Why Air Density Decreases with Increases in Humidity ~ D. Patrick Caldwell on The Joys of Flight

Wednesday, April 1, 2009

Why Air Density Decreases with Increases in Humidity

Air Density GuageMost 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.
I really appreciate comments so please feel free to comment on my posts. Whether you agree or disagree, I'd love to hear from you. Also, feel free to link back to your own blog in your comments. You can even subscribe to an RSS feed of the comments on this thread.

© 2008 — , D. Patrick Caldwell, Vice President for Research and Development, Emerald Software Group, LLC


  1. but water won't replace nitrogen n oxygen? will it? because in that case we won't be able to breathe !:D plz clarify ! thxxxxxxxxxxxxxxx

    1. dats d case of 100% humidity when we cant breathe in ideal cases not all d molecules of air will be replaced by h20 but only a few in thousands

  2. water vapour replaces Nitrogen and Oxygen? Then where does the O2 and N2 go? It has to be inside the atmosphere right? In this case the mean density remains unchanged, even from a practical stand point. Please clarify, i'm struggling with this for a very long time.

  3. D. Patrick Caldwell..Perfect explanation. Thanks!

  4. thank you! this saved me for my chem test!

  5. They should have explained it like that in the pilots handbook. Well done sir.

  6. Good.
    I am satisfied with this explanation.
    Thanks :)

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