## 4 Results and Analysis
### Characteristics of DP
#### 4.3.2 Characteristics of Seasonal DP  
Figure 8 shows that the variation characteristics of seasonal sand-moving wind frequency in Qiemo and Ruoqiang. Qiemo and Ruoqiang had the largest DPs (54.90 and 37.46 VU) in the spring, with RDPs of 42.27 and 30.20 VU and RDDs of 226.30\({}^{\circ}\) (SW) and 238.84\({}^{\circ}\) (WSW), respectively. Their RDP/DPs were 0.770 (medium ratio) and 0.806 (large ratio), respectively. The above results indicate that the wind directions of the two counties in the spring are fixed and not liable to variation. However, spring is also the season most susceptible to wind-sand disasters, mainly because of the largest DP in this season. In the summer, the DPs of Qiemo and Ruoqiang were 46.91 and 16.30 VU, respectively. Their RDPs were 15.49 and 2.04 VU, respectively. Their RDP/DPs were 0.330 and 0.125 only, respectively. The RDP/DP of Qiemo in the summer was extremely close to a small ratio, while that of Ruoqiang was already a small ratio, indicating that wind directions are variable in the summer. As shown in Figure 8, the DP in the direction opposite to the prevailing direction accounted for a considerable proportion in the summer, resulting in a smaller RDP and a greater effect on RDD in this season. The RDD of Qiemo in the summer was 213.22\({}^{\circ}\) (SSW), and that of Ruoqiang was 259.77\({}^{\circ}\) (W). These results are different from the overall manifestations. Although the summer had a lower intensity of sand drift activities than the spring, it is the season with the most complex wind regime among the four seasons. In the autumn, the DPs of Qiemo and Ruoqiang were 19.98 and 9.03 VU, respectively. Their RDPs were 15.45 and 6.64 VU, respectively. Their RDP/DPs were 0.774 and 0.735 only, respectively. Compared with the other three seasons, autumn has relatively stable wind directions. The RDDs of Qiemo and Ruoqiang in the autumn were 220.29\({}^{\circ}\) (SW) and 245.60\({}^{\circ}\) (WSW), respectively. These results are in good agreement with their respective annual RDDs. Overall, autumn has a low intensity of sand drift activities, a relatively stable wind regime, and simple wind directions. Qiemo and Ruoqiang had the smallest DPs (8.30 and 3.39 VU) in the winter,  
Figure 7: Characteristics of annual DP.  
with RDPs of 6.78 and 2.00 VU and RDDs of 225.18\({}^{\circ}\) (SW) and 237.31\({}^{\circ}\) (WSW), respectively. Their RDP/DPs were 0.816 and 0.589 (medium ratio), respectively. On the whole, there are relatively stable wind directions and infrequent sand-drift activities in the winter.  
The DPs of Qiemo and Ruoqiang decrease in succession from the spring to the winter, accompanied by a gradual decline in the intensity of sand drift activities. The prevailing directions of seasonal DP and annual DP are consistent. DP activities are intense in the spring and the summer. Wind directions are concentrated in the spring, making it convenient for the transport and accumulation of sand to the railway and resulting in traffic interruption. The complex wind regime makes it difficult to maintain stable wind directions in the summer. RDD is manifested as the combined action of several opposite directions, and sand drift activities in multiple directions pose great difficulties to sand control.