In general, the air pressure in winter is _ _ _ _ in summer _ _ _ in sunny days and _ _ _ _ in rainy days (choose "greater than", "less than" or "equal to"), and explain the reasons.

"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.

Cloudless, clear and dry are the results of air flowing from top to bottom in high pressure area. At this time, the dragonfly will fly higher. When the air flows from bottom to top, because the troposphere temperature is decreasing, a large amount of water vapor contained in the air will condense into small water droplets and form clouds. When the water drops get bigger, they will form rain. These are the characteristics of low pressure area.