D. Patrick Caldwell on The Joys of Flight

Most pilots understand why air density decreases with increases in temperature; however, a lot of us have a problem wrapping our brains around why increases in humidity yield decreases in air density.

The confusion probably comes from the fact that we all know that water is more dense than air (otherwise a water balloon would float around like a balloon full of helium). The first thing to understand is that water is the liquid form of H2O, ice is the solid form, and water vapor is the gaseous form. Humidity is water vapor and is a gas rather than a liquid.

You may be asking, "What difference does that make Patrick?" Well, Avagadro's Law states that equal volumes of gases, at the same temperature and pressure, contain the same number of particles, or molecules. Dry atmosphere comprises about 78% nitrogen (N2) and 21% oxygen (O2). The molecular weights of a nitrogen molecule and an oxygen molecule are 28 and 32 respectively.

Water vapor, on the other hand, has a molecular weight of 18. If you replace oxygen and nitrogen with water vapor, the total mass of a cubic foot of air must go down. Density is mass / volume. Thus, a decrease in mass reduces density. Adding water vapor to air (i.e., increasing humidity) decreases mass and therefore also decreases air density.

Have fun. Be safe. Happy Piloting.