Air density

In everyday life, few people think about what is the density of air and what value this indicator has in general in the existence of everything on the planet. Meanwhile, we see flying airplanes every day, hovering birds, flying up and falling objects and do not even think about the fact that it is the air density parameter that determines the presence of these phenomena.

Often, while spending vacations in different places, we say that in one place there was moist air, and in the other - dry. At the same time, we add that in the first case it was easy for you to move around, to breathe, and in the second you experienced a heavy movement, discomfort that was not experienced when staying in the usual climatic environment for you. At this time, we forget what the teachers told us while still in school at physics lessons - moist air has a density lower than air dry, and, consequently, its mass is less than that of dry air.

At first glance, this seems paradoxical if we take into account our sensory sensations, which we discussed above. Indeed, how can air, to which water in the form of steam is added, be easier than that which does not contain water?

But this is really so, and the answer to this paradoxical question, at first glance, the scientists knew for a long time.

For the first time the hypothesis that the density of moist air is lower than that of dry air was expressed by the great Isaac Newton in his famous book "Optics", which was published in London in 1717. Nevertheless, the hypothesis of the great Englishman could not succeed - until the eighteenth century, scientists did not simply not accept it, but did not even have a particular interest in this issue.

In order to somehow approach the understanding of the problem - why the density of air depends on its humidity - we should remember several known natural laws.

For example, at the very beginning of the last century, Amadeo Avogadro, the famous Italian physicist, established that, regardless of the type of gas, if we take a fixed volume, then at the same temperature and the same pressure, the number of molecules in this gas will be constant. This value, and later received the name of the constant Avogadro, also became known as the law he discovered for gases.

How this law manifests itself, how the density of air depends on temperature, pressure and humidity can be seen on a rather simple example.

As a rule, dry, pure (in the chemical sense) air, contains about 78% of nitrogen molecules, while the atomic weight of each of these molecules is 28. In the air , 21% belongs to oxygen molecules whose atomic weight is 32 One percent of the air comes from some other gases that are present in it, but for our calculation this figure will be considered insignificant.

Gas molecules, as is well known, have the property of free escape from the reservoir in which the gas is located. So, Avogadro established the following regularity: if we add water molecules to our volume of dry gas containing, as we agreed, the nitrogen and oxygen molecules, they will make our air less dense. This is very simple - water molecules have an atomic weight less than that of nitrogen and hydrogen molecules, it is 18. And since the number of molecules in a given volume of gas must be constant, the water molecules simply replaced the nitrogen and oxygen molecules in the air, replacing Them. In this way, the density of air moist is less than that of dry air.

In this example, however, there is one contradiction. It is that any layman can exclaim, how can this be if the density of water is higher than the density of the air. The answer is also simple: water is present in the air in the form of steam, which is lighter than nitrogen and oxygen, and therefore all the regularities derived by the brilliant Avogadro spread to such "water".

It is important to consider that the density of air depends more on temperature and pressure than on humidity. Therefore, moist air has a lower density than dry air only if the same values of temperature and pressure are maintained.