What does the weather forecast say about air pressure and weather?

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.

It should be said that because the atmosphere is in the open space around the earth, there is no specific boundary to limit its movement range, which makes it different from the gas in a closed container. For a closed container filled with air, as long as the gas in the container is not saturated, when we input water vapor into the container, the pressure of the gas will inevitably increase, but the atmosphere will not. When the atmospheric humidity in a certain area increases due to natural factors or human factors, the molecules of "wet air" (including air molecules and water vapor molecules) in this area will inevitably spread to the surrounding areas. Therefore, the content of "dry air" in the atmosphere in this area is less than that in the surrounding areas, while the content of water vapor is greater than that in the surrounding areas. In this way, the weight of the air column per unit bottom area in this area is smaller than that of the same air column in other dry air areas. This tells us that atmospheric pressure decreases with the increase of air humidity. As far as cloudy days and sunny days are concerned, the air humidity on cloudy days is actually higher than that on sunny days, so the air pressure on cloudy days is lower than that on sunny days.

You will learn this in high school.