{05}}\) was mainly distributed in [\(-\)80%, 0%], accounting for 79.7% of all its \(\Delta\)V\({}_{\text{Mdi}}\) data. The \(\Delta\)V\({}_{\text{Mdi}}\) values of LT\({}_{\text{23}}\) and FD were predominantly between [\(-\)100%, \(-\)20%), accounting for 78.1% of their \(\Delta\)V\({}_{\text{Mdi}}\) data. FD had a significantly smaller proportion of \(\Delta\)V\({}_{\text{Mdi}}\) in [\(-\)20%, 20%] than the two sets of LT. Overall, the distribution of \(\Delta\)V\({}_{\text{Mdi}}\) was negatively skewed by 40% at V\({}_{\text{Md800}}\). The deviation of V\({}_{\text{I}}\) from V\({}_{\text{M}}\) was the least for LT\({}_{\text{05}}\) and the greatest for FD, and the extent of deviation for LT\({}_{\text{23}}\) and FD was greater than that for LT\({}_{\text{05}}\) by 20%. The average (AVG), maximum (MAX), and minimum (MIN) values of \(\Delta\)V and \(\Delta\)V\({}_{\text{Mdi}}\) for the three visibility meters at V\({}_{\text{Md800}}\) are listed in Table 5. In general, V\({}_{\text{I}}\) was smaller than V\({}_{\text{M}}\) by 70-124 m, with a relative deviation of 17.4-21.8%. The MAX of \(\Delta\)V and \(\Delta\)V\({}_{\text{Mdi}}\) for the different instruments varied significantly, and the largest values reached 2000 m and 1000%, respectively, all corresponding to LT\({}_{\text{23}}\), which were significantly higher than those of the data of the other two visibility meters. The differences in the MIN of \(\Delta\)V and \(\Delta\)V\({}_{\text{Mdi}}\) for the different instruments were relatively small, and the absolute values of their MIN were considerably smaller than the MAX values. Statistical analysis revealed that the MAX and MIN of \(\Delta\)V and \(\Delta\)V\({}_{\text{Mdi}}\) for the different visibility meters appeared in March and April in spring. The MAX values occurred during fog dissipation or when dense fog turned into mild fog, that is, when visibility improved rapidly; the MIN values appeared at the beginning of the formation of fog or dense fog, that is, when visibility decreased rapidly.