Why is the center of the cold vortex low pressure?

The reason why the center of the cold vortex is low pressure is because the air expands with heat and contracts with cold.

When the temperature is high, the air rises and forms a low pressure on the ground. Cold vortex is the abbreviation of cold vortex, which refers to the cold vortex in the air (generally above 700 hectopascals). The center is colder than the surrounding vortex, and its strength increases with the increase of height. Spring and summer appear in North China, Northeast China and Northeast China.

Take the southwest vortex as an example. It is a kind of low vortex weather system which has the greatest influence on the weather in China. It originated in the southwest of China, hence its name. It is formed by the dynamic and thermal influence of the Qinghai-Tibet Plateau. It often appears one or two kilometers near the surface. Usually it first appears on the 500 HPA isobaric weather map of plateau, 700 HPA isobaric weather map of western Sichuan and Yunnan, and 850 HPA isobaric weather map near Sichuan Basin. The southwest vortex is cold and warm. The warm southwest vortex rarely moves, and the cloud precipitation weather is not strong. When cold air invades from the north or west, the warm southwest vortex can evolve into a cold or asymmetric low pressure. Moving eastward, combined with shear line and front, it can develop into Jianghuai cyclone; Moving to the northeast can affect North China and Northeast China; Moving to the southeast can affect South China, and even become an embryo generated by typhoons in the South China Sea.