Why is the air pressure high on sunny days?

What we usually call the atmosphere is the whole air layer around the earth. Besides nitrogen, oxygen, carbon dioxide and other gases, it also contains water vapor and dust. We call air with less water vapor (i.e. low humidity) "dry air" and air with more water vapor (i.e. high humidity) "wet air". Don't think that "dry" things are necessarily better than "wet" things. The molecular weight of water vapor is 18.0 16, so dry air molecules are heavier than water vapor molecules. Under the same conditions, the density of dry air is also higher than that of water vapor, only about 62% of that of dry air.

For dry air and wet air under the same conditions, the molecular density and average mass of gas in dry air are greater than those in wet air, and the average momentum of dry air molecules is also greater than that of gas in wet air, so the pressure of dry air with low humidity is greater than that of wet air with high humidity. The air is humid on cloudy days and dry on sunny days, so the cloudy weather is low and the air pressure is high on sunny days.

Because the total amount of atmosphere on the earth is basically unchanged, when the temperature in one area increases, it is often accompanied by the temperature decrease in another area, which makes it possible for the air in high temperature to spread to low temperature. The result of diffusion is often that the air pressure at high temperature is lower than that at low temperature. When we live in the northern hemisphere, it is the summer that receives the most solar heat, and the southern hemisphere is the winter that receives the least solar heat. At this time, because the air in the northern hemisphere will spread to the southern hemisphere, the air pressure in the northern hemisphere is lower than that in the southern hemisphere. However, because the total amount of atmosphere is basically unchanged, the pressure in the northern hemisphere will be lower than the standard atmospheric pressure, and the pressure in the southern hemisphere will of course be higher than the standard atmospheric pressure. Similarly, air diffusion in the opposite direction will make the pressure in the northern hemisphere higher than the standard atmospheric pressure in winter. So in the northern hemisphere, the air pressure in winter will be higher than that in summer.