Why does the weather suddenly cool down around Tomb-Sweeping Day every year?

Because it's spring. The warm air rich in water vapor in the south goes north, and when it meets the cold air that is still entrenched, the water contained in the warm air condenses into water droplets and falls down. But it is not absolute. For example, in the north, it doesn't rain at this time. Because before and after Tomb-Sweeping Day, the warm and humid air in the southeast of Fang Haiyang became active. The warm and humid airflow collided with the cold air from the north. Where cold and warm air conflict, rainy weather is formed. Therefore, the precipitation during the Qingming period is mostly frontal rain. During the Qingming period, the fronts are mainly distributed in the southern areas of the Yangtze River Basin, Huaihe River Basin and Yellow River Basin. This is also the reason why there are many rains in the Qingming period in the south of the Yangtze River. So there is a saying in the south called rainy season.