In everyday life, few people think about what air density is and what significance this indicator has in general in the existence of all things on the planet. Meanwhile, every day we see flying planes, soaring birds, objects flying upward and falling downward and do not at all reflect on the fact that it is the air density parameter that determines the presence of these phenomena.
Often, resting in various places, we say that in one place there was humid air, and in another - dry. At the same time, we add that in the first case it was easy for you to move around, breathe, and in the second you felt heavy when moving, discomfort that you did not experience when you were in the usual climatic environment for you. At this time, we forget what teachers told us while studying at school in physics classes - moist air has a density lower than dry air, and, consequently, its mass is less than dry.
At first glance, this seems paradoxical if we take into account our sensory sensations, which were discussed above. Indeed, how can air, to which water is still added in the form of steam, be lighter than that which does not contain water?
But this is indeed so, and the answer to this paradoxical, at first glance, question, scientists have known for quite some time.
The first 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. However, the hypothesis of the great Englishman did not know how to succeed - until the eighteenth century, scientists not only did not accept it, but generally did not have much interest in this problem.
In order to somehow come closer to understanding the problem - why the density of air depends on its humidity - we should recall several well-known natural laws.
For example, at the very beginning of the last century, Amadeo Avogadro, a famous Italian physicist, found that regardless of the type of gas, if you take its fixed volume, then at the same temperature and pressure, the number of molecules in this gas will be constant. This value later received the name of the Avogadro constant, the law he discovered for gases also began to be called.
How this law manifests itself, how air density depends on temperature, pressure and humidity can be seen in a fairly simple example.
As a rule, dry clean (in the chemical sense) air contains about 78% of nitrogen molecules, and the atomic weight of each of these molecules is 28. In the composition of air , 21% belong to oxygen molecules, the atomic weight of which is 32 One percent in the composition of the air is accounted for by some other gases that are present in it, but for our calculation this indicator will be considered insignificant.
Gas molecules, as you know, have the property of free exit outside the reservoir in which the gas is located. So, Avogadro established the following pattern: if we add water molecules to our volume of dry gas containing, as we agreed, nitrogen and oxygen molecules, then they will make our air less dense. This is explained very simply - 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, water molecules simply replaced the nitrogen and oxygen molecules in the air, replacing them yourself. In this way, the density of humid air becomes lower than that of dry air.
In this example, however, there is one contradiction. It consists in the fact that any layman can exclaim, how can this be if the density of water is higher than the density of air. The answer here 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 patterns deduced by the brilliant Avogadro apply to such "water".
It is important to consider that air density is more dependent on temperature and pressure than on humidity. Therefore, humid air has a lower density than dry air only if the temperature and pressure are the same.