Special climatic conditions

China is located in the northern hemisphere, and its climate is controlled by the westerly airflow of atmospheric circulation. Due to the uplift of the Qinghai-Tibet Plateau, the blocking effect on the westerly airflow is constantly strengthened, and the westerly airflow near the ground is divided into two streams, one of which goes north and the other turns south. Among them, the water vapor of the northern branch and the Arctic Ocean meets in winter to form a cold high-pressure air mass, which invades China from north to south, bringing rain and snow to the northern part of China, but in summer, more precipitation can only be formed through the Shankou area of Xinjiang, which is slightly lower in terrain. The south branch bypasses the Himalayas and joins the warm and humid airflow in the Indian Ocean, and enters Yunnan, Guizhou, Sichuan and southern Tibet along the north-south Hengduan Mountains and the Yarlung Zangbo River Grand Canyon, with abundant precipitation in summer. Although there is still a part of westerly airflow passing over the Qinghai-Tibet Plateau at an altitude of 4000 ~ 5000 m, the water vapor content is only 1/5 near the ground due to the height effect. Moreover, the water vapor carried by the westerly airflow comes from the Atlantic Ocean, and the precipitation in long-distance transportation has consumed most of the water. When we get to the Qinghai-Tibet Plateau, there is little water left, so most areas of the Qinghai-Tibet Plateau are extremely dry areas.

Another result brought by the barrier effect of Qinghai-Tibet Plateau is the formation of summer monsoon in southeast China. The weakening of the west wind provides the possibility for the Pacific water vapor to move from east to west. In summer, the tropical high over the Pacific Ocean landed in the east of China, especially the invasion of typhoon, which brought abundant precipitation to the southeast. At the same time, the confrontation between tropical high and northern cold high often leads to the extreme weather of continuous rainfall or drought in Central China.

Affected by the above airflow, China's climate is complex and changeable, and extreme weather phenomena occur frequently. Compared with countries with the same latitude, the annual temperature difference is greater than 5 ~ 2 1℃, and the annual and monthly precipitation variability is larger, especially in the northern region, which can reach 15% ~ 30%. Unstable monsoon leads to floods, frosts and sudden changes in weather, which often induce collapses, landslides and mudslides. Arid areas are more troubled by desertification, vegetation degradation and soil erosion.