Why Moist Air Less Dense Than Dry Air

Why Humid Air is Less Dense Than Dry Air

Take the Quiz

According to Avogadro's principle, parcels or pockets of air will always have the same number of molecules if they have the same temperature, pressure and volume.

1. Parcels or pockets of air always have the same amount of molecules if they have same temperature, pressure and volume.
2. They have the same amount of molecules even if one of them has heavier molecules than the other.

As you can see in the illustration above, if two empty bottles have the same temperature, pressure and volume, they will have the same amount of molecules even if the other one has heavier molecules.

'Dry' air or just regular air without water vapor is mostly composed of nitrogen and oxygen. In fact, the atmosphere is composed of 78 percent nitrogen and 21 percent oxygen -- a whopping 99 percent!
Nitrogen has a molecular weight of 14 g/mol. In the atmosphere, it comes in pairs. Thus, paired or diatomic nitrogen has a molecular weight of 28 g/mol.
Oxygen has a molecular weight of 16 g/mol. In the atmosphere, it too comes in pairs. Thus, paired or diatomic nitrogen has a molecular weight of 32 g/mol.

Water vapor or H2O has a total molecular weight of 18 g/mol.
Hydrogen has a molecular weight of just 1 g/mol. So, two of them equal to a molecular weight of 2 g/mol.
Since oxygen weighs 16 g/mol, adding two hydrogen atoms gives water vapor a total molecular weight of 18 g/mol .

With a molecular weight of just 18 g/mol, water vapor is ubstantially lighter than regular or 'dry' air, which is mostly made up nitrogen and oxygen.

Now the last thing to do is to find out what happens when water vapor or moisture is added to the atmosphere.

When water vapor is added to the atmosphere, it has to displace other gases for the amount of molecules to remain the same.

Remember, according to Avogadro's principle, parcels of air always have the same amount of molecules if they have the same pressure, temperature and volume .
So, when moisture or water vapor is added, other gases such as nitrogen and oxygen have to be displaced or 'move out of the way' so that the total amount of molecules in that air parcel can remain the same.
Since water vapor is much lighter than surrounding gases (nitrogen and oxygen), the total molecular weight of an air parcel decreases when water vapor is added.

One way we can illustrate how this works is by using empty jars.
The first jar just has regular or 'dry' air. The second has some water vapor, the two water molecules

As you can see, because water vapor has been added to the second jar, it now has a lower molecular weight than the jar with just regular air.
Notice how some of the molecules have also been displaced by water vapor. Again, this is due to Avogadro's principle.

If you have asked that question, you have already answered it yourself! Yes, it 'feels' heavier, but it isn't.
It feels heavier because it is not easy to breathe in a humid environment.
What is more, it is harder for sweat from the body to evaporate when the air is moist.
These and other factors give the sensation that moist air is more dense, but the opposite is true

Because of the unique environment of the southwest and central parts of the United States, there are often well-defined boundaries of moist air and dry air.
Since moist air is less dense than dry air, this creates instability and may gives rise severe weather such as supercell thunderstorms, hail, tornadoes, etc.