What is the origin of rainy weather?

From the middle and lower reaches of the Yangtze River in China to southern Japan, hot, humid and rainy weather often occurs at the turn of spring and summer. At this time, it coincides with the yellow ripe season of plum trees in the south of the Yangtze River, so it is called Meiyu or Huang Meiyu, and the English name is Plumrain because the utensils in this period are prone to mildew. In fact, the word "plum rain" existed in ancient times. In the middle of the 6th century, it was called "Plum Rain at Plum Maturity" in Brick Kiln of Liang Dynasty in China. Liu Zongyuan wrote in Plum Rain in the Tang Dynasty: "Shi Mei welcomes the rain, which is infinitely valuable in late spring". Su Dongpo, a great writer in the Song Dynasty, also wrote a poem, "Huang Meiyu broke at three o'clock and Wan Li came here to sail", which vividly described the relationship between the rainy season and the southeast monsoon. Meiyu usually begins in early June, and the start date is called entering plum or main plum; At the end of July, the end of the day is called plum or broken plum, which lasts for about a month. This period is called "Meiyu period". Due to the influence of climate, the date of entry and exit of plum blossoms is generally different every year. According to statistics, in the middle and lower reaches of the Yangtze River, the average plum blossom in June 10 in the 1950s, and the plum blossom in July 12, with a plum rain period of 33 days. In 1960s, the average plum blossom date was June 17, July 6, and the plum rain period was 20 days. There may not be plum rain for several years, which is called empty plum.

Why is it rainy in the season when plums are yellow? In June and July every year, the weather in the south gets warmer, and the humid warm air flows northward. When it meets the cold air from north to south, the cold air and warm air collide with each other, evenly matched and neutralized each other. A quasi-static front or a slowly moving cold front formed from the middle and lower reaches of the Yangtze River to Japan, resulting in continuous low temperature, low humidity and rainy weather in Jianghuai area. About a month later, the warm air in the south gradually became stronger, pushing the weaker cold air in the north far to the north. At this time, the rainy season passed and the weather cleared up. Cloudy, rainy, foggy and thunderstorm weather often occurs in rainy weather. In the Meiyu front belt from the East China Sea and the Yellow Sea to the south of Japan, the visibility is very low, and frontal cyclones often appear, causing strong winds on the sea surface and affecting the navigation safety of ships.