Rainfall dilutes pool chemistry levels and lowers the readings for pH, alkalinity, hardness, stabilizer, and chlorine. Rainfall does not contain chlorine. As well, rainfall brings with it small amounts of contaminants that are washed into the pool.
Because rainwater is generally acidic (see map showing pH of rainwater across U.S), then your pool water can become corrosive and cause damage to the plaster and metal parts of your pool. The extent of the chemistry issues you'll experience will depend on how much rain you get.
When preparing your pool for a storm, leave it uncovered. Installing any kind of cover across the pool will not do much to protect against dust and contaminants because storms often bring strong winds and heavy rain that can cause the cover to lift off your pool.
Remember, rain is acidic. Hence, pool overflowing from rain causes the Alkalinity and pH levels to lower. Very low pH causes pool water to turn very clear and acidic, which destroys your pool surface and equipment.
Effect on Pool Water pH
Since rain is diluting your pool, you may expect that it will reduce the acidity of your pool water. However, all rain in the US is acidic due to pollution, so rain actually decreases your pool's pH – in other words, the pool water becomes more acidic.
The extra filtering will help clean out the impurities rain has introduced into your pool's water. Pool pumps are made to withstand rain and it is beneficial to run your pump during or after rain.
"Heavy rain dilutes pool chemicals, especially salt and chlorine, which causes the pool to turn green. This means the water is not sanitised or healthy, so it's vital to address this.
With a rain storm, any number of contaminants can be washing into your pool – acid rain, pollen, insects, tree droppings, dust, sand and even phosphates. Any one or combination of these things in rain can make your pool cloudy.
One heavy rain is all it takes to throw off your pool's chemistry by unbalancing the pool water's pH and making it slightly more acidic. This additional acidity and nitrogen provide a breeding ground for algae that will turn your pool green.