Rainfall dilutes pool chemistry levels and lowers the readings for pH, alkalinity, hardness, stabilizer, and chlorine. Rainfall does not contain chlorine.
An inch of rainwater added to a typical 50,000 litre pool increases the amount of water by only about 1.5%, meaning dilution is not usually an issue. It's more about what comes into the pool with the rain.
If your pools calcium hardness level drops below the recommended 150 ppm the easiest and most common way to raise swimming pool calcium hardness is by adding a calcium hardness increaser such as calcium chloride to your swimming pool.
In addition to debris being blown into the water by the wind, the rainstorm can dilute the previously in-balance chemicals, impacting your pool water's pH and alkalinity.
After a hard rain or stormy weather, it's important to shock your pool with a chlorinated shock, such as Leslie's Power Powder Plus or Leslie's Chlor Brite.
Calcium Increaser (Generally after a major storm such as a Hurricane) Cyanuric Acid (Generally after a major storm such as a Hurricane) Phosphate Remover (Only necessary if the phosphate level has increased as a result of the storm) Algaecide (If your pool has started showing signs of Yellow, Green, or Black Algae)
Always test the pool after rain
Before you add shock, though, you should test the water. This will let you know if lighter rainfall has had an effect and, whether the rain's been light or heavy, it will allow you to adjust levels before adding the shock.
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.
Rain will decrease your Total Alkalinity level in the pool over time, requiring an addition of Alkalinity Increaser to correct. Pure rain water has a Total Alkalinity measurement near zero, and heavy rains will dilute your pool alkalinity level, falling by 5 or 10 ppm in a day.
If you know the water from your tap in your area isn't hard then you can drain part of the water in your pool and refill with fresh water. If you don't have the option or option 1 then your next option is to use a flocculant to bind the excess calcium and filter it with your sand filter.
If the calcium hardness level in your pool gets too high, the water will be hard. The first indication will likely be a cloudy pool. Normally, you can clear cloudy pool water with pool shock.
However, you can quickly and easily lower calcium hardness by removing water from the pool and replacing it with freshwater. Once you have your pool chemicals in balance, maintaining the pool is as simple as testing the chemicals in the water, adjusting them as necessary, and keeping the pool clean!
Rainwater is known to have acidic properties that can negatively affect your pool water chemistry balance. This can affect anything from the pH balance to alkalinity levels and more. The pH of rain across the US, which is all acid rain, is due to industrial gasses (i.e. NOx, SOx) which make nitric and sulfuric acid.
It's also a good idea to stay away from your pool when it's raining because your deck and other surface areas can be more slippery than usual. If you slide and fall around the pool, you can seriously injure yourself on a hard concrete or wooden deck, but you can also fall into the water unconscious.
Baking soda can: Help to clear cloudy water and restore the sparkle. Spot-treat algae. Make the pool water softer on your skin.
After Rainfall
In almost every case, you will have to add chemicals to a pool after it rains. Check the water's pH level first. However, if the pH level is too low (below 7.0), hold off on increasing the pH until after adding chlorine.
As a comparison, rainwater has a hardness of less than 5 mg/L.
You'll know your pH is too high when your pool water is cloudy, there is scale build up on your pool walls and your chlorine is no longer successfully sanitizing your pool. The process of proper re-balancing your pool water alkalinity can take more than one week, so be patient with this one.
The simple answer is yes, you can over shock a pool. This is when the chlorine level in the water becomes too high and can be harmful to swimmers. When you shock your pool, you are raising the chlorine level to 10 times its normal level. This is done to kill off any bacteria or algae that may be present in your pool.
If your pool is green and cloudy, it's likely an algae problem. Algae can persist in a pool even after shocking. A green pool – especially one that turned green overnight or after rain, can also be from a pool pump that isn't properly circulating water or an issue with your filtering system.
You can expect a heavy rain to introduce lots of organic matter and algae spores to your pool. The main time to deal with the mess is after the storm has passed. However, it's not a bad idea to pretreat your pool water. Adding some algicide to your pool water will help you get out ahead of the algae.
In times of very heavy rains, however, the pool's chemistry is overrun by the amount of rain and the black algae has a chance to grow in the pool. Black Algae is tough to remove from pools and it usually takes several treatments to remove it.
Rainwater can also help dilute your water and CYA levels, but the acidic nature of rain can upset your pool water's balance. Before diluting your water, test your water with test strips or a liquid test kit to determine your current CYA level.