How was Huangmeitian in the Jianghuai Valley formed?

Every year in June and July, when plums are ripe, there are many rainy days and high humidity, and things are prone to mildew and deterioration, and it is often rainy and rainy, so it is difficult to see continuous sunny days. This period is called Huangmeitian.

Between June and July every year, the warm and humid air flow in the south is already very strong, and it often extends northward to the Yangtze River basin and the area north of the Yangtze River. However, during this period, the cold air in the north still has considerable strength and is unwilling to quit this area. So cold air and warm air met in the Jianghuai area, as if the two roads of military forces would not give way to each other, and a big war began.

Because warm air is lighter than cold air, it slides northward along cold air, and a large amount of water vapor brought by warm air condenses into clouds in the process of sliding up, forming a long rain belt, but this rain belt is relatively narrow, generally only two or three hundred kilometers wide.

During my stay in Huangmei, it is still impossible to tell who wins and who loses. They tend to be stronger at one time and stronger at another. If the cold air in the north is strengthened, the rain belt will be pressed to the south, and it will rain in the south; If the warm air is stronger, push the rain belt to the north, and it will rain in the north. In short, the two armies confronted each other and were evenly matched. Therefore, the rain belt has been swinging from north to south in the Jianghuai basin, making the weather in this area wet and rainy.

After this melee, warm air finally defeated cold air. As a result, the warm air in the south is getting stronger and stronger, while the cold air in the north is shrinking. At this time, the rainy weather in the Jianghuai basin has ended and entered a hot summer.

However, this situation is not fixed. In some years, the rainy season is very long, such as 193 1 year, 1954, which lasts for two months. In some years, the rainy season is very short, such as 1934, 1978, which is not obvious. What is the reason?

It depends on the strength contrast between cold and warm air. In some years, the warm air is particularly strong, and there is a general strong momentum, which pushes the cold air to the north at once. In this way, the confrontation between cold and warm air in Jianghuai basin is short, and the plum rain is not significant. However, it has been very hot in recent years. The highest temperature in Shanghai 1934 reaches 40.2℃, and 1978 is 38. 1℃, although it is lower.

If the cold air is stronger, the opposite is true. Usually during the annual hot summer (July 7 or 8), the rainy season in the middle and lower reaches of the Yangtze River has ended, and it is a sunny day completely controlled by the warm air mass. However, if there is still cold air going south and inserted into warm air, it will force the moist warm air to rise upwards, causing convection and thunderstorms. Therefore, under normal circumstances, you can know that the cold air in the north is still relatively strong and the rain belt may be delayed for a period of time in the middle and lower reaches of the Yangtze River when you hear thunder and thunderstorm around the heat. This phenomenon is called "inverted yellow plum". Therefore, there is a saying that "it thunders a little in midsummer and hangs upside down as Huangmei".