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A pool with vapor over it at night

  1. May 9, 2006 #1
    I have a pool that has been above 80 degrees fahrenheit during the night. Recently I went for a swim just after sunset and noticed that I could see what appeared to be steam or some kind of vapor moving over or coming out of my pool. Now I thought it can't be steam, because steam is water in the form of vapor, which would have be to at 212 degrees fahrenheit! The air temperature is probably in the 60s at night, and I have a chlorine salt generating pool. Does anyone know what I am seeing and can anyone explain this to me? The only guess I have is that it has something to do with convection currents...but I don't know. I can't see how something can be evaporating out of the pool, because it's pretty cool at night; unless something is evaporating all the time, but I can only see it against dark backgrounds at night.
  2. jcsd
  3. May 9, 2006 #2


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    It's not steam. (Steam is actually invisible.) What you're seeing is actually droplets of water suspended in air.

    You are correct that your pool evaporates all the time, not just when the air is warm. In fact, it evaporates as long as the air above it is not 100% saturated already.

    When the air is already quite humid, the additional water vapor evaporating near your pool can cause super-saturation. Small droplets of water then condense out, and remain suspended in the air. What you're seeing... is fog.

    - Warren
  4. May 9, 2006 #3


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    Yep. Fog.

    At night, your pool water stays warm, while the air temp drops rapidly at night.

    The water evaporates (because it's warm) and rises, hitting the cool air and forming fog.

    As a side note: Counter-intuitively, the overnight temperature drop in a pool is NOT caused by the drop in ambient temperature of the environment. The vast majority of the temperature drop in a pool is actually caused by evaporation (which is an endothermic reaction - it steals heat). Best way to keep your pool warm is to prevent evaporation.
    Last edited: May 9, 2006
  5. May 9, 2006 #4


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    A side note:
    Think about that a little longer. If you listen to a weather report and the talking head tells you the relative humidity, what is being measured? What does that mean for your sentence above?
  6. May 9, 2006 #5
    Relative humidity is the ratio of the amount of water in the air to the amount of water when the air is saturated, right?

    So I was thinking about boiling water, but evaporation occurs all the time. Tell me if this is right: Evaporation occurs all the time because there are molecules at the top of the water that have enough kinetic enery to escape the liquid state. However, in boiling, all the molecules have enough kinetic energy to escape the liquid state. (but then why don't all the molecules immediately become a gas?)
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