By June 24, 2021Pool Safety

Written by Rob Hampton and published on

Have you ever wondered how rain affects your swimming pool water or if there is anything specific that you should do after a rain? As a pool owner, it’s a question worth asking because, after a rain, particularly heavy rain, the water of your pool changes both on a chemical and physical level. The amount of rain is important to consider. Light rainfall and showers will not impact your pool nearly as much as a rainstorm with heavy wind.

Have you ever wondered if there’s something you should do to your pool after it rains?  As a pool owner, it’s a definitely a question worth asking. Because after heavy rain, pools undergo a few changes on both a chemical and physical level.

Rainwater affects pH and Alkalinity levels, while excess rainfall adds extra water in the pool that you don’t need. Not to mention all the contaminants that rainstorms sweep into your water.

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

Rob is a pool-service industry professional with over 20 years of experience.

Does Rain Raise or Lower pH?

Rain does change the chemistry of the pool water but, keep in mind that minor amounts of rain aren’t going to change the pH that drastically. 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.

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 its 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.

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

Is Rainwater Good For Swimming Pools?

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 stabilizers. 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, when calcium levels are high, rainfall can be a benefit to diluting the water as well.

Rain Can Lower Pool PH

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.

Relevant Links

  • How Much Muriatic Acid Should I Add to a Swimming Pool?
    A guide to understanding pH and the effects of unbalanced pH levels in swimming pool water. What is pH? . Why does pH need to be balanced? Controlling pH
  • How to lower stabilizer in swimming pool
    What is stabilizer? How to adjust stabilizer level and diluting water
  • Advanced pool water chemistry.. A look at phosphates and stabilizer
    Resolving pool water chemistry issues.

This article is accurate and true to the best of the author’s knowledge. Content is for informational or entertainment purposes only and does not substitute for personal counsel or professional advice in business, financial, legal, or technical matters.

Questions & Answers

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

Answer: 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.

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

Answer: 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.

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