给我讲一下这道题 ---**Textual Information:** 17. (2024·江西萍乡·二模) 阅读图文材料，完成下列要求。 某年 2 月 20~22 日，青海湖北岸某观测站记录到两锋面过境引起的持续降雪天气。过程中，锋 A 于 20 日 22:30 左右率先过境，后与锋 B 相遇，两锋锋线合并为锋线 C。锋线 C 于 21 日 12:30 左右过境。下图示意该过程中的风场变化。 6 / 20 时间 (1)根据锋面高度变化指出锋 A 与锋 B 的冷暖性质，并比较势力强弱。 (2)分析该观测站此次持续降雪的产生过程。 (3)分析图中虚线框内风力与风向变化的原因。 **Chart Description:** * **Type:** Wind barb chart (showing wind direction and speed at different altitudes over time). It consists of two panels showing similar data for different time periods. * **Main Elements:** * **Coordinate Axes:** * Y-axis: Labeled "高度/km" (Altitude/km), with values ranging from 0 to 2 in increments of 0.2. * X-axis: Labeled "时间" (Time). The top panel shows times from 18:30 on the 20th to 2:30 on the 21st, in 1-hour increments. The bottom panel shows times from 10:30 on the 21st to 18:30 on the 21st, in 1-hour increments. * **Data Points/Series:** The chart displays wind barbs at various altitudes (0.2 km, 0.4 km, 0.6 km, 0.8 km, 1 km, 1.2 km, 1.4 km, 1.6 km, 1.8 km, 2 km) for each recorded time. Each wind barb indicates wind direction (pointing from the direction the wind is coming from) and wind speed (indicated by barbs and pennants). * **Annotations:** * Top panel is labeled "20-21 日". * The time "22:30" on the X-axis in the top panel is circled in red. * The time "12:30" on the X-axis in the bottom panel is circled in red. * A dashed rectangle encloses the wind barbs at 0.4 km, 0.6 km, 0.8 km, and 1 km altitudes for the times 11:30, 12:30, and 13:30 on the 21st in the bottom panel. * **Wind Barbs:** Varying directions and speeds are shown across the chart. For example, at 20:30 on the 20th, wind is from the northwest at higher altitudes and changes direction and speed at lower altitudes. At 12:30 on the 21st, wind direction changes significantly with altitude, generally shifting from north-northwest at lower levels to west-southwest at higher levels within the dashed box.