Why is there a rainy season in summer?

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

In early summer, there are often long rainy days in the Jianghuai basin. At this time, the utensils are easy to be moldy, so it is also called "moldy rain" or "mildew" for short; It is also called "Plum Rain" or "Huang Meiyu" because plums in the south of the Yangtze River are yellow and ripe. It is recorded in China's historical records. For example, The Book of Beginners quoted the compilation of Emperor Liang Yuan in the Southern Dynasties, "Plum blossoms have plum rains when they are ripe". Liu Zongyuan's Plum Rain in the Tang Dynasty: "Shi Mei welcomes the rain, and it is worth the late spring." Wait a minute. It is recorded in China's almanac that there is mildew rain all day: it is called "mildew" at the beginning and "mildew" all day long. The first day after the awn is planted, it will be moldy, but it will not be moldy on the first day after the light summer. Mold always enters between June 6~ 15 and leaves between July 8~ 19. There is an obvious rainy season in eastern China, with a long rainy period and relatively concentrated rainfall, which is caused by the displacement of the main rain belt from east to west to north, and is a unique phenomenon in the turn of atmospheric circulation in East Asia at the turn of spring and summer. After mid-June, the rain belt remained in the Jianghuai basin, which is Meiyu. The time that the rain belt stays is called "Meiyu season", the day when the rainy season begins is called "entering the plum blossom", and the day when it ends is called "emerging the plum blossom".

In addition, due to the high humidity in the air during this period, everything is easy to be damp and moldy, so people gave Meiyu an individual name, called "Moldy Rain". Xie Zaihang's Five Miscellaneous Torches in Ming Dynasty? "Tianbu No.1" describes: "Every year in March and April in the south of the Yangtze River, it rains bitterly, and everything is moldy, commonly known as Meiyu, and it is covered when it is green and yellow. From Xu Huai to the north, it is often dry in spring and summer. At the turn of June and July, it rained heavily and everything was moldy. " Li Shizhen, an outstanding medical scientist in Ming Dynasty, clearly pointed out in Compendium of Materia Medica: "Plum rain or mildew rain, when it comes to clothes and things, will produce black mold." .

Meiyu and rain belt

1. Meiyu

The middle and lower reaches of the Yangtze River in China are usually rainy from mid-June to early July every year. In the past few days, the sky has been gloomy and the precipitation has been constant, sometimes big and sometimes small. Therefore, there is a popular proverb in southern China: "Rain hits plum blossoms, and there is no sun for forty-five days". Continuous rain, high temperature and high humidity are the main characteristics of plum rain.

Very different from the climate in the same latitude area, Meiyu refers to the weather and climate phenomenon that occurs in a certain area and a certain season. It is found that Eurasia is between 20 N and 40 N, which is alternately controlled by subtropical high and westerly belt. The west coast of the mainland is controlled by the downdraft on the east side of the subtropical south pressure in summer, with sunny weather, partly cloudy and dry climate; In winter, under the influence of westerlies, warm and humid air is brought from the Atlantic Ocean, resulting in more precipitation and a mild and rainy climate. That is, the subtropical Mediterranean climate is dry in summer and humid in winter.

The east coast of the mainland is controlled by the west side of subtropical high in summer, and the downward flow is dry, but it absorbs a lot of water vapor from the warm and humid sea surface, bringing abundant precipitation and producing a subtropical humid climate. Due to the strong contrast between land and sea, a unique monsoon climate has formed here, which is characterized by rainy summer and dry winter, with rainfall concentrated in summer, which is just the opposite of the Mediterranean climate.

If compared with the east coast of Britain at the same latitude, it is also very different. There will be no rainy weather for a long time before and after the summer monsoon in the mid-latitude zone of the east coast of the United States, and people do not feel the feeling of sultry weather for a long time, so it is difficult to appear moldy. It can be seen that the precipitation season at the same latitude varies greatly. So there are only Huangmei in the middle and lower reaches of the Yangtze River in China, starting from the area east of Yichang and 29-33 degrees north latitude, as well as the southeast of Japan and the southernmost tip of the Korean Peninsula. In other words, Meiyu is a unique weather and climate phenomenon in East Asia and the middle and lower reaches of the Yangtze River in China.

2. Rain belt

Although Meiyu is a unique weather and climate in the middle and lower reaches of the Yangtze River, its appearance is not isolated, but closely related to the north-south displacement of a large-scale rain belt.

In the east of China, east of 1 10E, during the flood season from mid-May to early June, the main rain belt swings in Nanling Mountains and its south. In recent years, although it moved to the north of Nanling within a certain period of time, it often stayed in the south of 28N and 29° n from a waiting period (5 days) or the multi-year average of 10 days. This period is called "rainy season in the south of the Yangtze River" or "pre-flood season in South China".

In the middle and late June, the main rain belt moved northward within the range of 29° n-33° n (that is, Yichang in the west, the Yangtze River estuary in the east, and then across the sea to Japan; From the south of China Lake Basin to the south bank of Huaihe River), it is stable and less active. At this time, the area south of Nanling is outside the rain belt, and the rainy weather is over; The middle and lower reaches of the Yangtze River bid farewell to the sunny early summer and ushered in the rainy season. Heavy rain and rainstorm appeared from time to time and lasted until the beginning of July. This is a famous rainy season in the middle and lower reaches of the Yangtze River.

From mid-July, the rain belt moved northward again, reaching the area north of 33 N. Stagnation and wandering in the Yellow River, Huaihe River Basin, North China, Northeast China and other places have caused heavy rainfall again and again, which are called "Huanghuai rainy season" and "North China rainy season" respectively. At this time, the plum rains in the middle and lower reaches of the Yangtze River ended, the sun hung high and entered the hot summer season. This kind of weather will last until the end of August, and then the rain belt will quickly withdraw southward with the gradual activity of cold air. In less than a month, the rain belt will always retreat to the coastal areas of South China. The regular change of this rain belt shows that the plum rain in the middle and lower reaches of the Yangtze River is not an isolated and local weather and climate phenomenon, but an integral part of the main rainy season activities in eastern China, which reflects the stagnation of the main rain belt in the middle and lower reaches of the Yangtze River.

Normal Meiyu and Abnormal Meiyu

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 the stagnation of 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.

Meteorologically, the beginning and ending time of plum rain are called "entering plum" (or "standing plum") and "leaving plum" (or "breaking plum") respectively. In the middle and lower reaches of the Yangtze River in China, plum blossoms in mid-June and in early July every year, lasting for more than 20 days. But for each specific year, the beginning and end of plum rains and the intensity of plum rains are very different. As a result, plum rains are obvious in some years, not obvious in some years, and even empty plums appear. For example, the rainy season of 1954 is particularly long, lasting for more than two months, resulting in a rare waterlogging year in the middle and lower reaches of the Yangtze River. However, the rainy season of 1958 only lasted for two or three days, and there was a rare dry year in history.

(1) Normal Meiyu:

The normal Meiyu in the middle and lower reaches of the Yangtze River began in mid-June and ended in mid-July, which coincided with 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.

(2) At the beginning of Meiyu:

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. For example, since the cold air from north to south is still very frequent in the atmosphere near the ground at the beginning of the plum rain, the temperature is still relatively low and even a little chilly after the rainy day, which is the agricultural proverb: "If you eat Dragon Boat Festival brown, you should freeze for three times". 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 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.

(3) Late Meiyu:

Contrary to early plum rain, late plum rain is usually called late plum rain in meteorology. There are more opportunities for late meiyu than early meiyu. Because the solar term is already late, at the beginning of the late Meiyu, once the warm and humid air flows northward, its power is very strong, and the solar radiation is also relatively strong. When the air is heated, it is prone to strong convection, so there are often thunderstorms and showers in the later period of Meiyu. People also call this kind of Huang Meiyu "Huangmei". The duration of the late Meiyu is generally not long, with an average of only about half a month. However, the rainfall of this plum rain is sometimes quite concentrated.

(4) Special Meiyu:

During the period of 1954, a once-in-a-century flood occurred in the Jianghuai Valley of China, which was caused by the unusually long meiyu. This year, there were many spring rains in the middle and lower reaches of the Yangtze River in the second half of May before the beginning of Meiyu. Meiyu came very early and began in early June. The weather has been rainy and rainy, with heavy rain and heavy rain from time to time, which lasted for a long time until the beginning of August. When the rainy weather ended and turned to midsummer weather, it was near "beginning of autumn". This year, the whole meiyu period lasted for two months, and with the spring rain in May, it reached more than two and a half months. After entering the "light summer heat" and "great summer heat", the middle and lower reaches of the Yangtze River should have been sunny and hot, but it has been gloomy and difficult to see the sun. From time to time, pouring rain poured down to the ground, floods rolled in, and "cold" hit people in many areas. In this year, the rainfall in the middle and lower reaches of the Yangtze River from May to July is generally 800- 1000 mm, which is close to the annual rainfall in normal years in this area. The rainfall in some areas is as high as 1500-2000mm, which is equivalent to the rainfall in the same area for one and a half years, leading to flooding. China has a vast territory, and local floods often occur. Some may be caused by typhoon rainstorm, and some may be caused by several consecutive rainstorms brought by other weather systems, but their duration is not long, the flood recedes quickly, and the influence range is relatively small. It is extremely rare that the rainfall time like 1954 reaches more than two months, resulting in a basin-wide flood in the Yangtze River basin. This rare flood is usually associated with abnormal plum rains. Like the flood in 1998, it was also caused by the extremely long plum rain.

(5) "Short plum" and "empty plum":

Contrary to the exceptionally long plum rains, in some years, plum rains are very inconspicuous. It is like a passer-by, staying in the middle and lower reaches of the Yangtze River for ten days, and then going to journey to the south. Moreover, there is not much rain during this period, and it is rare to have one or two heavy rains. This situation is called "short plum". What's more, there has been no continuous rainy weather in the Yangtze River basin since the early summer of some years. Most days are sunny, warm as spring, and cool in the morning and evening, so the weather of "Huangmei season is dry and loose" appears. Moldy clothes, which often happens in rainy season, almost never happen. As soon as the cool weather is over. Then it became midsummer. This year is called "empty plum". The probability of "short regret" and "empty plum" appears, with an average of l-2 times in ten years. In the years of "short plum" and "empty plum", summer drought often occurs, and some years can also cause serious drought.

(6) pour Huangmei:

In some years, Huangmei days in the middle and lower reaches of the Yangtze River seem to have passed, the weather turned fine and the temperature rose, showing the characteristics of midsummer. However, a few days later, hot and humid thunderstorms and showers reappeared and remained for quite some time. This situation is like Huangmeitian retrogression, returning to the middle and lower reaches of the Yangtze River, so it is called "Huangmeitian". There is thunder in summer, and Huangmei is upside down. This is a weather proverb widely circulated in the middle and lower reaches of the Yangtze River. It means that after the plum rain passes, if there is thunder in the "light summer heat", the plum rain will be reversed. There is some truth in this statement. Because after the meiyu, the weather in the middle and lower reaches of the Yangtze River is usually more and more stable, and thunderstorm is a symbol of weather instability. Besides, in the "little summer", cold air usually no longer affects the Yangtze River basin, and the occurrence of thunderstorms is often related to the southward movement of a small cold air in the north, which is conducive to the re-establishment of rain belts in the middle and lower reaches of the Yangtze River. Of course, "pour Huangmei" does not necessarily appear after the thunder in summer. Generally speaking, the duration of "pouring Huangmei" is very short, ranging from about one week to ten days and a half months. However, during the "Huangmei" period, due to many thunderstorms and showers, the rainfall is often quite concentrated, which needs attention. Because "inverted yellow plum" is a kind of plum rain, it usually turns into sunny and hot weather after it ends.

From the various plum rains described above, we can see that Huang Meiyu, which is usually considered to be similar, is actually diverse, and the differences between them are sometimes quite large. In terms of "entering Mei", the earliest is May 26th, and the latest is July 9th. The earliest date of "plum blossom" is June 16, and the latest date is August 2, with a difference of one and a half months. The longest year of plum rains lasts for more than two months, which can cause rare floods, while the short year has only a few days, and some even have "empty plums", which brings serious drought. It can be seen that plum rain is a complex weather and climate phenomenon, which is far from being as simple as "entering the plum blossom" and "leaving the plum blossom" as stipulated in the lunar calendar. Compared with normal plum rain, early plum, late plum, extra-long plum rain, empty plum and severe yellow plum all belong to abnormal plum rain.