Sunny day heating - faster with or without cover

[Bart]

The rain has finally stopped and the sun is out, but the pool is cool. I was wonder if I'd get a higher rise temperature if I left the cover on the pool or took it off. In my mind I can see pro and cons of each, but I wonder if anyone has done any home testing on this?

It's a blue "bubble" cover.

Thanks!

 

[JasonLion]

On a bright sunny day with sunlight directly on the water you are almost always better off with the solar cover off. The solar cover blocks some of the sunlight, removing it allows more to reach the water. The only exception is if it is both very dry (no humidity) and very windy.

 

[gullzway]

Thanks Jason, I was about to post this same question. I also have a blue solar cover and have been leaving it on unless swimming. Pool is still 70 degrees. I'll try leaving it off during the day, but with only 4-5 hours of direct sunlight I may be wasting my time.

 

[chem geek]

As described in this post, a typical white plaster pool can gain up to 0.7ºF per hour in temperature from direct noontime sun. However, as described later in that thread, typical 1/4" of evaporation will lower the temperature by about 5ºF or if consistent over 24 hours roughly 0.2ºF per hour. So there is a net gain so long as the cooling from evaporation isn't particularly high.

As Jason noted, very dry air and wind can significantly increase evaporation rates. Specifically, for typical water/air temperature differences, 10% relative humidity evaporates about 50% faster than 66% relative humidity while as little as 5 MPH wind (at the pool surface) increases evaporation by 220%. So just a consistent light breeze could nearly wipe out all gain from direct sunlight.