Average water temps

[laurandavid09]

Of pool water during the hottest part of the year in your area.

The area I am most interested in is the Houston area.

Thanks.

 

[mas985]

Pool temps generally will follow the average daily temperature within a few degrees so just look up your area's historical temperatures on wunderground.com. A solar cover will increase that average by 3-5 degrees.

 

[257WbyMag]

Lived in a rent house with a pool many years ago in Spring and as I recall, about the hottest that I remember it being was around 85 during the bitter part of the summer. This was a much bigger pool than the one that I have now and mine will get to about 88 degrees max.

Lots of things can affect the temperature though. How much direct sun it gets is a biggie. Also, the more water it has, the less temperature fluctuation you are likely to see.

 

[polyvue]

Pool temps generally will follow the average daily temperature within a few degrees so just look up your area's historical temperatures on wunderground.com. A solar cover will increase that average by 3-5 degrees.

Mark, is this your own data... or do you consider this universal?

I ask because the average mean temperature in Sacramento in July/August is only about 75 degrees F. but it seems the pool water temperature is consistently much higher than the mean from late March through late September. With continual use of a solar cover my pool's temp would exceed 95 degrees, so I remove it for most of July to get it down to a more pleasant 86-94 or so. Full sun, all day, in the month preceding and following the summer equinox [*]... no solar panels in use, partial sun on equipment pad (maybe 1/2 day).


EDIT *An interesting notion, to be sure -- but I meant to say "summer solstice"! END EDIT

 

[chem geek]

From what I've seen monitoring what people have reported on various sites, the water temperature without a pool cover tends to be fairly close to the average day/night temperature, usually somewhat higher but not by a lot. It does absorb sunlight during the day and can warm up several degrees during that time, but then it cools off at night from evaporation, conduction, convection, and radiation. A solar cover usually adds around 10-15F. So you'd expect perhaps 80F or so without a cover and 90-95F when using a cover, but it sounds like you getting 86-94 without a cover and 95+ with a cover. Maybe your use of the cover up to June did heat the water to 90-95F and removing the cover only slowly has the water temperature drop. Does it continue to drop (albeit slowly) throughout the month when you have the solar cover off?

Maybe your situation is due to having very little wind such that the evaporation is less, but your relative humidity is low which should have a fairly high rate of evaporation.

Our next door neighbor doesn't heat his pool nor use a cover and it doesn't get to 75F with our average July temperature of 68F.

 

[polyvue]

So you'd expect perhaps 80F or so without a cover and 90-95F when using a cover, but it sounds like you getting 86-94 without a cover and 95+ with a cover. Maybe your use of the cover up to June did heat the water to 90-95F and removing the cover only slowly has the water temperature drop. Does it continue to drop (albeit slowly) throughout the month when you have the solar cover off?

The cover was removed because the water was uncomfortably warm and there was a gradual cooling off toward the end of the month, so your explanation makes sense. This was not a controlled experiment, of course --- I just scanned my logs for water temperature and cover usage on a weekly basis, so don't have sufficient data that would support any particular hypothesis. If we modify Mark's statement a bit to stipulate no cover usage or solar panels, zero precipitation, a narrow range of relative humidity, wind and barometric pressure (evaporation rates) perhaps it can be universalized. A one-minute thought experiment would indicate that the water must be fairly close to a (moving) average mean. Or, in language I loosely attribute to duraleigh's perspective on treatment dosing: "close enough!"

It would be interesting to know if the same observation proves correct in other latitudes and longitudes with higher humidity, variable wind and precipitation, ground temperatures etc... and what effect larger bodies of water have on this, per 257WbyMag's suggestion.

 

[tjt040774]

Here in SE Texas, our pool is in full sun the majority of the day. During the middle of the summer when the air temps are hitting the triple digits, the water is in the mid 90's.

 

[mas985]

There are many factors that go into the actual pool temperature, primarily solar heat gain and evaporative losses. The evaporative losses are dependent on dew point temperatures and wind speed so those areas with higher humidity will have higher than average pool temps. Those with dryer air will have lower than average pool temps.

There are several ways you can estimate the evaporation rates but this one seems to be the most accurate. Once evaporation rates are known, and if you assume they are 60% of the total heat loss, then you can estimate the total heat loss for the pool. Combining that with a heat gain calculation using tables from this source, you can find a pool temperature equilibrium.

As an example, if you compare Polyvue in Sacramento, CA to tjt040774 in Huntsville, Tx in the month of July:

PolyVue (no Cover, no solar):

Average Air Temp: 79 F
Average Dew Temp: 48 F
Average Water Temp: 82 F
Morning Water Temp: 78 F
Evening Water Temp: 86 F
Average Water Loss: 472 Gallons/Week

PolyVue (with cover, no solar):

Average Air Temp: 79 F
Average Dew Temp: 48 F
Average Water Temp: 94 F
Morning Water Temp: 91 F
Evening Water Temp: 98 F
Average Water Loss: 405 Gallons/Week

tjt040774 (no cover, no solar)

Average Air Temp: 84 F
Average Dew Temp: 72 F
Average Water Temp: 93 F
Morning Water Temp: 89 F
Evening Water Temp: 96 F
Average Water Loss: 510 Gallons/Week

So because Huntsville has a much higher dew point temperature, the average pool temperature is higher but Polyvue's is fairly close to the air temperature without a cover. But again, there are a lot of variables that go into the pool temperature so the results can vary by quite a bit.

Notes:

For the pool cover, I assumed 15% solar blockage but 50% heat retention. Pool solar heat gain is 80% of incident. Evaporation is 60% of total. Zero wind speed.

 

[polyvue]

But again, there are a lot of variables that go into the pool temperature so the results can vary by quite a bit.

I guess the variables you allude to must be shifting my Sacramento July pool temp higher than what would be expected by the analysis. The biggest surprise for me is the estimated evaporation ("average water loss") during a week with the solar pool cover in place... only 14% water conservation achieved with a pool cover?

Sheesh. Maybe I'll install solar panels and give up the pool cover entirely during summer.

 

[mas985]

I'm not sure exactly where in Sacramento you are but according to this, average temp in July was 79 degrees with a dew point of 48 degrees. There are micro-climates in the area so the averages can vary by quite a bit.

Also, the reason that there is little water savings is because of the water temperature rise. A pool cover would save water if the water temperature didn't rise. Also, I don't have a lot of very good data on solar cover effectiveness. Mostly from manufactures claims. Also, it depends upon if you keep in on all the time or just night time. In effect though, the cover is saving 50% of the water. Without a cover and at a same 94 degree temperature, you would lose 800 gallons/week.

 

[polyvue]

I used the Average Mean, which is reported at 76 degrees F. from this page:

http://www.wunderground.com/history/air ... story.html

My location, just north of downtown, would be (IMO) a bit warmer and less humid than Rancho Cordova... closer to Sac Intl Airport than Exec Airport temps.

Hopefully, this question isn't too obvious, but how does the water evaporate though a solar cover?

 

[mas985]

That location is fine if you live near the airport but it is a bit more accurate if you can choose a PWS near where you actually live from this map.

 

[polyvue]

This July 2009 data for Sacramento (South Natomas) is reported from a weather station and is formatted like that used in your analysis.

 High: Low: Average: 
Temperature: 105.1 °F  56.5 °F  77.2 °F  
Dew Point: 64.4 °F  44.4 °F  53.8 °F  
Humidity: 78.0% 17.0% 47.4% 
Wind Speed: 11.9mph from the SSE - 1.7mph  
Wind Gust: 17.9mph from the South - - 
Wind: - - South 
Pressure: 30.03in  29.69in  -

Source: http://www.wunderground.com/weatherstat ... span=month

My question regarding evaporation thru a solar cover was serious. I think there must be some mistake in the water loss shown between the two conditions (100% use of solar cover vs. 0% usage)

 

[mas985]

The higher dew point increases the average water temperature by a degree or so to 84 degrees. [EDIT] Forgot to add wind speed of 1.7 MPH. With that, the average temperature drops to 74 degrees.

The cover is actually saving about 405 Gallons per week but it depends upon your reference. 50% water savings is usually what is quoted for solar covers but I believe that is for a constant water temperature. So that also means it is retaining 50% more heat and increases the water temperature which in turn increases the evaporation. If you compare with and without a cover but at the SAME water temperature, the water loss is half as much with the cover. So without a cover but at 94 degrees water temp, you would lose 810 gallons per week (assumes heat comes from somewhere else such as solar). But with the cover and SAME water temperature, the water loss is 405 gallons or half the loss. The better the solar cover, the more heat it retains and the higher the water temperature increases the remaining evaporation component.

I have solar panels without using a cover and I can lose close to 800 gallons per week or 2.5"/week with an average pool temperature of 87 degrees. With a cover and using much less solar for the same temperature, I would lose about half that amount.

 

[bk406]

Not to hijack the thread too much, but these water loss levels will sound really high to most people (but they are actually correct). It furthers my contention that a lot of people (read most) that fear a water leak is in all actuality, evaporation. A 1 inch water loss in a ~450 sq ft pool is around 300 gallons. So, an 800 gallon a week loss in that size of pool is around 3 inches. At that level of water loss, I can promise you that a lot of people would worry about a leak. Nice analysis, Mas!

 

[mas985]

bk, I think you are correct. If a pool has fairly high temperatures relative to the dew point, you can expect quite a bit of evaporation. I have measured over 3/8" in a day on my own pool.

 

[polyvue]

Not meaning to flog a dead horse here, but perhaps there's something I'm not getting.

I don't question the figure for 400... or even 1000 gallons of water loss in an uncovered pool, but exactly how does water that is covered with a impermeable membrane (the solar cover) evaporate? If not through the cover, then, from where?

 

[bk406]

Well, my solar cover doesnt exactly fit tight like saran wrap. The edges of the cover on one side dont go all the way to the edge of the pool.
Even with my cover on, on a crisp fall morning, the water vapor looks like dry ice fog coming off. I still get pretty good heat retention though.

I would guess that most people who have solar covers have gaps around the perimeter of the pool. I think you are assuming a solar cover is more like a seal when its really not. Also, water evaporates 24/7, so it's evaporating when the cover is off during the day too.

 

[polyvue]

Well, my solar cover doesnt exactly fit tight like saran wrap. The edges of the cover on one side dont go all the way to the edge of the pool.
Even with my cover on, on a crisp fall morning, the water vapor looks like dry ice fog coming off. I still get pretty good heat retention though.

I would guess that most people who have solar covers have gaps around the perimeter of the pool. I think you are assuming a solar cover is more like a seal when its really not. Also, water evaporates 24/7, so it's evaporating when the cover is off during the day too.

Thanks for the reply... I wouldn't say mine fits like saran wrap, but there's very little perimeter gap (it abuts the water tile pretty snugly and creases in areas at the center). Assumption is that the cover is on 24/7 and has no gaps wider than 1/4 inch along the perimeter. I can think of only two vulnerable areas --- the skimmer and the auto fill, but can't believe that these areas would allow hundreds of gallons of water to evaporate in a week.

Re: Steam on fall mornings. I wonder if the vapor you observe is water droplets/condensation on top of the cover. My question purposefully evaded the question of heat retention -- just interested in how water loss might happen from a pool surface that is 99%+ covered. I don't think it can.

 

[mas985]

I believe that the 50% factor for the solar cover assumes that the cover is used only half the time so the other half of the time there is evaporation. But in general, you are correct in that if the pool was nearly 100% covered for 24 hours a day, the evaporation rate should go down quite a bit but probably not to zero. Also, I had to guess at the cover type, if you have the spa covered and the total surface area the pool.