Ask Jack: Humid Air
Q: Why is humid air less dense than dry air?
Matt, LaGrange, Ga.
A: At first it might seem strange that humid air is actually less dense than dry air at the same temperature. Most of the time we say things like, “humid air has more water vapor than dry air.” A more accurate way to describe humid air would be: When the air is humid, water vapor molecules have replaced some of the nitrogen and oxygen molecules that account for approximately 99% of the air’s molecules.
This is an example of Avogadro’s Theory. In 1811 the Italian scientist Amadeo Avogadro hypothesized that equal volumes of gases with the same temperatures and pressures contain equal numbers of molecules. In subsequent years different scientists confirmed this in various ways and we now talk of Avogadro’s Theory or Avogadro’s Law.
As explained on Page 81 of The AMS Weather Book, when air is not confined to a container, water molecules will replace nitrogen and oxygen molecules as water evaporates into the air. The nitrogen molecules in the air have a molecular weight of 28 and the oxygen molecules a molecular weight of 32. Both of these are heavier than water molecules, which have a molecular weight of 18. Since the lighter water vapor molecules are replacing some nitrogen and oxygen molecules, humid air is less dense than dry air.
Note: My July 2011 Flight Training Magazine column about density altitude links to this page. I made two errors in that column. I have published corrections on this Web site.
Chapter 4 of the AMS Weather Book explains why as the density of air decreases, either from being warmed or becoming more humid or often both, the faster it will rise under the right conditions to form showers, thunderstorms, and even hurricanes. Chapter 8, on thunderstorms, and Chapter 10, on hurricanes, go into more detail on how this works.