How Does Rain Affect Chemicals and pH in a Swimming Pool's Water?

Updated on May 21, 2017
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Rob is a licensed pool service industry professional with over 20 years of experience.

Does rainfall change the water chemistry?

Yes, it does but keep in mind that minor amounts of rain aren't going to change the chemistry much. Rain is acidic. It has a lower pH level than what we keep our pool level at. We want to keep the pH in our pool between 7.4 and 7.6. The reason for this number is that the human eye has a pH of 7.5. Major fluctuations from this can cause serious discomfort. I have heard too many times of people complaining that "My eyes are burning, the chlorine is too high" Very rarely is chlorine an issue if your eyes are in discomfort while swimming. It is most likely that the pH is low, or acidic. Normal rainwater has a pH of around 5.0. So depending on how much rainfall, it can lower the pH of the pool.

Chlorine molecule
Chlorine molecule

Do I need to raise the pH every time it rains?

Not necessarily. It depends on the amount of rain and also your current water chemistry. If you have a light rainfall (.25 to .50 inches) of rain, it may temporarily lower the pH in the pool, but not by much and will come back up on it's own. If you always maintain proper water chemistry, especially pH, Alkalinity and Stabilizer, A bit of rain in the pool is nothing to worry about.

A large amount of rain however, such as a strong thunderstorm or a rain that drop between 1 and 3 inches in a short amount of time will require of course, draining the pool a few inches and re-testing the chemicals. The pH will be lower and the Chlorine may be slightly diluted.

Are there benefits to rain in the pool?

YES, rain can be beneficial to pool water in certain instances. There are certain chemicals in pool water that the levels can not be lowered without diluting the water. These are stabilizer and calcium. You may want to look at my article HERE on lowering stabilizer. Basically if you use chlorine tablets, they have stabilizer in them and over time, can over stabilize the pool water and will need to be diluted. Also calcium levels that are high, rainfall can be a benefit to diluting the water as well.

Conclusion

There is much more to pool water chemistry than most people know. It is much more than just Chlorine and pH. If you keep your water balanced on a professional level, your pool will not turn green after a rainfall. I suggest a pro test kit, because the test strips simply are not reliable. Accuracy is important in maintaining a good chemical balance. Please visit the suggested links on this page for more information on water chemistry.

Questions & Answers

  • If my phosphate level is too high, will my pool turn green after a heavy rain?

    If the phosphate level is high, the pool will most likely develop algae, even if it doesn't rain. If it rains, this will dilute the chlorine, and there is a better chance that it could turn green. Add phosphate remover anytime the phosphate level is over 100ppb.

  • I am floccing my in-ground pool, and it has started to rain. How will rain affect the process?

    Once the floccing agent has been added with the water circulating for a few hours, turn the pump off to allow everything to settle to the bottom (usually 24 hours). Rain will not affect this process at all. Everything should still sink to the bottom.

© 2013 Rob Hampton

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