Meiyu is a unique weather and climate phenomenon in the middle and lower reaches of the Yangtze River in early summer. It is the result of stagnant current in the Yangtze River basin during the northward movement of the main rain belt in eastern China. Meiyu is over and midsummer is coming. The change of this season every year is roughly the same as the movement of the rain belt with the season, forming a certain climate regularity. However, the annual plum rains are not completely consistent, and there are great interannual changes.

Normal Meiyu in the middle and lower reaches of the Yangtze River begins in mid-June and ends in mid-July, which is in the two solar terms of "ear seed" and "summer solstice". The meiyu period is about 20-30 days, and the rainfall is between 200-400 mm. Before and after "Little Summer", the main rainfall belt moves northward to the Yellow River and Huaihe River basins, and then to Shandong and North China.

The Yangtze River basin began to change from rainy and humid weather to sunny and hot summer. According to statistics, this normal plum rain accounts for about half of the total.

cause

Every year from late May to early June, cold air from the north and warm air from the south meet in South China, forming a quasi-static front in South China. By the end of June, the influence of warm air was strengthened, and the quasi-static front moved northward to Jianghuai area, becoming the quasi-static front of Jianghuai (also known as Meiyu front).

Because the warm air from the south carries a lot of water vapor, when it meets the cold air mass, it will produce a lot of convection activities. Because the cold and warm air forces are similar during this period, the front stays in the Jianghuai area. Simply put, it is related to the western Pacific subtropical high: in rainy season, the cold air in the north often goes south, and the warm and humid airflow in the southwest goes deep into the north, and the cold and warm air meet.

Rain belts usually form. In normal times, it will rain, but it happened that the western Pacific subtropical high system moved northward, pushing this rain belt to the above areas.