What are the south branch trough and the north branch trough in meteorology? What is a branch ditch?

"Trough" is a visual term in meteorology. On the weather map, the trough in the sky is a long and narrow area extending from the low pressure area. The air pressure in the trough is lower than that on both sides, higher on both sides and lower in the middle, which is similar to the trough in life, so the long and narrow area extending from this low pressure area is called "trough".

In winter half a year, the upper trough cloud system moves eastward from the south side of the Qinghai-Tibet Plateau, affecting the eastern part of China. This kind of high-altitude trough is called "south branch trough". The south branch trough began to appear in the middle of 10, and disappeared in the middle of June the following year. Most of these troughs originate in the Mediterranean Sea, Caspian Sea, Arabian Sea and North Africa, and generally move eastward to the Bay of Bengal, which will affect China. The south branch trough is an important weather system that affects the southern part of China, which not only brings precipitation, but also often causes disastrous weather such as heavy rain, hail and strong wind.

In winter, the western Pacific subtropical high leaves Chinese mainland, and the cold northwest airflow is strong. Influenced by the northwest airflow, when the cloud system of the southern branch trough moves eastward from the south side of the Qinghai-Tibet Plateau to the east of Hengduan Mountains and to the south of the Yangtze River basin in Yunnan, Guangxi, Guangdong and Hunan, it merges with the cloud system of the static front in South China and Kunming, resulting in a large area of dense clouds, which often causes continuous rainy weather in the south of the Yangtze River. When the cloud system of the southern branch trough is shield-shaped, moving eastward to South China will form a southern cyclone, which will also cause disastrous weather such as heavy rain, hail and short-term gale.

The "Northern Branch Trough" is a westerly trough that appears on the northern branch of westerly jet. It is divided into two branches in the northern part from the westerly belt in the middle latitude when it is on the Qinghai-Tibet Plateau. Because the mid-latitude westerly belt is divided into two branches when passing through the Qinghai-Tibet Plateau, westerly jet is also divided into two branches. The westerly trough appearing on the northern branch westerly jet is called the northern branch westerly trough. The westerly trough located in the northeast of China belongs to the north branch trough.

In winter, the average active position of the north branch trough is located near 45-50 degrees north latitude, which is often connected with the Antarctic vortex. When the deep-developed north branch trough begins to affect the northern part of China, especially the northeast, it will often bring strong winds and cool weather to the northeast, northwest and north China with the violent development of Mongolian cyclones, and there will often be disastrous weather such as snowfall and even blizzard in the northeast. This is the cold current that is often encountered in northern winter. If the latitude at the bottom of the north branch trough is low, once the south branch trough moves eastward, it will guide the cold air in the north to meet the warm and humid airflow in front of the south branch trough, forming a large area of rain and snow in the south.