Why is Moist Air Lighter Than Dry Air?

Air contains mostly nitrogen and oxygen molecules. Each of these molecules is made of two molecules that are bonded together. A molecule of oxygen weighs 32 atomic mass units (amu), and a molecule of nitrogen weighs 28 atomic mass units. If these balloons were both filled with dry air they would have the same weight. Avogadro’s Law states that if the temperature and the pressure remain constant so must the number of molecules. If water vapor molecules are added to one of the balloons some oxygen and nitrogen must be lost to keep the same total number of molecules. Water molecules weigh only 18 atomic mass units. Since water molecules weigh less than the oxygen or nitrogen molecules they displace, the whole sample of air is lighter. Another way to say this is that the moist air is less dense. The difference in density of air masses is what drives atmospheric circulation around the globe.