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Saturation of water vapor in the atmosphere

  1. May 19, 2010 #1

    Entropee

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    So I have a quick question about something I'm a bit confused on. I was under the impression that, unlike liquids, gasses become more saturated in COLDER temperatures, rather than warmer. My geology teacher told us that the WARMER it is in the atmosphere the easier it is for water vapor to become saturated; thus when it cools down the water vapor condenses and phase changes into a liquid and it rains. If anyone could tell me why this is, or if it is false information it would be greatly appreciated!
     
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  3. May 19, 2010 #2

    russ_watters

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    Slight wording issue there. It should be: "The warmer it isn the atmosphere, the easier it is for the air to hold water vapor". It takes more water vapor for it to become saturated when it is warmer than when it is colder.
     
  4. May 19, 2010 #3

    Entropee

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    Oh ok that makes much more sense, thanks man!
     
  5. May 20, 2010 #4
    http://www.ems.psu.edu/~fraser/Bad/BadClouds.html" [Broken] with that phrasing.
     
    Last edited by a moderator: May 4, 2017
  6. May 20, 2010 #5

    D H

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    A bit too pedantic, perhaps. Even at 50 C, the saturation pressure is 7.4 kilopascals. In other words, at sea level, air at 100% relative humidity, a temperature of 50 C (122 F) is 12% water vapor, 88% something else. That 88% something else is what drives the temperature.

    On the other hand, he is correct. For a given temperature, the only thing that restricts the amount of water vapor in a packet of air is the amount of water vapor already in the packet. Imagine two other planets with drastically different pressures compared to Earth. One has a much less dense atmosphere, the other a much denser atmosphere. The saturation pressure of water at 50 C will be 7.4 kilopascals on the Earth and on both of those other planets.
     
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  7. May 20, 2010 #6
    The geological distribution of water is really a deal more complicated than simple saturated/unsaturated vapour pressure.

    In order to have a saturated situation you have to have a source of water in the solid or liquid phase in equilibrium with vapour phase.

    In cold regions there may be no liquid water so the only source is the vapour pressure exerted by the solid phase. This may not be enough to saturate the atmosphere.

    In warm/hot regions there may be sufficient water to saturate the air over the oceans, but not over land, especially over deserts.

    So there is considerable variation of saturation around the world atmosphere and consequential transport between air masses.

    A second complicating factor is that much of the aerial water ( in fact all that we can see since water vapour in invisible) exists as a disperse system of liquid in gas. So you have to define how you are going to account for this water.
     
  8. May 20, 2010 #7
    I tend to agree, that http://fraser.cc/BadScience/BadScience.html" [Broken] is a little too pedantic, here insisting "air holds" will often be interpreted as "nitrogen picks up" rather than "a parcel of atmosphere contains".

    But I don't get what you're saying. The 88% of dry air drives temperature? To start with, there's about 300 times more mass in the sea than the whole atmosphere. And water is known for its great heat capacity. Even just concentrating on the atmosphere, at 50C isn't the saturation pressure 12kPa (and rising by about .6kPa/K)? Giving (ideally) 12% water vapour by volume, only 8% by weight, but 14% by normal heat capacity.. plus, maintaining saturation with varying temperature involves latent heat of vaporisation (adding 0.4% per K to the weight of atmosphere, whilst in unit weight terms the latent heat is three orders of magnitude greater than the per degree heat capacity, swamping the normal heat capacity with another 10J/gK) so the water vapour represents 92% of the total effective heat capacity of this air. (I accept you chose 50C as an upper bound, but clearly water vapour is a significant component of the representation of heat in the atmosphere.) Furthermore, isn't the temperature "driven" by varieties of radiation to which the dry air is almost entirely transparent? (Sure, alterations of some components of dry air like CO2 exert a long-lived climatic "forcing" whereas water vapour equilibrates on a short timescale, but this seems a long way from justifying the "air holds water vapour" phrasing..)
     
    Last edited by a moderator: May 4, 2017
  9. May 20, 2010 #8

    Entropee

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    Blah... I know just enough chemistry to understand the terms heat of vaporization, heat capacity, specifit heat, etc, and how those values are obtained, but I can't seem to put all this together into an explanation I can understand yet. Thats not any of your faults though, just rather frustrating for me at the moment.
     
  10. May 21, 2010 #9

    D H

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    Whoa! You are reading too much into what I said. I was not talking about the climate, or even weather. I was talking about a little parcel of air. My objection to Fraser's pedantry is that he is making a tail wagging the dog argument.

    I picked an over the top situation (100% relative humidity, 50C, standard pressure). Even at that extreme, water vapor is smallish portion of the air, whether measured by mass, moles, or heat capacity. A better extreme would be 35 C rather than 50 C because 35 C is the highest recorded dewpoint. This drop of 15 degrees reduces the saturation vapor pressure by 55% (to 0.055 atm).
     
    Last edited by a moderator: May 4, 2017
  11. May 21, 2010 #10
    What exactly are you saying?

    By the way my rough calculation is that if a parcel of air is kept in equilibrium with liquid water then, even at 10°C, water vapour still accounts for half of the heat which this parcel shuffles.
     
    Last edited: May 21, 2010
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