The Short answer? Yes! For the very simple fact that rain adds water to your pool, this means that your chemical levels are being diluted. More water = lower concentrations of chemicals. But that's not what all rain brings to your pool. Aside from bringing the water volume up, there are some things that rain brings to the table that we should be aware of.
Acid Rain and Swimming Pools
This term has been floating around for some time now and it's actually quite true. Of course, this isn't the comic book style melt your face type of acid rain, but it's more of the rain water being slightly acidic and in turn, will cause your pool's pH levels to drop.
The best way to prevent drops in your pool's pH levels due to acid rain is to ensure that your total alkalinity levels are where they should be. If this is the case then all acid rain can do is reduce your total alkalinity levels but will leave your pool pH levels where they are.
Leaves and other Debris
Rain brings wind with it and wind, for some reason unknown to everyone, somehow brings leaves directly to our pool. I mean, really, I've need tiny pools with more leaves than a giant yard after a brief shower. If that's not a mystery of the universe then I don't know what is.
Leaves and other organic debris bring with them phosphates and nitrogen, which is coincidentally algae food. Many pool owners will usually encounter algae blooms after heavy rain and when leaves and other debris are deposited to the pool. So to combat this, once the sun is out, fish out all of the debris with your trusty leaf rake or leaf skimmer then allow your pool water to circulate. It's also a good idea to give your pool a good dose of phosphate remover after clearing your pool of leaves.
Runoff from the deck or lawn
While not apparent, rain also brings with it the possibility of runoff from your deck or lawn. Unless you have an elevated pool or a really effective drainage system around your pool, runoff can drastically change your pool water's chemistry. The reason for this is simple, the water that hits your deck or your lawn will bring with it all of the dirt, phosphates, chemicals, biological matter, and other contaminants to your pool water.
What to do after rain?
The first thing you should do after a rain shower or a storm is to clean your pool. Scoop out all of the visible debris and allow your pump to run so that the tiny ones that you've missed will be filtered out.
Next, check the water levels. This is an easy fix, simply turn your filter (or 3-way valve) to waste and drain out the excess water. Or even better, you can take this chance to vacuum to waste, throwing out the excess water along with the debris that has settled to the bottom of your pool.
Once the water levels are back to where there should be and there is no visible debris left, time to check and balance your water to ensure that your pool is sanitised. Important levels to check are pH, Alkalinity, and chlorine levels, but just to be safe, check them all, you're already there anyway right?
Depending on the amount of rainfall, it may be a good idea to shock your pool after the rain has cleared. If your chlorine levels held steady during the rain then there wouldn't be a need to shock the pool. But if your free pool chlorine levels have dropped to zero after a rainstorm, it might be a good idea to shock the pool.
Need help with shocking your pool? Check out our guide to shocking your pool here.
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Happy swimming :)