Humidity and air density. Humid air is lighter, or less dense, than dry air. How can the air become lighter if we add water vapor to it? Scientists have long known about this fact. The first was Isaac Newton, who stated that humid air is less dense than dry air in 1717 in his book, Optics. To see why humid air is less dense than dry air, we need to turn to one of the laws of nature the Italian physicist Amadeo Avogadro discovered in the early 1800s. He found that a fixed volume of gas, say one cubic meter, at the same temperature and pressure, would always have the same number of molecules no matter what gas is in the container.
Imagine a cubic meter of perfectly dry air. It contains about 78% nitrogen molecules, which each have a molecular weight of 28 (2 atoms with atomic weight 14) . Another 21% of the air is oxygen, with each molecule having a molecular weight of 32 (2 atoms with atomic weight 16). The final one percent is a mixture of other gases, which we can safely disregard. Air can hold a maximum of about 4% water vapor, and an average of probably 2%. Molecules are free to move in and out of our cubic meter of air. The water molecules, which replace nitrogen or oxygen, have a molecular weight of 18. (One oxygen atom with atomic weight of 16, and two hydrogen atoms each with atomic weight of 1). This is lighter than both nitrogen and oxygen. In other words, replacing nitrogen and oxygen with water vapor decreases the weight of the air in the cubic meter; that is, it's density decreases.