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Condensation on outside of glasses

  1. May 8, 2012 #1
    On a warm day, why do droplets of water appear on the outside of a cold glass of water? I don't really understand partial pressure and saturated vapour pressure, but I know it has something to do with this. Like when partial pressure = SVP, the water will boil, but at what point will water vapour condense...?
     
  2. jcsd
  3. May 9, 2012 #2
    You would want to look up dew point.
    And relative humidity is applicable also.
    Come back with a few more questions if you need to.
     
  4. May 10, 2012 #3
    My understanding is that warm air can hold more water vapor than cold air. So if you have warm air with a lot of water vapor getting cooled (perhaps by a cold glass) to a temperature where it cannot hold as much water, it has to dump the excess water out. Onto the glass, in this case.
     
  5. May 10, 2012 #4
    I agree with Lsos.
    But I always thought that the cold glass would cool down the gas (air) around it into liquid form, thus creating the water droplets on the outside of the glass.
     
  6. May 10, 2012 #5

    russ_watters

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    Staff: Mentor

    Since your house is always at room temperature, this has nothing to do with the capacity of the air to hold moisture. It is all about humidity. When the air is more humid and the dew point is above the surface temperature of the glass, dew will form. That's basically the definition of "dew point".
     
  7. May 11, 2012 #6
    Isn't dew point the point where air cannot hold any more moisture (water-to-air saturation temperature)...in which case it has everything to do with the capacity of the air to hold moisture?

    I just glanced at the Wikipedia explanation at dew point and it seems to exactly fit what I described....

    If the dew point is below the room temperature in your house, for example, it means that the air CAN hold more water. However, if the cold glass cools the air to below that dew point, then that colder air CANNOT hold more water. Indeed, it already has too much, and it gets rid of it.
     
    Last edited: May 11, 2012
  8. May 11, 2012 #7

    russ_watters

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    Staff: Mentor

    Yes, your explanation is correct, but I was trying to convey to the OP that his first sentence, which starts off "On a warm day...." was off the mark. I probably could have done it better.
     
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