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Temperature, humidity and atmospheric pressure

The greater the humidity, the greater the atmospheric pressure.

Junior high school physics tells us: "The change of atmospheric pressure is closely related to the weather. Generally speaking, the air pressure in sunny days is higher than that in cloudy days, and the air pressure in winter is higher than that in summer. " It is often difficult for teachers to explain this narrative clearly. In my opinion, this problem can be attributed to the relationship between temperature, humidity and atmospheric pressure. Let me talk about my preliminary understanding.

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. In this way, the content of "dry air" in the atmosphere in this area will be less than that in the surrounding areas. The water vapor content is higher than that in surrounding areas. This is just like when cottonseed is mixed with soybeans, the density of the mixture is lower than that of soybeans, so the density of wet air in this area is lower than that of dry air in other areas. In this way, the weight of air column per unit area in this area is lower than that in other dry air areas, which tells us that with the increase of air humidity, the atmospheric pressure decreases. As far as cloudy days and sunny days are concerned, the air humidity on cloudy days is actually higher than that on sunny days.

We know that the "collision" of gas molecules is the fundamental cause of gas pressure. Therefore, it can also explain the problem that atmospheric pressure changes with air humidity. According to the basic theory of gas molecular motion, the average velocity of gas molecules is:

Then the average momentum of gas molecules (only considering their size)

It can be seen that the average momentum of gas molecules with large average mass is also large (some literature ① says that "the average speed of dry air is also greater than that of humid air", which is incorrect). For air and wet air under the same conditions, the molecular density and average mass of gas molecules in air are greater than that of wet air, and the average momentum of dry air molecules is also greater than that of wet air, so the pressure of dry air with low humidity is greater than that of wet air with high humidity.

When we heat a closed container filled with air, its pressure will definitely increase. For the atmosphere, things are different. When the atmospheric temperature in a certain area rises due to some factors, it will inevitably lead to the expansion of air volume, and air molecules will inevitably spread around. When the temperature is high, gas molecules will definitely move faster, which becomes a factor to promote the increase of pressure. On the other hand, as the temperature rises, gas molecules will diffuse around. Then the number of gas molecules in this area will decrease, thus forming a factor to reduce the pressure. The actual situation is the result of the interaction of the above two opposing factors. As for which of these two factors plays the main role, we might as well look at the actual situation that the pressure of the mainland and the ocean changes with temperature. We say that the temperature in the mainland is higher than that in the ocean in summer, and the pressure in the mainland is lower than that in the ocean because the air in the mainland diffuses to the ocean. In winter, the temperature of the mainland is lower than that of the ocean, and the pressure of the mainland is higher than that of the ocean. Therefore, in the two factors of temperature change and molecular diffusion, diffusion plays a major and decisive role. It should be pointed out that the diffusion mentioned here refers to the lateral flow of air, because the vertical flow of air cannot change the weight of the vertical air column (some literature ② describes the pressure change caused by temperature as the result of air fluctuation, which

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 the atmosphere is basically unchanged, the pressure in the northern hemisphere is 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. Of course, the change of atmospheric pressure is very complicated, but we still have to explain what is said in middle school textbooks.