Why does it rain in Tomb-Sweeping Day every year?

Every year before and after Tomb-Sweeping Day, it is often rainy, so Du Mu, a poet in the Tang Dynasty, said in a poem: "It rains a lot during the Qingming Festival, and pedestrians on the road want to die." Why does it always rain in Tomb-Sweeping Day?

First, when winter goes and spring comes, the cold air gradually weakens, and the warm and humid air on the ocean begins to move northward actively. Before and after Qingming, cold and warm air often meet, thus forming rainy weather.

The second is that the air pressure is much lower in spring. The clouds in the low pressure move fast, the wind is strong and the rain is urgent. Whenever the low pressure passes, there will be rainy weather.

Thirdly, before and after Tomb-Sweeping Day, there was a lot of water vapor in the atmosphere, which easily condensed into Mao Mao rain at night. For these reasons, there are many rains during the Qingming Festival.