How cold does winter pool water really get?

[SoDel]

The answer seems obvious enough but I figure I’d ask anyway. For Winter chem adjustments I’ve assumed a low of 32 — after all not much going to happen any lower than there. Where I live, the temps will go well below freezing, but rarely for more than just overnight, maybe a full 24 hr period max. So I’ve looked for an answer to a seemingly straightforward question and it surprised me, not much seems to be out there as far as how to even start to go about figuring it out (which maybe I should take as a clue that it’s as straightforward as it seems lol). I’m aware it doesn’t really make much difference in the grand scheme but I hate unanswered questions in my mind.

Assuming a covered pool with the water level drained to about a foot below grade, is there some way to relate water temps as a function of outdoor temps or of ground temps or is it all just as it seems, like assume the water will or at least may eventually get as cold as it is outside? Thanks for any insight!

 

[Newdude]

The only way to get a round about guess is to average the last 5 to 7 day's highs and lows.

If the temp range is consistent, it's darn near dead on, but if either end of the temp window is high/low it will mess up the result by a couple degrees. For example, 1 or 2 days of warmer temps at the end may not be enough to raise the pool temp accordingly.

 

[Pauls234]

I found taking the average of the high and low to be surprisingly accurate. I've never paid close attention in the winter (it's a giant ice cube most of the time) but in the summer I'd often do that using the forecast, and even with that extra variable it was accurate enough. "I'm having a pool party next Saturday, what will be the approximate temp that day?" (Or put another way, what time do I need to make sure the heater is on to get to 88* by noon?)

 

[mgtfp]

Just logg your water temperature over the course of a year and plot the data in a graph. Just a couple of measurements per per week, sometimes in the morning, sometimes in the evening (or get a Bluetooth floating thermometer that does the job automatically for you), will give you a good understanding of what to expect on average and how much it usually fluctuates.

 

[JamesW]

The density of water increases until it hits 4 degrees Celsius at which point the density begins to decrease.

So, the entire pool will get to 4 degrees Celsius and then, the top layer will begin to lose heat and the top layer will get to 0 degrees Celsius.

At 0 degrees Celsius, any further heat loss results in the water turning into ice.

So, if you see ice, the water is probably at zero degrees at the top and up to 4 degrees Celsius at the bottom.

If you do not see ice, then the temperature is higher.

You can look at your local weather history and check local bodies of water (like a lake) to get an idea about water temperatures in the winter.

 

[JamesW]

Water/Sea Temperature in Sussex County for Today, October and 2023 - Delaware - United States - Watertemp.ORG

Sussex County's water temperature for today, this month, historical average sea/ocean temperatures and climate data for the year

www.watertemp.org

 

[SoDel]

Thank you all. Very helpful. I did go back through my logs and compared with historical daily air temp data and the correlation seems very loose, but close enough, maybe +-5 degrees F a lot of the time. There is some confounding factor(s) which I’m guessing are mainly solar / wind. A little surprising, the fact that it’s in the ground still doesn’t appear to provide much heat sink / source. My guess there is the ground isn’t as good a conductor of heat as I would have thought, and/or the water is able to gain and lose heat from its surface magnitudes faster than interaction with the ground.

Anyway, thanks again, very helpful — I’ll go with the assumptions that at some point it does freeze or close enough on top and gets pretty darn close to that underneath, and frozen water doesn’t have much effect lol, so 32 F is as low as need be accounted for (does that make sense?) .

 

[JoyfulNoise]

In any given season, the water temperature will trend towards the dew point temperature. At that point evaporation all but stops. However, that assume that there is no air flow (wind). This is why here in “the desert” pool water can cool off significantly overnight … at the cost of lots of water lost to evaporation (leading to high water bills ). In more humid climates, the dewpoint is much higher and water will not cool off much even after the sun goes down.

 

[SteveTech]

Last's years winter was rough where I am and I ran the pool all winter, the lowest I saw on the temp monitor in the pentair app was about 26-28 (top of pool was frozen)