Rainfall dilutes pool chemistry levels and lowers the readings for pH, alkalinity, hardness, stabilizer, and chlorine. Rainfall does not contain chlorine.
Rain almost immediately causes the pH (Potential Hydrogen) in the pool water to rise while also reducing the TA (Total Alkalinity) slightly via dilution. Higher pH will cause more of the chlorine in a pool to become inactive or “fall asleep” reducing it's effectiveness.
Check pH and Alkalinity Levels
Acid rain can cause your pH to drop. However, this is what alkalinity's job is. When rain tries to lower the pH, the alkalinity will take the big hit. That means, your alkalinity levels might see a more drastic change than your pH levels, which is a good thing — thank you, alkalinity.
The most common cause of a consistently low pH level in pools is using chlorine tablets or stabilized forms of chlorine. These have a pH level of around 3. Acidic rainfall, heavy leaf debris, and dirt/mulch in the pool can also lower the pH level.
High chlorine levels decrease the pH of your pool's water, making it more acidic. The more acidic the water, the higher the likelihood of corrosion. This corrosion can affect metal piping, equipment, and the surface of your pool (tiles, liners, concrete, etc.).
You can lower the pH in your pool naturally by directing the downspouts from your house into the pool. If a pool becomes too full due to backwash it dumps water. Since rain is about 5.6 pH it is going to bring down the pH of the water naturally. The problem that you will have with rainwater is its low alkalinity.
Normal, clean rain has a pH value of between 5.0 and 5.5, which is slightly acidic. However, when rain combines with sulfur dioxide or nitrogen oxides—produced from power plants and automobiles—the rain becomes much more acidic. Typical acid rain has a pH value of 4.0.
Most rainwater has a pH of 5.6 to 5.8, simply due to the pressence of carbonic acid (H2CO3). The latter is formed from dissolved CO2 gas and H2O. The source of the CO2 is the atmosphere, which presently contains about 380 ppm CO2. Values of pH below 5.6 or so are considered "acid rain", values above are "basic rain".
When rainwater and 'run off' enter the pool, they can change your water's pH, calcium hardness, total dissolved solids (TDS), alkalinity, and other chemical levels as well as bring contaminants such as dirt and debris into the pool.
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.
Answer: It should not matter much. It could help to filter debris and contaminants the rain washes into the pool. However, lightning can be a concern with the filter running.
"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.
One common method that is used to increase the pH is to send the water through a calcium carbonate filter, which neutralizes the acid and increases the pH of the water. Another common method is to inject a sodium carbonate solution into the water.
Rain water is naturally slightly acidic, witha pH of about 5.0.
Not all rain can be considered pure water. Pure water is neither alkaline nor acidic. As rain falls from the atmosphere the impurities it collects changes the pH of the rain water, making it slightly acidic. The pH of water determines if it is acidic or alkaline.
Treat the Pool to Lower pH Level
When you've determined that your pool pH is too high, there are two ways you can balance it: dry acid or muriatic acid.
A high pH level can be caused by several factors, the main culprits being additional chlorine stabilizers and sudden increases in temperature. In addition, high pH runs a risk with your chlorine, as your chlorine will no longer disinfect fully. There are also physical consequences of high pH for swimmers.
Non-stabilized chlorines like calcium hypochlorite (cal hypo), sodium hypochlorite (liquid chlorine), and salt chlorine generators all tend to raise the pH of the pool. One common habit in the pool business is to add some acid to "offset" the pH rise that liquid chlorine causes.
Give your pool a good shock treatment 1 to 2 days before the storm hits. You can bring the chlorine level up pretty high to prolong the pool being depleted of chlorine.
It is designed to be filled with water at all times. If the pump is operated dry, it will burn out the motor and cause you to have to replace an expensive piece of pool equipment. If a pump is allowed to operate dry, it will build up heat that will melt the pump and possibly surrounding plumbing fixtures.
Should the green be due to pollen, there may be little to do in the way of minimizing the discoloration short of erecting a building around the pool. Fortunately, assuming there are no allergies to the pollen, it is safe to swim in a pool with that as the cause for green water.
Rain delivers algae spores and debris to pool water and it is these things that cause problems. Algae spores are usually present in the air. Riding the air currents hither and yon in order to sometimes land in hospitable locations is part of algae's reproductive strategy.