What's the weather like in the Yangtze River valley? What caused it? (geographical reasons, etc. )

The Yangtze River originates from the Qinghai-Tibet Plateau, and its upper reaches belong to the plateau and mountainous climate. Its characteristics are as follows: ① With the increase of altitude, air, water vapor and sand dust decrease, while direct solar radiation increases, especially ultraviolet radiation; But the effective radiation has also increased. In the snowy plateau, the reflectivity increases and the radiation absorbed by the ground decreases, so the net radiation is smaller than that of the plain at the same latitude. ② Low temperature, large daily range and small annual range; (3) On the windward side of the wet air flow, the precipitation increases, while in the interior and leeward side of the plateau, the precipitation decreases greatly; ④ The wind is strong.

The middle and lower reaches are located in the eastern monsoon region of China and belong to the subtropical monsoon climate.

Cause of formation: Located between the largest continent and the largest ocean, the thermal properties of land and ocean are significantly different. In summer, the Eurasian continental depression is connected, the ocean subtropical high extends westward and advances northward, and the southeast monsoon from the subtropical high in the North Pacific brings abundant precipitation. In winter, the northerly wind from the powerful Mongolian high pressure affects the local area. Because the wind shear meets the requirements of monsoon, it is a monsoon climate.

Features: ① It is hot and rainy in summer: In summer, the sun is high and the temperature is high, and the southeast monsoon blowing from the tropical ocean surface brings abundant precipitation. ② Mild winter with little rain: the average temperature in Leng Yue is above 0 degrees Celsius, and the winter is mild. The reasons are: the local latitude is low, affected by Kuroshio, far from the source of winter wind, and the winter wind is weakened by topographic relief. The monsoon is extraordinary.

But there is also a special rainy season in the middle and lower reaches of the Yangtze River. In mid-June, the banks of the Yangtze River, the so-called Jianghuai area. At this time of the year, there will be a rainy season in Jianghuai. There are many kinds of rainy season, which are generally divided into normal rainy season, early rainy season, late rainy season, long rainy season, "short plum season", "empty plum season" and inverted plum season.

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. If the cold air is strong, the cloud and rain area will move southward with the cold air; If the warm air is strong, the cloud and rain area will move northward with the warm air.

In some years, the rainy season starts very early and suddenly comes at the end of May and the beginning of June. Meteorologically, the plum rain that began before "Mangzi" is generally called "early plum rain". The early plum rains will bring some abnormal phenomena. Because in the early rainy season, cold air from north to south is still very frequent in the atmosphere near the ground, the temperature is still relatively low and even chilly after the rainy season. At the same time, there is no obvious damp phenomenon. Farmers in some areas in the middle and lower reaches of the Yangtze River call Huang Meiyu, where the temperature is relatively low, "cold water Huangmei". In the future, with the extension of rainy time and the strengthening of warm and humid air, the temperature will gradually increase, the humidity will continue to increase, and the inherent characteristics of Meiyu will become more and more obvious. The chance of early plum rain is roughly once in ten years. This kind of early plum rain often presents two situations. One is to start early and end late, even in late July. The rainy season lasts for forty or fifty days, and some years last for two months. The other is to start early and finish early. By the end of June, the middle and lower reaches of the Yangtze River had entered midsummer. Due to the early arrival of midsummer, droughts of different degrees often occur in the middle and lower reaches of the Yangtze River.