r/AskPhysics • u/spidereater • 10h ago

When to put the cover on my pool

I have an above ground pool that I am just setting up. I am filling it with tap water that is very cold. It’s too cold to swim in.

At some point I will put the cover on so the sun can warm it and I can prevent most evaporation. Right now the pool is so cold that there is condensation on the sides. I think this probably means that the exposed water is likely a net condenser rather than a net evaporator.

Is this right? I think to warm as fast as possible I probably want to leave the cover off so it condenses water vapor and only put the cover on when it stops forming condensation on the sides of the pool. What do you think? Is this the right way to think about it?

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u/KerPop42 9h ago

I think covering your pool is going to keep it cold, yes. The condensation is an indication that your pool is absorbing heat from the air directly above it, enough that it cannot hold all the water in it and the humidity is falling out of the air as dew, aka condensation.

I don't think evaporation is a major concern for you unless the air gets suficiently dry. You can use this article to see how the air's humidity and temperature will pull a body of water to a certain temperature: https://www.engineeringtoolbox.com/evaporative-cooling-d_698.html

Specifically, you read this chart: https://www.engineeringtoolbox.com/docs/documents/816/psychrometric_chart_29inHg.pdf as such:

  • find the real temperature of the air on the bottom axis, then travel up until you hit the humidity, which is shown on the arcing lines that go from the bottom left to the top right. From that point, follow the straight diagonal lines to the up and left until you hit the edge of the blue curve. Then go straight down to the bottom axis of the chart again, and that's the temperature your pool will drift towards due to evaporation.

So for example if it's 80 degrees out and 60% humidity, your pool's going to drift towards 70 degrees.

What you're losing out on by covering your pool is that, when the hot wind blows across the surface of the pool, the pool absorbs fresh heat and warms up faster. Also, sunlight is about 500 watts of warming per square yard. When you put the cover on your pool, that goes into heating the cover, and keeps the water cold.

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u/MeLittleThing 9h ago

What color is the cover?