What is the density of air at sea level?
The density of dry air at standard sea level pressure is 1.204 kg/m3 at 20° Celsius. The temperature is important. The density of moist air will vary but is obviously slightly greater than the above number. Actually, the above comment about moist air seems obvious but is wrong, so please read the correct explanation below: Moist Air is Less Dense... As you may have noticed, moist air is less dense than dry air. It may seem reasonable to try to argue against that simple fact based on the observation that water is denser than dry air... which is certainly true, but irrelevant. Solids, liquids and gasses each have their own unique laws, so it is not possible to equate the behavior of liquid water with the behavior of water vapor. The ideal gas law says that a certain volume of air at a certain pressure has a certain number of molecules. That's just the way this world works, and that simple fact is expressed as the ideal gas law, which was shown above in equation 1.
Note that this is the gas law... not a liquid law, nor a solid law, but a gas law. Hence comparisons to a liquid are of little help in understanding what is going on in the air, and may simply result in more confusion.
According to the ideal gas law, a cubic meter of air around you, wherever you are right now, has a certain number of molecules in it, and each of those molecules has a certain weight.
Most of the air is made up of nitrogen molecules N2 with a somewhat lesser amount of oxygen O2 molecules, and then other molecules such as water vapor.
Since density is weight divided by volume, we need to consider the weight of each of the molecules in the air. Nitrogen has an atomic weight of 14, so an N2 molecule has a weight of 28. For oxygen, the atomic weight is 16, so an O2 molecule has a weight of 32.
Now along comes a water molecule, H2O. Hydrogen has an atomic weight of 1. So the molecule H20 has a weight of 18. Notice that a water molecule is lighter weight than either a nitrogen molecule or an oxygen molecule.
Therefore, when a given volume of air, which contains only a certain number of molecules, has some water molecules in it (which are very light weight), it will weight less than the same volume of air without any water molecules.