The subtropical high is at an altitude of 5 kilometers. Is it ground or sky?

Count the ground. Subtropical high refers to two large pressure systems over the northern hemisphere. One is the low pressure at high latitudes and polar regions, which is called polar vortex; The other is the high pressure in the middle and low latitudes. Because its range of activities is mainly in subtropical areas, it is called subtropical high, which is called subtropical high for short. The westerly winds prevail in the middle and high latitudes between the polar vortex and the subtropical high, which are generally called westerly belts. Due to the influence of ocean and land distribution, subtropical high is often divided into several high-pressure circulations, including North Atlantic subtropical high, North Pacific subtropical high, North Africa subtropical high and North America subtropical high. Among them, the North Pacific subtropical high, which has the most direct impact on China, is also called the Pacific subtropical high because it mainly moves over the central and western Pacific Ocean. It is usually flat and oval, extending long in the east-west direction and narrow in the north-south direction. Under the action of subtropical high circulation, the surrounding air moves clockwise, with easterly airflow in the south and westerly airflow in the north. In the subtropical high pressure system, the upper air diverges and sinks from the outside to the inside, and the lower air diverges and sinks from the inside to the outside, so the sinking dry-hot airflow prevails in the subtropical high pressure system.

Subtropical high is a deep warm high, which extends from near the ground to the upper troposphere. Generally, we take the subtropical high above 5500 meters on the ground, that is, the isobaric surface of 500 hectopascals as the representative, and use the contour line of 5880 potential meters to reflect the changes of the intensity and position of the subtropical high. The subtropical high changes with the seasons, gradually strengthens from winter to summer, and moves northward. From summer to winter, on the contrary, the subtropical high weakens from strong to weak and gradually recedes to the south. In the east-west direction, the subtropical high also changes. With the swing from east to west, sometimes the subtropical high extends westward and goes deep into Chinese mainland.

The western Pacific subtropical high has an important influence on the weather in China, especially the summer precipitation. China is a typical country with monsoon climate. Precipitation in most parts of the country is mainly concentrated in the summer half year. From winter to summer, with the seasonal northward advance and strengthening of the western Pacific subtropical high, the summer monsoon is guided to advance northward with abundant water vapor, and the rainfall belt formed by the intersection of cold and warm air currents on the north side of the subtropical high gradually moves northward, thus making all parts of China enter the rainy season from south to north. The subtropical high is located in the south from May to June, and the main rainfall belt is in the south, that is, South China and Jiangnan enter the rainy season; By the end of June, the western Pacific subtropical high had an obvious northward uplift process, and the precipitation in the first flood season in South China ended, and the Yangtze River basin and the Jiangzhun area entered the rainy season. After early July, the subtropical high continued to push northward, and most parts of northern China entered the rainy season one after another. At this time, the western Pacific subtropical high often moves around 30-40 degrees north latitude, and extends westward to control the middle and lower reaches of the Yangtze River and the Jianghuai area in China. July-August is also the high temperature and summer drought season in the middle and lower reaches of the Yangtze River and the Jianghuai area because of the sinking dry and hot airflow of subtropical high. At the same time, South China is just south of the subtropical high. Under the guidance of easterly airflow, frequent tropical storms and typhoons affected South China, and South China entered a rainy period in the late flood season. After midsummer, the subtropical high began to withdraw southward, the summer monsoon gradually weakened, and the cold air force in the polar regions gradually strengthened. When the rainy season ended in the northern region, precipitation in most parts of the country began to decrease significantly. However, from September to June of 10, due to the zonal extension of the western Pacific subtropical high to the mainland of China, under certain topographic conditions, rainy and continuous autumn rain weather will form in the west of China, while autumn weather will often appear in the east of China.

In addition to regular seasonal changes, the western Pacific subtropical high sometimes changes abnormally. The large-scale severe drought and flood disasters in flood season in China are generally related to the abnormal changes of subtropical high in the western Pacific. For example, when the subtropical high in the western Pacific develops strongly and its location continues to be southward, the main rainy areas will stay in the Yangtze River and Jianghuai area for a long time, causing floods in the Jianghuai basin. This is the case with the 199 1 Jianghuai flood. On the contrary, when the subtropical high moves northward ahead of schedule and its position is unusually northward, the main rainfall belt in flood season in China is also northward, and it will be rainy or even flooded in the north. However, the Jianghuai Basin is hot and dry. 1994 the floods in Songhua river and Liaohe river and the drought in the Yangtze river basin are all related to the abnormal change of subtropical high.

Since the flood season this year, the development of the western Pacific subtropical high is unstable, and there is more process precipitation from south to north in China. At the same time, because the subtropical high moved northward earlier in early summer, the meiyu in Jianghuai basin was also obviously ahead of schedule, and the location of the meiyu belt was also farther north than in previous years. After entering July, the subtropical high did not move northward, and the rain belt swung in the Jianghuai area, which led to the end of the Meiyu obviously later than normal. Plum rains and precipitation in the middle and lower reaches of the Yangtze River and the Jianghuai River are on the high side, and floods have occurred in some areas. So 5000 is the bottom line.