Why is the air pressure higher on sunny days than on cloudy days?

Generally speaking, the air humidity on cloudy days is relatively high, so there is more water vapor in the air. On sunny days, there is less water vapor in the air. Because the density of water vapor is lower than that of air, the air pressure on sunny days is higher than that on cloudy days.

Of course, in places controlled by low pressure, the airflow is generally upward, which can easily turn into clouds and rain. Where the air pressure is high, the airflow is downward, and Wan Li is generally a clear Wan Li. So cloudy places are often controlled by low pressure, and clear skies in Wan Li are often controlled by high pressure.