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Gas, vapor, clouds any difference?

  1. Oct 28, 2013 #1
    Gas, vapor, clouds .... any difference?

    "Most of the water in the atmosphere is in the gas state called water vapor." (http://www.srh.noaa.gov/srh/jetstream/atmos/ll_rainman.htm)

    This quote was taken from a site dealing with weather.

    Does this mean that the average temperature of the water in the atmosphere is 100 degrees Celsius or more? Or is this statement confusing the gas state, with a particular way that liquids can exist, namely as clusters of molecules which are too small to be seen, and which, although their density is greater than that of air, are small enough to stay afloat in air for the same reason that particles of dust stay afloat, which, I assume, is due to being light enough to be lifted by random air currents.

    Comments?
     
  2. jcsd
  3. Oct 28, 2013 #2

    Nugatory

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    Water vapor can exist quite happily in the air at temperatures lower than 100 Celsius. The significance of the boiling point is not that water vapor cannot exist below it, but rather that liquid water cannot exist above it.

    So most of the water in the atmosphere is indeed water vapor, water in its gaseous form. Some, however, does condense out into tiny droplets of liquid water (or crystals of ice), which if tiny enough will stay afloat as clouds, contrails, and fog.
     
  4. Oct 28, 2013 #3
    Water vapor can (and does) exist at temperatures below 100 degrees.
    Even below 0 Celsius. So you don't have to imagine some exotic form of liquid water in the atmosphere.

    However there is water in the form of liquid droplets too. For example in some clouds.
     
  5. Oct 28, 2013 #4

    SteamKing

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    'Humidity' is a measure of how much water vapor the atmosphere contains.

    If you put a saucer of water out, the water will eventually evaporate into the atmosphere, even though the temperature is nowhere near 100 C.

    Do not confuse the process of 'evaporation' with the process of 'boiling'.
     
  6. Oct 28, 2013 #5

    Borek

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    Clouds are not vapor, but gas is.
     
  7. Oct 28, 2013 #6
    So, most of the water in the atmosphere is in the form of individual water molecules, not clustered together with other water molecules in the form of tiny micro-droplets? And these -- presumably what we see as clouds -- are not called water vapor? (In other words, 'water vapor' is just a synonym for 'water in the gaseous state'?)

    What proportion of water in the atmosphere is individual water molecules (i.e. gas/water vapor), and what proportion is 'floating liquid', i.e. condensed micro-droplets?

    And ... if individual water molecules can condense into tiny liquid floating micro-droplets, can any other common gas do this (at near STP)?

    Could we imagine very different temperatures and pressures where we could get micro-droplets (and perhaps rven 'rain') of, say, carbon dioxide molecules?
     
  8. Oct 28, 2013 #7
    One other question: when students are told that 'liquids take the shape of their container' but 'gases expand to fill their container' ... this presumably doesn't mean water in the form of micro-droplets? It really means 'liquids where the molecules have clustered together in such great numbers that they are not buoyed up by any surrounding fluid".
     
  9. Oct 28, 2013 #8

    Nugatory

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    yes. cloud and rain formation require fairly large volumes (it's much easier to form droplets on a convenient solid surface, as in distillation) so there may not be any realistic examples on earth... but it's easy to imagine.
     
  10. Oct 28, 2013 #9
    There are many, many, different molecular species which constitute what we call air: methane, CO2, oxygen, nitrogen, CO, water, NO, NO2, HNO3, HCl, N2O5, ClO, ClNO3, and others. All of them can be made to condense out if we raise the pressure and lower the temperature enough. Also, all of them will dissolve in water droplets to a small extent.
     
  11. Oct 28, 2013 #10

    Borek

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    Clouds on other planets (especially the gas giants) have nothing to do with water. While water has some very particular properties, on some general level it behaves the same way other gases do. In every mixture of gases in the correct range of pressures and temperatures there will be clouds and rains.
     
  12. Oct 29, 2013 #11

    sophiecentaur

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    Having read through the above, I feel I should point out that the water vapour in the atmosphere is no different from all the other gas molecules up there. The partial pressures of all the gases go to make up the total pressure in the atmosphere. Air is not 'like a sponge'. Water does totally its own thing. The partial pressure of water vapour, in the presence of liquid water will be the vapour pressure corresponding to its temperature and that's what will govern the amount of water vapour. (I am, of course, talking in terms of the equilibrium state of a fixed mass of air over a water surface; the weather is constantly giving non-ideal conditions).
     
  13. Oct 30, 2013 #12

    cjl

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    Water vapor is clear - what you see as clouds is actually a collection of small water droplets. Water vapor is everywhere in the atmosphere though, not just in the clouds.
     
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