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Humidity and heat?

  1. Feb 13, 2007 #1
    Ok, let me know if this belongs in a different forum or not, but here's a question: is the difference in air density between hot and cold air less pronounced when the air is humid?

    A friend of mine has an apartment that's pretty cold this time of year. And her apartment's pretty dry. So she bought a humidifier for her bedroom. She notices that in her bedroom, the heat is pretty evenly distributed from floor-to-ceiling. BUT, in other rooms, the air near the floor is much colder than the air near the ceiling.

    So it *could* just be the apartment-- it might just be drafty near the floor in other rooms. Or it could be that the humidifier is blowing air around the room so as to keep it more consistant-temperature, while in other rooms the air has more of a chance to settle into hot/cold zones. But might it have something to do with the humidity?

    And while I'm on the subject-- I'm assuming that humid air (being more substantial?) will retain heat longer, as well as take longer to heat up?

    DaveE
     
  2. jcsd
  3. Feb 13, 2007 #2

    Gokul43201

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    Heat transfer in air (in a house/apartment) is dominated by convection. My guess would be the fan on the humidifier is what's doing it.

    Yes, humid air, having a slightly bigger heat capacity (I assume that's what you mean by "substantial"; the density of humid air is lower than that of dry air), will have a wee bit longer thermal time constant (maybe 1% longer, if you go from really dry to really wet).
     
  4. Feb 13, 2007 #3

    chemisttree

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    Humid air is less dense than dry air at any given temperature above dewpoint. One potential reason for the observed temperature difference is that the more humid air is displacing warm dry air downwards. It should equal out eventually.
    It takes more energy to heat humid air than dry air. It is why desert temperatures can vary in the extreme. 90's in the daytime and freezing at night...
     
  5. Feb 13, 2007 #4

    Gokul43201

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    Based on that last statement by c-tree, and my growing doubts, I shall recant my 1% estimate - I'm probably misremembering something.
     
  6. Feb 14, 2007 #5

    chemisttree

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    Desert temperature extremes are probably due to radiative cooling effects much more so than dry air conditions. The specific enthalpy of moist air is almost the same as dry air according to the website "Engineering Toolbox"

    I quote:

    The Enthalpy Difference
    The enthalpy difference when heating air without changing moisture content can be expressed as:

    dhA-B = cpa tB + x [cpw tB + hwe] - cpa tA + x [cpw tA + hwe]

    = cpa(tB - tA) + x cpw (tB - tA) (2)

    ...
    Note! The contribution from the water vapor is relatively small and for practical purposes it may often be neglected. (2) can then be modified to:

    dhA-B = cpa( tB - tA) (2b)

    http://www.engineeringtoolbox.com/heating-humid-air-d_693.html
     
  7. Feb 16, 2007 #6

    GCT

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    Where's the humidifier at, on the floor? What type of a humidifier is it, if it's heating the colder air on the floor, then that would explain your friend's case.
     
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