What is "rainy day"?

Intermittent rain in Mao Mao (middle and lower reaches of the Yangtze River)

(Meiyu) From the end of spring to the beginning of summer, rainy weather often occurs from the Jianghuai valley to southern Japan. At this time, it coincides with the yellow ripe season of plums in the south of the Yangtze River, and it is also a period when clothes are prone to mildew, so it is called plum rain, commonly known as mildew rain. Meiyu begins in mid-June and ends in early July, lasting about 20 days. However, the rainy season starts and ends at different times every year, and the duration is different, from more than one month to only a few days. There has been no plum rain for several years, which is called empty plum. The weather changed obviously before and after Meiyu. It was sunny before the plum rain. Meiyu period, abundant rainfall, high relative humidity, short sunshine time, usually rainy, but sometimes there are heavy rains and heavy rains. After the plum rain, the weather was fine and dry, and the temperature rose sharply, entering the midsummer. Meiyu is caused by the quasi-static front formed by the cold and warm air in the middle and lower reaches of the Yangtze River in late spring and early summer. Every spring to early summer, the cold high pressure in the north gradually weakens, but the cold air still often reaches the middle and lower reaches of the Yangtze River southward. At the same time, the subtropical high in the western Pacific strengthens northward, bringing warm and humid air. When the ridge line of subtropical high moves northward to the north of 20 north latitude and stays between 20 and 25 north latitude, cold and warm air will contain each other in this area, forming a quasi-static front. Therefore, the intensity of the western Pacific subtropical high and the seasonal variation of the ridge line position determine the time of plum blossom and plum blossom, the duration of plum rain and the amount of plum rain every year. It directly affects the summer drought and flood in Jianghuai Basin and North China.