You are correct -- mea culpa. Just read up on it.
Humidity is a measure of the percentage of water vapor in the air. An increase in humidity has a surprising effect on air density: As humidity increases, air density decreases. In damp air, the large, heavy oxygen and nitrogen molecules are replaced by lighter water molecules, resulting in less density -- in essence, lighter air. Physicist Paul Doherty explains it this way: "We think of humidity as something that's added to the air on a hot, muggy day. So you might think that a ball would go farther on a dry day than on a humid day. But for every water molecule that we add to the air, we displace a heavier nitrogen or oxygen molecule. Since the addition of humidity actually makes the air less dense, a ball will go farther on a humid day than it will on a dry day." The changes in air density related to humidity are not large: Compared to dry air at the same temperature and pressure, there's only about a 1 percent reduction in density for a humidity of 80 percent.