Why is there so much rain after a long summer?

As the name implies, long summer refers to the beginning of summer. However, the temperature varies from place to place, and the time of entering summer is actually inconsistent. According to the climatological standard that the five-day average temperature in summer is higher than 22℃, the southern Sichuan basin has just entered summer before and after the long summer; The temperature in other basins is about 20℃, and it is still in the late spring season when "there is no one outside to ask the flowers to fall, and Ran Ran is covered with greenery all over the world"; The low-altitude valley in southwest Sichuan felt the heat of summer as early as mid-April, and the temperature in long summer has reached above 24℃, which can be described as hot summer. Yi Wei has a long summer's theory of "seeing with electricity" But as far as Sichuan is concerned, even in the northwest of the basin, where the first thunderstorm is the latest, the perennial thunderstorm begins in the middle of April, so it is not necessary to wait until the long summer. After the long summer, it is the critical period of water demand for large-scale cultivation of mid-season rice in the basin. The early and late arrival of rainstorm and the amount of rainfall are closely related to agricultural production. At this time, if there is no heavy rain, those farmland without water irrigation can't rake pears and sow seeds. According to the statistics of climate data, the average rainstorm for many years in the east of the basin began in the middle and late April and in the west in the middle and late May. The rainfall in the southeast of the basin in May is100-200mm, and that in the northwest is 75-100m. In the western and central parts of the basin, the rainstorm started late, with little rainfall, and summer drought often appeared. During this period, just like harvesting crops in early spring in the basin and planting crops in Da Chun, special attention should be paid to the influence of changeable weather. Harvest in time in sunny days and sow in rainy days. Even in rainy days, the harvest in Koharu should be protected from germination, mildew and drought, and the harm of low temperature below 20℃ to early rice should be prevented.