Pool water evaporation...

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frasermom

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May 21, 2008
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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???
 
frasermom said:
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???
Click to expand...

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.
 
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!
 
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
 
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.
 
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.
 
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
 
Mmmmm.... back to the drawing board on this one. :eek:
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.
 
Refreshing this older thread as I am having the same trouble right now. Last week I have seen a significant drop in the water level, like 1/2 an inch a day. My first thought was a leak, so i looked at all the obvious places and do not see anywhere that there is water.
I have a vinyl liner inground pool. The tempts this last week have been seasonally low and I am wondering if this is the case. Today I will jump in with some food dye and my goggles and look for anything strange. I also am using the bucket trick to compare water levels.
 

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babykinz said:
Refreshing this older thread as I am having the same trouble right now. Last week I have seen a significant drop in the water level, like 1/2 an inch a day. My first thought was a leak, so i looked at all the obvious places and do not see anywhere that there is water.
I have a vinyl liner inground pool. The tempts this last week have been seasonally low and I am wondering if this is the case. Today I will jump in with some food dye and my goggles and look for anything strange. I also am using the bucket trick to compare water levels.
Click to expand...

I have noticed my water level dropping quicker than normal over the last week or so here in Rome, GA too. Like you, we have had cooler than normal temps. I looked for leaks over the weekend and found nothing.
 
The evaporation rate for water increases as follows:
  • Hotter water temperature
  • Colder air temperature (only a factor if there is some humidity)
  • Lower humidity percentage (for same temperature; variation better described by dew point temperature)
  • Higher wind (a HUGE factor)
Based on this page for August weather in Augusta, GA, it does look like August 1-3 was lower than normal in temperature (in this page it looks like July was pretty normal). Using data from this week and daily detail, I can calculate the evaporation rate of the water, though the wind should be the wind at the surface of the water which with typical barriers (fences, etc.) will be lower than I show below. "Night" is early AM of the day indicated (i.e. the coldest usually around 4 AM). I assume a water temperature of 86ºF.

Date ........... Temp ...... DewPt ...... Humidity ...... Wind ...... Evap (in/day)
Jul 27 Night .. 77ºF ....... 69ºF ......... 72% .......... 0 MPH ..... 0.18"/day
Jul 27 Day .... 95ºF ....... 72ºF ......... 47% .......... 9 MPH ..... 0.72"/day
Jul 28 Night .. 82ºF ....... 73ºF ......... 74% .......... 0 MPH ..... 0.13"/day
Jul 28 Day .... 96ºF ....... 66ºF ......... 37% .......... 14 MPH ... 1.37"/day
Jul 29 Night .. 71ºF ....... 60ºF ......... 68% .......... 6 MPH ..... 0.82"/day
Jul 29 Day .... 88ºF ....... 57ºF ......... 35% .......... 7 MPH ..... 0.99"/day
Jul 30 Night .. 70ºF ....... 57ºF ......... 63% .......... 7 MPH ..... 0.99"/day
Jul 30 Day .... 91ºF ....... 58ºF ......... 33% .......... 6 MPH ..... 0.86"/day
Jul 31 Night .. 73ºF ....... 68ºF ......... 83% .......... 6 MPH ..... 0.65"/day
Jul 31 Day .... 84ºF ....... 70ºF ......... 63% .......... 9 MPH ..... 0.79"/day (rain/thunderstorm and cooler later in the afternoon)
Aug 1 Night .. 69ºF ....... 66ºF ......... 90% .......... 11 MPH .... 1.10"/day
Aug 1 Day .... 89ºF ....... 69ºF ......... 69% .......... 10 MPH .... 0.50"/day
Aug 2 Night .. 69ºF ....... 67ºF ......... 93% .......... 8 MPH ..... 0.82"/day
Aug 2 Day .... 77ºF ....... 70ºF ......... 76% .......... 7 MPH ..... 0.68"/day

Looking at the overall history in July to early August, it looks like the average wind (day and night combined) went up from 4 MPH to 7 MPH with higher gusts as well (and not having calm nights) and the dewpoint dropped from 69ºF to 66ºF and these two factors largely account for the increase in evaporation rates. This would go from 0.46"/day to 0.77"/day. If the wind is cut back from fences by 43% to 2.3 MPH to 4 MPH then this is 0.33"/day to 0.52"/day which might be what you have been seeing.
 
WoW! Thanks Chem, that's a lot of information right there.
I did the bucket test and the pool had dropped 1/4 inch in 6 hours while the bucket remained the same. I did not run the pump at all, but did get in and dropped food coloring around the fitting and skimmmer, but did not see it being sucked in. I did notice the water in the pool has got cool, by 4 degrees.
 
Update:
Food dye might not be a good choice, or maybe I need a syringe. I did notice the jet on the stairs , the inner part popped out and sand spilled on the steps, maybe a few tablespoons. My husband thinks its sand coming from the filter, I think its coming from behind the stairs. He says , since we live in Georgia it would be the red clay coming through not sand. I think they might have used sand behind the stairs and the bottom of the pool itself.
Is it possible filter sand could spill out of a jet like that, and only one jet?
 
I have to agree that where I live in PA I also experience much more evaporation with cooler nights. I have not added water for well over a month, but the last few evenings have been quite cool and so, for whatever reason, I had to add quite a bit of water. Also, I've resumed using my heater (did not use it for close to 3 weeks due to renovation and new plaster), and so the water is warmer than it would be normally. I've always found that on hot humid days and relatively warmer evenings, there is very little evaporation, but as we move into fall I really notice it. Every year, though, I always get paranoid about leaks - have done bucket tests several times over the years and never found that the decrease of level is different between the bucket and pool. One obvious factor that has not been mentioned is, of course, the amount of rainfall. We haven't gotten much of that over the past few weeks. Where I live up on a hill we also get a lot of wind, which is a big factor in evaporation rate as Chem Geek pointed out.
 
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