View Full Version : How much water evaporates in winter, if any ?

05-29-2009, 05:26 PM
I live in central Massachusetts,
I took the cover off the other day and I lost 6 inches over the winter. I lost 6 inches last year but I had a lot of issues in the summer of 2007 as noted in my post last year in this forum. I got a lot of help in this forum and bought a DE filter which cured all of my iron issues. Last year, I found a tear on the bottom of the pool which I fixed but I probably did that scooping out all the leaves last spring.
I have a brand new winter cover and no water got into the pool over the winter and I did not siphon any pool water out of the pool. My skimmer and return were both plugged all winter.
This is not a surprise: I called 2 pool places and got different answers. One said "6 inches can be normal " and the other said "you got a leak"
2 questions)
1) What is normal evaporation over winter (Sept 1 to June 1 pool is closed)
2) Does water evaporate in the winter? .

Another thing that I do not understand: I read on the web that evaporation happens in low humidity and not high humidity which is opposite as to what I would think but it might make sense as a solar cover would raise the humidty which would slow evaporation. But the winter cover would at times raise the humidity in the pool on hot days but most days it would stay low as it is in the days of fall and spring. Sorry about the tangent.

chem geek
05-29-2009, 07:25 PM
The rate of water evaporation is proportional to the absolute difference between the saturation vapor pressure of water in air at pool water temperature and the actual vapor pressure of water in air at air temperature. The relative humidity percentage is the ratio of the actual vapor pressure to the saturation vapor pressure. So if the air and water temperature were the same, then at 100% relative humidity there will be no evaporation. This should make perfect sense logically since 100% relative humidity means that the air cannot hold any more water, so how could any water evaporate into it?

The rate of water evaporation is also dependent on temperature with higher temperatures resulting in faster water evaporation. This is further accelerated when the water temperature is higher than the air temperature -- basically, the water temperature is related to the saturation vapor pressure (for the formula) while the air temperature is related to the actual vapor pressure.

Finally, the rate of water evaporation is GREATLY accelerated by wind, especially near the surface of the water. A 5 MPH wind at the surface of the water (so higher wind further above the water) results in roughly 3 times the rate of evaporation.

There is a myth that the evaporation is very different between the day and the night. In fact, the vapor pressure of water in the air doesn't change a lot between day and night because as air temperature cools (so the saturation vapor pressure is lower since the air can hold less water), the relative humidity rises. The net result is roughly the same absolute amount of water in the air (there is a little less at night after dew is deposited). The saturation vapor pressure at water temperature doesn't change so the amount of water evaporation day and night is roughly the same. Of course, you may see more of the evaporation at night because the evaporated water cools as it mixes with the air and then becomes visible (i.e. small water droplets are formed as the cooled gaseous water liquifies when the air is saturated with water locally), but evaporation is happening during the day as well except that you don't see it because the water remains in a gaseous state.

So there are really only two questions you need to answer to know if there is a lot of water evaporation over the winter. 1) Does the air have more absolute amount of water in it and 2) Is the water temperature still warm. If you stop heating the pool and let the water cool during the winter, then the rate of water evaporation will drop considerably. As for the first question, let me take my own city of San Rafael, CA as an example where I will compare the air temperature and humidity in July with that of January.

Jul Day -- Temp 74.5F; Rel. Humidity 49%; 10.72 mm Hg vapor pressure
Jul Night -- Temp 51.5F; Rel. Humidity 85%; 8.28 mm Hg vapor pressure
Jul Avg. -- Temp 61.9F; Rel. Humidity 68%; 9.64 mm Hg vapor pressure

Jan Day -- Temp. 58.4F; Rel. Humidity 43%; 5.38 mm Hg vapor pressure
Jan Night -- Temp. 37.2F; Rel. Humidity 77%; 4.34 mm Hg vapor pressure
Jan Avg. -- Temp. 47.1F; Rel. Humidity 60%; 4.96 mm Hg vapor pressure

Let's say that a pool is heated in July to 88F. With no wind, the rate of evaporation during a July day would be 0.28" per day and during a July night it would be 0.31" per day. If the pool were still heated in January (unlikely), then the evaporation average for day/night would be 0.35" per day (24 hours). However, if the pool were at the average temp in January of 47.1F, then evaporation would only be 0.04" per day.

So, bottom line, is that if the pool water is unheated and gets cool, then evaporation over the winter becomes very low -- assuming no wind.

As for a pool cover, if it is solid (not mesh), then evaporation is pretty much stopped since a small amount of water evaporates to saturate the air under the cover and that's all. So if you lost any water, then you've got a leak.

As for central Massachusetts, this map (http://www.grow.arizona.edu/images/water/panevap.gif) show an annual evaporation of unheated water in Massachusetts of around 35" per year (about 0.1" per day). If I look at average temperatures in Worcester here (http://countrystudies.us/united-states/weather/massachusetts/worcester.htm), it gets cool when you have the pool closed so evaporation would be lower. Also note that there is rain so I would think that would make up for a lot of the evaporation during the winter if there was no cover.


05-30-2009, 09:00 AM
Thanks Richard that was very good and very detailed info.
I filled the pool and am trying to get the iron out now. Once, it is clear. I will look again for a leak and if I cannot find it, I will call pool place.

05-30-2009, 12:06 PM
Now to put it in super-simple terms. I live in North Central New Jersey and our weather isn't much different than Mass. A little hotter in the summer and maybe not so cold in the winter. I use a mesh safety cover.

When I close, I drop the water a full inch below the bottom of the bushing around the returns--about 15-18" below the edge of the pool. When I opened this spring, my pool was over-full. I consider the two screws in the exact middle of the skimmer to be the "full" mark and it was at least 2" higher.

Now that's a bit much--I usually have to add water in the spring but not nearly as much as I let out in the fall.

My point being, in the North East, if you lose water over the winter you most definitely have a leak--the chances of it being evaporation are pretty much nil. Snow melt and rain are going to far out-weigh any evaporation loss.

That's my humble opinion.

05-17-2010, 02:25 AM
I concur. I am also in North Central New Jersey (actually I am in Central North New Jersey) but I use a solid cover. I have to thread a needle to get below returns and stay above the light while avoiding the tile trim on swim-out and steps. There is only one level that works. I close at that level and it is just the same when I open within an inch or so.

Edit: Wow, I didn't realize this thread was this old. Forgive the noob--I was just looking around and wanted to get my first post up.

05-17-2010, 09:36 AM
That's OK, Durk. Welcome to the forum.

05-27-2010, 01:26 PM
I took my solid cover off of my 18x36 inground pool this past Saturday. 3 of the straps had broken because of the snow in February so there was about a 3 inch by 8 foot gap on the one side which was a way for water to evaporate for the last 3 - 4 months that had never been there in the past 7 years. I was shocked at the amount of water that had evaporated. I had to add 23 inches of water & I only drained about 9 inches out in the fall. I actually thought I may have had a leak. It is holding so I am pretty sure there is no leak.