Question re: water temperature loss due to evaporation

Jump to latest Follow Reply
bertschb

bertschb

Bronze Supporter
Dec 11, 2021
392
Arizona
Pool Size
13600
Surface
Plaster
Chlorine
Salt Water Generator
SWG Type
Pentair Intellichlor IC-40
I'm no scientist and I haven't stayed at a Holiday Inn Express lately so I need a little help understanding the effects of water temperature loss due to evaporation. Here's my hypothetical question...

If my pool water was 80 degrees and the ground surrounding my pool was 80 degrees and the ambient temp was a constant 80 degrees 24 hours a day and the pool was in the shade all day, would the water temperature drop below 80 degrees purely from evaporation? It sounds like kind of a stupid question but if evaporation causes a loss of heat energy it would seem the water temp should drop. But, how can it drop below ambient? Be gentle. I'm way out of my wheelhouse here.
 
This one had my head spinning for a minute. If I’m remembering things correctly…

If everything is at a constant and exact equilibrium temperature, there should not be enough available energy to perform an evaporation phase. I believe this could only realistically take place in a vacuum.
 
bertschb said:
I'm no scientist and I haven't stayed at a Holiday Inn Express lately so I need a little help understanding the effects of water temperature loss due to evaporation. Here's my hypothetical question...

If my pool water was 80 degrees and the ground surrounding my pool was 80 degrees and the ambient temp was a constant 80 degrees 24 hours a day and the pool was in the shade all day, would the water temperature drop below 80 degrees purely from evaporation? It sounds like kind of a stupid question but if evaporation causes a loss of heat energy it would seem the water temp should drop. But, how can it drop below ambient? Be gentle. I'm way out of my wheelhouse here.
Click to expand...
Put water on your arm, just a bit. Fan that arm. Lower than ambient temperature.
pooloperationmanagement.com

Swimming Pool Energy: Common Sources of Energy Loss

Learning about swimming pool energy conservation is a large part of becoming a Certified Pool Operator®. Find out how pools can waste energy.
pooloperationmanagement.com pooloperationmanagement.com
 
1poolman1 said:
Put water on your arm, just a bit. Fan that arm.
Click to expand...
Would we feel the cooling if the humidity was say 95% and the water and ambient temp was 98.6 degrees (same temp as our skin)?

If the water temp can drop through evaporative cooling even if everything was stabilized at 80 degrees (air, water, soil), I wonder how far it would drop.
 
bertschb said:
Would we feel the cooling if the humidity was say 95% and the water and ambient temp was 98.6 degrees (same temp as our skin)?

If the water temp can drop through evaporative cooling even if everything was stabilized at 80 degrees (air, water, soil), I wonder how far it would drop.
Click to expand...
Having been in New York during July in those very conditions I can say yes, you do feel cooling, just not as much. There is a very complicated (at least to me) formula for heat loss due to evaporation that takes into account relative humidity. All I know is what I felt. Was doing some volunteer construction work. Would literally pour water on my shirt (head, back) and then have to stand in front of a fan to get some relief. But, it did work.
 
So here is an update...

Our high temps over the past five days have been 110-115. Overnight lows 79-83 and that is for a very short time. I would expect the water temp to be in the 90's but it's 82 right now. Soooooo, either evaporation really does lower water temperature below ambient and/or the ground surrounding my pool is what is contributing to most of the temperature loss. I read something last year that indicated pool water temps would be roughly the average of the high and low temps for the day. Clearly that is not the case.
 
Your morning water temperature will be roughly equivalent to your low air temperature. By afternoon the pool temperature will increase based on how much UV impact your pool gets. Surrounding ground temperature may have some cooling effect right now, but by late June early July it will be much warmer.
 
  • Like
Reactions: proavia

Enjoying this content?

Support TFP with a donation.

Give Support
bertschb said:
I read something last year that indicated pool water temps would be roughly the average of the high and low temps for the day. Clearly that is not the case.
Click to expand...

Maybe in a very shallow body of water.

A pool is a large thermal sink with a relatively small surface area.

Any evaporative cooling only happens on the surface of the water and all the water under it provides a large thermal sink so the overall temperature change is minimal.

Your 13.6K pool contains 112,880 lbs of water and thus takes 112,880 BTUs of cooling to make a 1 degree change in the pool.

The pool water temperature will lag the ambient temperature by a significant amount of time depending on how many BTUs its surface area can absorb or radiate.
 
bertschb said:
So here is an update...

Our high temps over the past five days have been 110-115. Overnight lows 79-83 and that is for a very short time. I would expect the water temp to be in the 90's but it's 82 right now. Soooooo, either evaporation really does lower water temperature below ambient and/or the ground surrounding my pool is what is contributing to most of the temperature loss. I read something last year that indicated pool water temps would be roughly the average of the high and low temps for the day. Clearly that is not the case.
Click to expand...

Brian,

In AZ you can expect to lose a lot of water overnight due to evaporation especially if any wind is present. There is enough of a temperature inversion (warm water and cooler air) as well as extremely low humidity (teens) to drive A LOT of evaporation. That evaporation will cool the pool water.

If you go outside in the late evening when the air temp is lower than the pool temp, pull the cover off your auto fill pot and see how fast it is running.
 
JoyfulNoise said:
If you go outside in the late evening when the air temp is lower than the pool temp, pull the cover off your auto fill pot and see how fast it is running.
Click to expand...
I have a water flow meter on my auto fill so I can monitor how much water is going through my RV water softener. I've gone through 420 gallons since I did my most recent re-gen and that was about a week ago maybe?
 
  • Like
Reactions: JoyfulNoise
mknauss said:
Surrounding ground temperature may have some cooling effect right now, but by late June early July it will be much warmer.
Click to expand...
If the water temp goes up a lot in July, that would lead me to believe it's due to the ground temp warming. We've already had a long string of hot weather with very warm overnight temps so that will remain the same. Plus, evaporation should also be about the same. The amount of daylight starts to decline in a week. All that is left is the ground warming up slowly as ambient temps stabilize at higher levels over longer periods of time.

I realize I'm overthinking this but as a new pool owner I'm trying to minimize the use of our solar cover. We have it in the garage right now which is where we'd like to keep it as long as possible. If we ever build a pool, it will absolutely have a built in automatic pool cover!
 
Our solar cover is removed Memorial Day and put back on shortly after Labor Day. I then remove it around thanksgiving for the winter and put back in mid March for the spring.
 
  • Like
Reactions: JoyfulNoise
Even though evaporation is upright now, putting the solar cover on now will only lead to a non-refreshing bath water temp pool. With the low humidity, we are probably losing 1/2" to 1" of water per day. 92 degree plus water is not overly refreshing. Chances are refillwater averages in to low to mid 80's right now.

Ground temp will continue to increase for a while yet.
 
Interesting post. I actually have an auto-cover. I was curious about how much evaporative cooling I would get if I left the cover open overnight. So, I tried it.

Last week, pool water temp got up to 90 degrees on June 7. So, I left cover open overnight, (I normally close the cover every night) water temp dropped from a high of 90 degrees to 82 degrees when I checked at 8:12 am on June 8. I measured the water level at a specific point to then compare the next morning. I lost overnight 1/4". Of course, ymmv.
 
Thought I'd follow up on this post. Our high temps have been 110-115 and lows 85-90 for the last week (and 5 degrees lower than that for the last month). Water temp at 7:00AM has been 84-85. This is our first summer in the Phoenix area and I expected water temps to be in the low 90's by now so I'm really surprised. Our water temperature is lower than the overnight lows! What the heck????
 
bertschb said:
What the heck????
Click to expand...
Very low dewpoint.
Once (if) Monsoon humidity settles in, the low pool water temperature will rise some.

My evaporation has been very high this late spring early summer. Our low temperatures here have been lower than normal until just the last few days.
 

Enjoying this content?

Support TFP with a donation.

Give Support
You must log in or register to reply here.
Thread Status
Hello , This thread has been inactive for over 60 days. New postings here are unlikely to be seen or responded to by other members. For better visibility, consider Starting A New Thread.
Top Bottom
Back
Jump to latest Follow Reply