Why is the weather cloudy again today?

Haha, it's like this in Xi'an. I used to travel to Xi'an for a long time on business.
Xi'an is geographically surrounded by the Qinling Mountains and is surrounded by mountains. Therefore, there is little wind and it is difficult for pollutants to dissipate.
So when there are clouds, the sky is cloudy, and when there are few clouds, the sky is also gray.

Atmospheric pre ssure is closely related to the weather. Generally speaking, areas with high air pre ssure on the ground tend to have sunny days, while areas with low air pre ssure on the ground tend to have rainy days. The high pre ssure and low pre ssure mentioned here are relative and do not refer to the absolute value of atmospheric pre ssure. If the air pre ssure in a certain area is higher than the air pre ssure in the surrounding areas, it is called a high pre ssure area; if the air pre ssure in a certain area is lower than the air pre ssure in the surrounding areas, it is called a low pre ssure area. On the same horizontal plane, if the air pre ssure distribution is uneven, air will flow from high pre ssure areas to low pre ssure areas. Therefore, the air pre ssure in a certain area is high, and the air in that area flows horizontally to surrounding areas. The air above the high pre ssure area is about to fall. Since atmospheric pre ssure increases with decreasing height, when the air at high altitudes descends, the pre ssure it experiences increases, its volume decreases, its temperature increases, and the condensation in the air evaporates and dissipates. Therefore, the central area of ​​high pre ssure is not conducive to the formation of clouds and rain, and it is often sunny. If the air pre ssure in a certain area is low, the air from surrounding areas will flow into the area in the horizontal direction, causing the air in the area to rise. The rising air will expand due to the reduction in pre ssure, the temperature will decrease, and the water vapor in the air will condense. , therefore, it is often cloudy and rainy in the center of low pre ssure. Since air pre ssure is closely related to weather, each meteorological post observes the local atmospheric pre ssure at a uniformly specified time every day and reports it to the meteorological center as one of the basis for weather forecast. This is not due to geographical location. It is the same everywhere. This is because the air generally contains water vapor. The atmospheric pre ssure is the sum of the partial pre ssure of water vapor and the partial pre ssure of dry air. On cloudy days, the water vapor increases and the water vapor partial pre ssure increases. The pre ssure also increases, so the partial pre ssure of the air decreases, and the barometer drops