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What Happens to Air When We Humidify It?

I presented a topic to my students at Suffolk County Community College the other night that stimulated some very healthy discussion. I simply asked my students, "What happens to the density of air when we humidify it?"

As you might have expected, my students looked at me with puzzled looks of sheer confusion. One brave student chimed in and said, "Professor, why are you asking us such a ridiculous question? We all know that water is heavier than air, so if you add water to air, the density of the air will go up."

I then asked the class if everyone was in agreement and, as you might have expected, they all were. Here's the deal.

Air is comprised mainly of oxygen and nitrogen. An oxygen molecule weighs 32 Atomic Mass Units (1 AMU weighs 1.66053892 x 10^-27 Kg) and a nitrogen molecule weighs 28 Atomic Mass Units. On the flip side, a molecule of water weighs only 18 AMU. Now here's the kicker.

When we humidify air, we are adding water molecules to the air. In order to keep the pressure of the air constant (At atmospheric pressure), the number of molecules in a given volume of air cannot change. So, if we add a molecule of water to the air, a molecule of something else must leave. This "something else" is most likely a molecule of oxygen or nitrogen, which weighs more than the water we are adding.

So, we are removing a heavier molecule and replacing it with a lighter one, making the weight of the given volume of air lighter. And, since weight per unit volume is referred to as density, the density of air is lower when it is humidified.

Cool, huh?


  • Harvey Ramer
    Harvey Ramer Member Posts: 2,217
    Very cool! Now all we need is a separator calibrated somewhere between 18 and 28 AMU, then we can dehumidify at will without bringing air to the dew point. :)

    How much area would it take to create a laminar flow and skim the water vapor off the top?

    One of my customers is a former NASA engineer. They built a clean room to assemble the electronic components for the space craft. The room was a 100' cube. one wall was comprised entirely of hepa filtration. The opposite wall had the fans to move the air out thus creating a laminar flow. They set the comfort conditions and ended up with a rude surprise! The top 5' of space turned into a cloud and it started drizzling, lol.. The had to sacrifice a little bit of comfort to make it stop. :p

  • That's your homework assignment for tomorrow!

    How are things going with you?
  • RobG
    RobG Member Posts: 1,850
    That explains why it is harder to breathe in humid environments than on a nice brisk chilly day. Less oxygen on the humid day. I never thought of it like that.
  • KC_Jones
    KC_Jones Member Posts: 4,656
    I have a '72 Ford LTD with a number of modifications. It loves running in the spring and fall, the summer not as much. The cool dense air makes a noticeable power difference. Now I know why the air is denser. Cool stuff indeed!
    2014 Weil Mclain EG-40
    EcoSteam ES-20 Advanced Boiler Control
    Boiler pictures updated 2/21/15
  • Jack
    Jack Member Posts: 1,045
    yes, try flying a small Cessna or other prop plane in the middle of winter and then on a nice hot humid July day. In the winter the prop has real bite and the performance (rate of climb) is terrific. Summertime you really pay attention to those obstacles at the end of the runway to make sure you can clear them. Big, big difference in performance.
  • DanHolohan
    DanHolohan Member, Moderator, Administrator Posts: 15,341
    That also explains clouds.
    Retired and loving it.
  • Harvey Ramer
    Harvey Ramer Member Posts: 2,217

    That's your homework assignment for tomorrow!

    How are things going with you?

    I'll pass! I have quite enough useless information in my head already :)

    Things are going great! I use the things I learned in your seminar almost every day.

    How are things with you? Are you getting buckled down for a cold winter out there on Long Giland ?

  • NJ, Designer
    NJ, Designer Member Posts: 53
    edited September 2014
    Really Cool, Thanks, So attic fans do suck out the humidity?

    When I'm designing a house with a dehumidifier should it always be on the highest level?

    What about humid basements how come the humidity doesn't rise to the upstairs quicker....

    Thank You Students......... :)
  • Techman
    Techman Member Posts: 2,144
    Hi Professor.I did my homework and I was ready for you, but my dog ,Freon, ate my homework papers, dam mutt.
  • icesailor
    icesailor Member Posts: 7,265

    Then, there's the issue of that same hot and humid day at the airport where the less dense warm moist air gives you less lift. you're over your envelope for weight, and you try to take off. As you go down the runway and past the threshold, the ground effect air is compressed because of the height, but you still can't get enough speed and lift to get any higher. And the trees are coming. You're running out of horizontal and vertical room.

    Not usually good.
  • icesailor
    icesailor Member Posts: 7,265
    Throw this in the mix.

    We ice sailors say that mountain sailing in the Winter in a NW wind is heavier than sailing along the Coast. Where the wind is steadier and usually slightly warmer. With mountain or inland sailing, it is claimed that the winds are "heavier" and are stronger. The temperatures can actually be the same. The mountain wind, from the NW is dryer but gusty and erratic. The sea shore air is wetter but more consistent. The perception is that the colder, gustier air is stronger than the steadier shore air if the temperatures are the same.

    Another reason to have some air flow. Of course, if the OAT is 25 degrees, and you're ripping across the ice at 40 or 50 MPH, it all seems rather cold and windy. With the apparent wind over 60 MPH.