Why is the pressure low in cloudy weather?

What we usually call the atmosphere is the entire layer of air surrounding the earth. In addition to containing nitrogen, oxygen, carbon dioxide and other gases, it also contains water vapor and dust. We call air that contains little water vapor (that is, low humidity) "dry air", and air that contains more water vapor (that is, high humidity) is called "moist air". Don't think that "dry" things are necessarily lighter than "wet" things. In fact, the molecular weight of dry air is 28.966, while the molecular weight of water vapor is 18.016, so dry air molecules are heavier than water vapor molecules. Under the same conditions, the density of dry air is also greater than the density of water vapor. The density of water vapor is only about 62% of the density of dry air.

It should be said that since the atmosphere is in an open space around the earth, there are no specific boundaries that restrict its range of movement, which makes it different from gases in closed containers. For a closed container filled with air, as long as the gas in the container has not reached a saturated state, then when we input water vapor into the container, the pressure of the gas will inevitably increase. This is not the case with the atmosphere. When the atmospheric humidity in a certain area increases due to natural or artificial factors, the "moist air" molecules (including air molecules and water vapor molecules) in the area will inevitably spread to the surrounding areas. As a result, the "dry air" content in the atmosphere of this region is smaller than that of surrounding areas, while the water vapor content is larger than that of surrounding areas. In this way, the weight of an air column per unit bottom area in this area is smaller than the weight of the same air column in other dry air areas. This also tells us that the atmospheric pressure decreases as the air humidity increases. As far as cloudy days and sunny days are concerned, in fact, the air humidity on cloudy days is greater than that on sunny days, so the atmospheric pressure on cloudy days is smaller than that on sunny days.