What are weather troughs and shear lines?

On the high-altitude weather map, the long and narrow area extending from the low pressure is called the high-altitude trough, with the middle height lower than the two sides. In a low trough, the connecting line at the maximum curvature of the contour line is called the trough line, and the trough line is approximately north-south or northeast-southwest, which is called a vertical trough, while the east-west trend is called a horizontal trough, and the contour line opening is called an inverted trough. Generally, there is a strong upward movement of warm and humid southwest airflow in front of the trough, so there is more rain and less rain, and there is a dry northwest downward airflow behind the trough, so it is sunny. Usually, it refers to the wind direction or wind speed discontinuous line of 700 hectopascals and 850 hectopascals, which is formed by the high moving speed of the northern section of the trough and the slow moving speed of the southern section. The airflow on the shear line is a cyclone circulation with obvious horizontal convergence, which is conducive to upward movement and easy to produce rainy weather. The shear line is most likely to appear in the middle and lower reaches of the Yangtze River ● The dividing line between two horizontal airflow in different directions and the cyclone (counterclockwise in the northern hemisphere) is called the shear line. On the weather map, it is shown as a discontinuous wind direction line. In meteorology, the shear line generally refers to the weather system on the isobaric surface at low altitude of 850 HPA or 700 HPA. On the shear line, the airflow often has horizontal convergence and upward movement, which is easy to produce rainy weather. Shear lines may appear all year round, but they are most frequent in late spring and early summer. Spring activity is in South China, which is called South China Shear Line. At the turn of spring and summer, it is mostly located in the Jianghuai basin, which is called the Jianghuai shear line. From mid-July to August, it mainly appears in North China, which is called North China Shear Line. The distribution of precipitation on the shear line is very uneven, and heavy rains often appear in areas with strong support and abundant water vapor supply. Discontinuous line of cyclone shear wind field. There are large-scale weather systems ranging from1000 to 2000 kilometers, and mesoscale weather systems ranging from tens to hundreds of kilometers. In China, it occurs all year round, whether in the plain or the plateau, especially in late spring and early summer, and it is one of the main precipitation weather systems. According to the type of wind field, it can be divided into cold front, warm front, quasi-static front and north-south shear. The distribution of precipitation on the shear line is very uneven, and heavy rains often appear in areas with strong convergence and sufficient water vapor supply. The eastward movement of southwest vortex along shear line is often the main reason for enhancing convergence.