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Thread: Pool water evaporation...

  1. #1
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    Pool water evaporation...

    Hello all...hope you all had a trouble free pool season. Live in NJ so our pool season is winding down. We're closing this weekend. These cool nights and not so warm days really make for no more swimming. Not to mention schools back in!!!

    Quick question about evaporation...hope I don't sound too stupid
    Do cool nights...I mean upper 50's and warm, sunny days...very low 70's make pool water evaporate more quickly???
    IG 17x26x36 Designer Ell
    vinyl liner
    Hayward c-3000 (4) cartridge filter
    Dolphin diagnostic pool cleaner

  2. #2
    Senior Member donaldm823's Avatar
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    Re: Pool water evaporation...

    Quote Originally Posted by frasermom
    Hello all...hope you all had a trouble free pool season. Live in NJ so our pool season is winding down. We're closing this weekend. These cool nights and not so warm days really make for no more swimming. Not to mention schools back in!!!

    Quick question about evaporation...hope I don't sound too stupid
    Do cool nights...I mean upper 50's and warm, sunny days...very low 70's make pool water evaporate more quickly???
    I live in your same area (near Philadelphia). The cool nights directly cause water evaporation-if your pool water is warm with the night air cool you can see water mist (looks like steam) rising over the water. The evaporation rate is directly proportional to the difference between the pool water temp and the air temp (assuming humidity is constant).

    I use a solar cover at night (put it on around 6:00PM) which stops all of the evaportation as well as maintains the heat, so I generally only lose 2-3 F at night in pool temp. I use a pool heater to heatup the pool back to the 86F range (2 hrs of heating) in the morning and then take the solar cover off at noon time.
    Don
    SW Florida
    10,000 gal in ground concrete pool
    Pentair Cartridge filter
    Pentair Intelliflow pump
    Pentair SWG
    using BBB for pool chemistry/Lamotte ColorQ tester for daily testing

  3. #3
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    Re: Pool water evaporation...

    Thanks for replying...would use a solar cover if we were going to leave the pool open for any longer but we're just hangin' in there until the w/e so we can get it closed. Of course after we close it the temp here at the Jersey shore will skyrocket to 90...LOL LOL LOL and my kids will be crying the blues bc the pool is closed...forgetting that i have had to twist their arms to go in it for the last three weeks!
    IG 17x26x36 Designer Ell
    vinyl liner
    Hayward c-3000 (4) cartridge filter
    Dolphin diagnostic pool cleaner

  4. #4
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    Re: Pool water evaporation...

    Actually, at the same relative humidity and bulk pool water temperature, hotter air would cause faster water evaporation. During the day with the sun on the pool, evaporation is accelerated even if the air is warmer than the pool. You can see evaporation rates around the country by clicking on the map in this link. The rate of evaporation is a function of the temperature at the water surface (the air/water boundary), the relative humidity of the air (or alternatively, the vapor pressure of water in air at that temperature), the temperature of the air, air flow above the surface, and water flow below the surface. Evaporation is greatly accelerated by air movement above the water surface (which keeps the humidity lower; otherwise the humidity at the surface would rise and evaporation would slow down) and by circulation of the water below the surface (which keeps the temperature up; otherwise the water at the surface would cool and evaporation would slow down). So a breeze or wind and keeping the pump running both increase the rate of evaporation. The temperature of the air is a factor as evaporation is limited by diffusion and diffusion is faster at higher air temperature.

    As a general rule, the relative humidity at night is higher since the temperature drops and the water in the air doesn't go anywhere so there is usually less evaporation at night than during the day. Also, the cooler temperature at night cools the water surface which further slows down evaporation. The reason you see the steam at night is that the air is more saturated with water already so as warmer water-saturated air rises from the pool surface, it cools down forcing water to form droplets which are visible as steam; during the day the water evaporates into lower humidity so is not as visible.

    The bottom line is that if one wants to limit evaporation, then the daytime is when it's most important. However, when it comes to the water temperature dropping due to heat loss, that is usually much greater at night because evaporation cools the water surface AND at night there is no sunlight adding heat to the water nor warmer air to compensate for (or to slow down) this heat loss.

    Richard
    16,000 gallon outdoor in-ground 16'x32' plaster pool; Pentair Intelliflo VF pump; Pentair IntelliTouch i9+3s control system; Jandy CL-340 square foot cartridge filter
    12 Fafco solar panels; Purex Triton PowerMax 250 natural gas heater (200,000 BTU/hr output); automatic electric pool safety cover; 4-wheel pressure-side "The Pool Cleaner"

  5. #5
    Administrator JasonLion's Avatar
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    Re: Pool water evaporation...

    Cool nights lead to lower humidity, which means more evaporation. Where I am in Maryland we go from an inch a month in July to an inch every couple of days in late September (with the water heated).
    19K gal, vinyl, 1/2 HP WhisperFlo pump, 200 sqft cartridge filter, AutoPilot Digital SWG, Dolphin Dynamic cleaning robot
    TFP Admin. Creator of PoolMath and Pool Calculator. Other handy links: Support this site, TF Test Kits, Pool School

  6. #6
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    Re: Pool water evaporation...

    Jason, do you mean that the cooler nights force the water out of the air (as dew, for example) so that the next day when the temperature rises there is less relative humidity so the evaporation is greater? That makes sense. What I was referring to was that during the night itself, the relative humidity is high so the evaporation rate at night is low.
    16,000 gallon outdoor in-ground 16'x32' plaster pool; Pentair Intelliflo VF pump; Pentair IntelliTouch i9+3s control system; Jandy CL-340 square foot cartridge filter
    12 Fafco solar panels; Purex Triton PowerMax 250 natural gas heater (200,000 BTU/hr output); automatic electric pool safety cover; 4-wheel pressure-side "The Pool Cleaner"

  7. #7
    Administrator JasonLion's Avatar
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    Re: Pool water evaporation...

    Right. In mid summer we often have humidity in the 90% range during the day. Come September and cooler weather the humidity starts to drop and by the end of September it can be in the 20-30% range during the day.

    Cold night are strongly associated with rapid evaporation around here, though I'm not at all sure which is a cause and which is an effect. My very limited weather knowledge suggests that high humidity helps keep the temperature higher at night by limiting direct radiation into space, just as much as cold nights tend to lower humidity through condensation/dew. Though I could easily have that wrong.
    19K gal, vinyl, 1/2 HP WhisperFlo pump, 200 sqft cartridge filter, AutoPilot Digital SWG, Dolphin Dynamic cleaning robot
    TFP Admin. Creator of PoolMath and Pool Calculator. Other handy links: Support this site, TF Test Kits, Pool School

  8. #8
    Senior Member donaldm823's Avatar
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    Re: Pool water evaporation...

    Bottom line-I lose water at night in Sep unless I use a solar cover at night. The water loss stops when I start using a solar cover at night. I lose very little water level during the day (I run the pool pump during the day).

    Not faulting chemgeek's logic, but my pool water level measurements do not follow his explanation. Jason's experience with water loss in MD is pretty much mine also
    Don
    SW Florida
    10,000 gal in ground concrete pool
    Pentair Cartridge filter
    Pentair Intelliflow pump
    Pentair SWG
    using BBB for pool chemistry/Lamotte ColorQ tester for daily testing

  9. #9
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    Re: Pool water evaporation...

    Mmmmm.... back to the drawing board on this one.
    If you can find out from a weather report what your temperature and relative humidity are during the day and at night, that might be useful, but this isn't that important to figure out.
    16,000 gallon outdoor in-ground 16'x32' plaster pool; Pentair Intelliflo VF pump; Pentair IntelliTouch i9+3s control system; Jandy CL-340 square foot cartridge filter
    12 Fafco solar panels; Purex Triton PowerMax 250 natural gas heater (200,000 BTU/hr output); automatic electric pool safety cover; 4-wheel pressure-side "The Pool Cleaner"

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