My sis and BIL have an in-ground pool they love, and spend practically all their leisure time in and around it in the Summer. Since even Summer in Michigan can often get cool, especially at night, they often keep their pool heater cranked so it’s always warm- no shock to the system jumping in their pool.
Which is great if it’s say, 68 degrees F out. But a couple weeks ago we had a heat wave, which for Michigan meant a week of temps in the mid to high 90s. We went over to sis and BIL and hung out in the pool, which was like bathwater-- water temp low 80s. It was more refreshing than the ambient air temp, but barely. I said to BIL “you keep the pool heater going even in this weather?” and he said “no, that’s just how hot it gets in this weather. Can’t cool it down”.
Which got me wondering-- Arizona gets up to 115F regularly, and I just saw on the news that Palm Springs is going to have highs from 115-121F all week! Which got me thinking- at temps like that all week, I imagine a smallish pool is going to climb up to temps in the 90s-- almost hot tub temps. Do pools at a certain outside temp become unusable? Do they have coolers for their pools? Do they often keep their pools enclosed indoors with A/C if they can afford it?
A pool at a higher temperature than body temperature would surely be dangerous. In such an environment, the body would have no way to shed heat. And if the ambient temperature stays above body temperature for long enough, eventually the pool will be, too (how long this would take would depend on how big the pool was, how hot and humid the environment was, and what the initial temperature of the pool was).
How to Cool Down Your Pool Water #1. Aerate Your Water #2. Run Your Pump at Night #3. Stop Using Solar Covers #4. Add Some Shade #5. Install a Pool Chiller or Pool Cooler
It’s certainly possible to cool your pool, if you’re willing to pay for it.
Many years ago, I was staying at a resort in Vegas, that had an outdoor pool. It was mid-July, so quite hot, but they cooled the pool. In a shallow area, you could stand so the water came up to about your waist. You could actually feel the heat differential along your body, as it got warmer from your waist to your head. Kinda freaky.
Before I got rid of my pool, the water used to become quite warm by mid-summer. But, I always preferred it warmer. By mid-August, the water temperature was like a nice, warm bath. Then, once you got out, the dry air would chill you as it evaporated the water on your body.
That is normal thermal behavior in any pool, cooled or no. Likewise the surface at the shallow end is warmer than the surface at the deep end.
To be sure if they are pumping in cooled water the differential will probably be greater than would naturally occur. Net of where the cooled water is introduced into the pool.
From my time as a kid with a pool in SoCal, one thing that does happen on high solar days, and in warmer weather regardless of surface insolation is the rate of evaporation increases. Which has the effect of cooling the bulk pool water as the most energetic = hottest water molecules are boiled off into the sky, leaving the relatively less energetic ones behind.
A pool is a large thermal mass of water and maybe 3/4ths of the water’s surface area is in contact not with the atmosphere but instead with the buried concrete container and the surrounding Earth. The latter of which neither warms nor cools appreciably in the course of a brief heat wave.
True, but in my (Canadian) experience, I’d never felt it so clearly, all along your body. The degree of difference matters a lot in how well you feel it. It wasn’t just water vs. air, even the air closer to the water surface was distinctly cooler than the air at the top of your head.
Southern Arizonan here. My pool temp got up to 92F last week. It is down to 85 this morning. It cycles through about 3 degrees every 24 hours, and can have an overall upward or downward trend depending on the weather. Last week was very hot. However, monsoon rains arrived with several events of downpours over the last week, causing the pool temp to drop. (Monsoon rains are often lots colder than the temps at ground level, and have a noticeable cooling effect)
I don’t do anything to cool my pool…When it is 104F outside where I am working, jumping into water that is 85F is refreshing…but 82 would be better!
Yeah, I guess even if the pool water temps got hot enough to be uncomfortable, or at least not refreshing, you could take turns jumping in, climbing out, and letting evaporative cooling do its thing. That probably works even better in Cali and Arizona than it does in Michigan, where on the semi-rare occasions when it does get really hot here, it tends to be extremely humid as well.
Right now at 11am the beach near me (SoFL), the near-shore surface ocean water temp is 85F/29.5C and the air temp is 89F/31.5C. It’s more humid than Arizona but less so than e.g. Michigan.
The water is quite refreshing to get into. Being cooler than your skin, and given the tremendous difference in heat capacity between water and air, it pulls heat out of your body very effectively. But not so effectively that you can’t “get used to it” and you simply get colder and colder past the point of refreshing and down into discomfort.
And when you get back out, you dry pretty quickly in the warm air. Refreshing to enter and refreshing to leave. As long as there’s not too much breeze it’s just about an ideal combination IMO.
This is kind of getting at what I’m wondering in my OP question. Water transfers heat more effectively than air. That’s why 85F water feels cooler than 85F air-- since the temperature differential is still around 13 degrees less than body heat, the water can still do a good job drawing heat away from your body. But at around what temp does the heat transfer ability of water start to actually work against you, and possibly be more dangerous than the same air temp?
Skin temp is a bit less than the proverbial 98.6. I don’t have an answer off the top of my head and can’t be arsed to google right now. But you body is a net heat exporting machine. And you will eventually suffer heat stroke if that heat production cannot be shed to the outside world. For round numbers an adult human just sitting quietly gives off the same amount of heat as a 60W incandescent lightbulb. Just spread over a larger surface area.
The rate of heat transfer is proportional to temperature difference, and to the specific heat of water versus air, plus a term for transport if either the air or water is moving past your body.
Given adequate water humans don’t promptly overheat in 120F air because of sweating. Anyone who ever swam competitively can tell you that yes, you can sweat like a dog in a swimming pool.
Humidity plays a huge role in human thermal control. In conditions of high enough humidity sweat doesn’t work and you can die of bodily overheating in the shade despite unlimited water. The “heat index” beloved of news media is an attempt to combine the effects of temp and humidity into a single human physiology-relevant number.
So I’m not sure there’s a simple calculation that can say “water at temp X is as bad as this list of a dozen temperature/humidity combinations”.
Our apartment pool isn’t heated (which I guess means it’s technically “solar heated”). In San Jose when we have a week or so or 100-degree days the water temp might get close to 80, but most of the summer it’s in the low 70s. Most of the residents are elderly so that’s way too cold for them, which means I have the pool mostly to myself for swimming laps or just to hop in to cool off.