What Does Rain do to Swimming Pools?

We all know that modern rain is filled with pollutants from the industrial world we live in.  In countries such as the United States, acid rain is common all over the country.

As a pool owner this acid rain can be problematic for your pH, alkalinity and other chemical levels.  It’s important to know exactly how rain affects your pool and what action you need to take when it does rain.



Does rain affect pool pH?

The ideal pH for swimming pools is between 7.4-7.6.

Most rainwater in the USA has a pH of around 5 (acidic).  The North East has among the most acidic rain in the country, in many areas just below 5.

If you have ever noticed your eyes stinging when swimming in an outdoor pool, it could be because the water is too acidic. The more rain there is, the more it will affect the balance of your pH levels.

A light rain under 1/2 inch might lower your pool’s pH temporarily but it should bounce back again on it’s own.  It is always a good habit to check your pool chemistry on a regular basis regardless.

A heavy rain between 1-3 inches in a short time period requires attention to maintain the proper pH balance, alkalinity and chemical levels.  The best cause of action is to drain the pool a few inches, adjust the chemical levels and re-test.


Does rain affect pool alkalinity?

Rainwater has an alkalinity close to 0.  Heavy rain can lower the alkalinity level in your pool by 5-10ppm per day.

A swimming pool with low alkalinity and low pH can become corrosive.  This can be damaging to metal surfaces such as ladders, rails and lights.  It can affect equipment such as the internal components of your pump and filters.  It can deteriorate your masonry surfaces and liner.


Does rain cause algae to grow?

Rain does not directly cause algae but it can promote the environment for it to grow.  Rain water can introduce a range of organic contaminants such as high levels of phosphates and nitrates that algae thrives on.  If the chlorine level also lowers at the same time then this can give algae good grounds to establish.


When rain raises the pools water level

If there is heavy rainfall, including runoff from your deck etc. then your water level and cause problems for the skimmer.  If the water gets high enough then the skimmer can stop working completely; leaves, bugs and debris can settle, dropping to the bottom of your pool and making for a tough cleaning job.

If the water level gets too high consider back-washing/pumping out the water.  Be careful not to drain below the skimmer as this can lead to the pump running dry.


What to do when it rains

The general advice is to monitor your swimming pool, it’s pH, alkalinity and chemical levels.  Most light rainfall isn’t going to significantly affect your swimming pool, particularly if you are proactive about maintaining your pool’s levels anyway.  If there is heavy rain then you will want to carefully check the affect on your chemicals and take steps to keep things topped up.  You may want to consider contacting a local pool service company to clean and balance your pool for you.