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A few questions about steam, water vapor, and humidity

  1. Oct 8, 2010 #1
    I'm trying to understand the differences (if there are any) between steam, water vapor, and humidity. First of all, is there any difference between the meanings of the terms steam and water vapor? Is there a difference between the terms "gas" and "vapor" in general?

    Also, I was under the impression from my physics class that steam could not exist below 100 C, (at 1 atm) as it would turn back into liquid water. So then how does any humidity (which I'm guessing isn't any different from the steam/water vapor you get when you boil water) exist under 100 C? And just how cold can it get before it must condense back into water?

    The thing that got me thinking about all this was when my windshield fogged up this morning as I was driving to school. I put the air on to defog it and then I wondered two things: 1) how the water vapor (steam) could exist in such cold air to begin with and 2) how does blowing room-temperature air through the vents of my car under the windshield cause the condensation to go away? Why does this make it evaporate?

    Another semi-related question: I've heard people say you should use hot air and other people say you should use cold air to defog your windshield. Can someone please explain which makes more sense and why?

    Thanks a lot, and sorry for asking so many questions.
  2. jcsd
  3. Oct 8, 2010 #2


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    Staff: Mentor

    You're on the right track: steam is a vapor that is saturated (at 100% RH).

    Condensation is mostly unrelated: it occurs on a surface that is below the dew point.
  4. Oct 12, 2010 #3
    Suggest you look your terms up in Wilipedia:



    These are not necessarily scientifically precise terms....

    I don't think steam is necessarily saturated water vapor....see the Wiki article above 'SATURATED STEAM'

    it's just visible water vapor and while it is 100 degrees C upon emission from boiling water,at the standard pressure ( I have forgotten is it 1 ATM???).... anyway, it quickly drops below that temperature in typical, say kitchen, environments...but may still appear as water vapor mist.

    You might also check on fog (which is also water vapor).....and clouds....

  5. Oct 12, 2010 #4
    Here's an example which may not be precisely accurate, but conveys the idea...

    Say a power plant gives off hot water vapor...maybe from a smokestack....steam (water in the form f heated mist) ...it likely rises for a while and drfits around, may hang as a low (visible) cloud then cool and sink and become ground fog....which is now cooler water vapor...or it may just increase ground level humidity and not be visble.....and it comes into conatct with a cool surface and condenses and appears as water droplets....
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