What does steam pressure mean in the weather?

About the relationship between atmospheric pressure and weather, some data have this description:

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.

In this way, there is no more explanation and explanation.

For junior high school students

It is a bit difficult to understand this problem.

So most students choose to remember the conclusion directly.

But many students are also very interested in the reasons.

Just want to know why.

Because the knowledge involved in this problem is more complicated.

I will put the main explanation on my blog.

"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 definitely wetter than "wet". 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.

When we heat a closed container filled with air, its pressure will of course increase (PV = NRT). However, for the atmosphere, the situation is 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, with the increase of temperature, when gas molecules diffuse around, the number of gas molecules in this area will decrease, thus forming a factor to reduce pressure. However, 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, because the air in the mainland diffuses to the ocean, so the pressure in the mainland is higher than that in the ocean. In winter, the temperature in the mainland is lower than that in the ocean, because the air in the ocean will spread to the mainland, and the pressure in the mainland is higher than that in the ocean. Therefore, in the two factors of temperature change and molecular diffusion, diffusion plays a major and decisive role.

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, due to the air diffusion from the northern hemisphere to the southern hemisphere, the atmospheric pressure in the northern hemisphere is lower than that in the southern hemisphere. However, because the total amount of atmosphere is basically unchanged, the atmospheric pressure in the northern hemisphere is lower than the standard atmospheric pressure, and the atmospheric 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 atmospheric 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. Some materials describe the relationship between atmospheric pressure and weather like this:

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.

In this way, there is no more explanation and explanation.

For junior high school students

It is a bit difficult to understand this problem.

So most students choose to remember the conclusion directly.

But many students are also very interested in the reasons.

Just want to know why.

Because the knowledge involved in this problem is more complicated.

I will put the main explanation on my blog.

"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 definitely wetter than "wet". 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.

When we heat a closed container filled with air, its pressure will of course increase (PV = NRT). However, for the atmosphere, the situation is 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, with the increase of temperature, when gas molecules diffuse around, the number of gas molecules in this area will decrease, thus forming a factor to reduce pressure. However, 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, because the air in the mainland diffuses to the ocean, so the pressure in the mainland is higher than that in the ocean. In winter, the temperature in the mainland is lower than that in the ocean, because the air in the ocean will spread to the mainland, and the pressure in the mainland is higher than that in the ocean. Therefore, in the two factors of temperature change and molecular diffusion, diffusion plays a major and decisive role.

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, due to the air diffusion from the northern hemisphere to the southern hemisphere, the atmospheric pressure in the northern hemisphere is lower than that in the southern hemisphere. However, because the total amount of atmosphere is basically unchanged, the atmospheric pressure in the northern hemisphere is lower than the standard atmospheric pressure, and the atmospheric 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 atmospheric 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. Some materials describe the relationship between atmospheric pressure and weather like this:

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.

In this way, there is no more explanation and explanation.

For junior high school students

It is a bit difficult to understand this problem.

So most students choose to remember the conclusion directly.

But many students are also very interested in the reasons.

Just want to know why.

Because the knowledge involved in this problem is more complicated.

I will put the main explanation on my blog.

"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 definitely wetter than "wet". 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.

When we heat a closed container filled with air, its pressure will of course increase (PV = NRT). However, for the atmosphere, the situation is 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, with the increase of temperature, when gas molecules diffuse around, the number of gas molecules in this area will decrease, thus forming a factor to reduce pressure. However, 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, because the air in the mainland diffuses to the ocean, so the pressure in the mainland is higher than that in the ocean. In winter, the temperature in the mainland is lower than that in the ocean, because the air in the ocean will spread to the mainland, and the pressure in the mainland is higher than that in the ocean. Therefore, in the two factors of temperature change and molecular diffusion, diffusion plays a major and decisive role.

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. -Yu's blog.