Most pool owners don’t know how rain affects swimming pool water. The pool water may look clean and clear but the rain can dilute the chemistry and leave the pool unsanitary.
Light rain has little effect on pool water as the volume of rainfall is very small compared to the total volume of water in the pool.
Heavy rain can affect swimming pool water by introducing contaminants, altering the pool chemistry, diluting the salinity and increasing the water level. It’s important to test your pool water after heavy rain and adjust the water chemistry as required. Don’t swim in the pool until the water chemistry is right.
Table of Contents
How Rain Affects Swimming Pool Water
Rain Creates a Layer of Unsanitized Water
Rain affects pool water by floating on top of the pool and doesn’t mix in with the pool water. The layer of freshwater does not contain any chlorine until the pump circulates the pool water and mixes it up.
Saltwater pools have a similar sanitation issue. Saltwater is heavier than freshwater so when it rains on a saltwater pool there will be a layer of fresh water on the surface. The freshwater is sucked into the skimmer box and through the filter and chlorinator. When fresh water passes through the chlorinator there is no chlorine production. This is only a minor issue until the rainwater is mixed into the pool water and the chlorinator can generate chlorine again.
Rain Affects Pool Water Salinity
Rainfall is fresh water and will dilute the salinity level of a saltwater pool. This is a common issue after the water level is adjusted down by pumping water into the drain. After a period of rain test the water salinity and add extra pool salt if required. When adding pool salt try to dissolve it by brushing it around the bottom before activating the pump. Undissolved salt can clog the filter and plumbing increasing pressure and lowering the water flow.
Rain Dilutes pH and Stabilizer
pH is a scale in chemistry to indicate the level of acidity or alkalinity of an aqueous (water-based) solution.
Swimming pool water must be between pH 7.2 and 7.6 which is slightly alkaline and never acidic.
Always test the pool water after heavy rain. Rain is acidic and causes the pH to drop. It’s important to re-balance your pool chemistry as soon as possible to prevent damage to your swimming pool surfaces and equipment. Unbalanced pool chemistry causes calcium and metal stains, cloudy or green pool water, and many other problems. It’s always better to treat the water quickly rather than leave it to turn in to a bigger problem.
Don’t swim in water with a low pH. The mildly acidic water may sting your eyes, irritate your skin and annoy you nasal cavity. Your skin could be left feeling dried out and itchy the next day. Children’s skin is more sensitive than an adult and will be more severely affected by low pH water.
Sodium carbonate (which is commonly called soda ash) is the most widely used substance to raise pH. There are many branded products with names such as “pH Increaser”, “pH Raiser”, etc. which all contain basically the same ingredients (sodium carbonate or sodium bicarbonate) in varying concentrations. Any of these will get the job done. Just make sure you follow the instructions on the container and don’t add too much on the first application. If you add too much chemical you will increase the pH too high and need to purchase another chemical to reduce the pH. Add the chemical to the water above the outlet jets to ensure it’s well mixed into the water and leave the pump running for at least an hour.
Rain Dilutes Chlorine in the Pool
Light rain will not have a noticeable effect on the chlorine level in most pools. However, heavy rainfall will dilute the chlorine level which allows algae and bacteria to grow and take over the pool.
In a saltwater pool the chlorinator will produce more chlorine when the pump operates. This will replace the chlorine in the diluted water.
In a freshwater pool add extra chlorine after heavy rain to boost the level.
Many pool owners choose to shock their pool after heavy rain to kill all bacteria and contaminants.
You must always circulate the water in the pool after adding shock so the best time to add is just before the pump comes on. Make sure the pump is going to run for at least 4 hours to thoroughly mix in the shock chemical. I do not recommend to swim in a pool straight after adding shock chemicals because the high free chlorine will sting your eyes and itch your skin.
Adding shock in the evening will guarantee your pool is sanitized and fresh ready for swimming the next day.
Rain Affects Pool Water Alkalinity
Rain affects pool water by lowering alkalinity.
A low total alkalinity in pool water may lead to the following issues:
- Rapid changes in pH
- Green water
- Irritated eyes and itchy skin for swimmers
- Stains forming on the pool surfaces
- Pitting, etching or erosion of the pool surfaces
- Corrosion of metal equipment parts (ladder, vacuum pole etc.)
Sodium bicarbonate is used to raise total alkalinity in pool water. This is simple process of adding the required quantity to the water. Simply follow the instructions on the container but be careful not to add too much chemical as you may raise the alkalinity too high. It is recommended to raise the level in small increments and re-test over several days to prevent raising the level too high.
If you need to raise both Total Alkalinity and pH together use sodium carbonate (soda ash). However, sodium carbonate can cause cloudy pool water so only add small amounts at a time.
Rain Affects Pool Water Hardness
Calcium Hardness is the measure of all calcium compounds dissolved in the pool water. The recommended level of calcium is as low as possible to minimize any problems with calcium type deposits forming on the pool surfaces or in the salt water chlorinator cell.
The normal range for Calcium Hardness is 200 – 400 ppm
Water Hardness causes calcium carbonate scale to accumulate on pool surfaces and inside the filtration system.
Calcium carbonate scale is a hard crystalline substance that bonds to your pool surfaces and equipment like concrete. Many people don’t notice the scale in their pool as the deposits are off-white or clear and not easily seen on plaster and aggregate surfaces. Brush or sand the pool surface to clean away small deposits of scale. Larger deposits require emptying the pool and acid washing the surface.
The accumulation of scale is more serious in your pool filtration system, heater and chlorinator cell as it will damage the equipment. The equipment requires regular inspection and periodic acid cleaning to remove these crystalline calcium deposits.
Please read our article on How to Clean Scale from a Pool
Rain Increases the Water Level
Rainfall increases the water level in the pool. When the water level reaches the top of the skimmer box it no longer works effectively.
When the pump operates the weir flap is drawn open and water flows into the skimmer box. However, the skimmer box will not skim the surface of the pool as the water is drawn from below the surface of the pool. As a result, any debris floating on the surface will float past the skimmer box and remain on the surface. After the pump has switched off the floating debris will sink to the bottom. The debris will decay causing organic stains on the bottom of you pool.
To lower the water level in your pool turn the pump valve to waste (drain) and run the pump for a minute or two to remove water from the pool.
Please read our article on How to Drain Water from a Pool
Rain Washes Contaminants into the Pool
Algae spores, bacteria, dirt and many other contaminants are floating in the air. Rain affects pool water by washing contaminants from the atmosphere into your pool. These contaminants float in the surface layer of rain water until circulated around the pool.
Running the filter during rain removes the surface layer of water which contains the contaminants. This is one reason why it’s better to run the pump several times a day to skim the pool surface before contaminants sink deeper into the water.
Read our post on How to Clean a Pool after a Storm
Protecting Your Pool from Lightning
Many pool owners are totally unaware that their new swimming pool fence may need to be earthed to comply with electrical safety standards. There is the possibility that a person can suffer an electric shock from a pool fence if there is an electrical current in the pool water caused by lightning or an electrical fault.
An equipotential bond (earthing) may be required to all metal structures around the pool including steel reinforcing in the pool shell, pool light fittings, support structures, handrails and fences.
As aluminum pool fences are one continuous structure the fence only needs to be earthed in one location. However, glass pool fences may need to be earthed at every metal panel fitting as they are all individually attached to the ground.
Pool owners should make their own enquiries at their local council for pool fencing requirements and the installation of an equipotential bond. Only use a licensed electrical contractor to install an equipotential bond.
Frequently Asked Questions
Should I run my pool pump when it rains?
Yes, it’s perfectly acceptable to run the pump when it rains.
There is no need to switch off a pool pump when it’s raining. In fact, filtering the pool during rain removes the contaminants from the water introduced by the rainfall. I recommend setting your pump to run for two periods of 4 hours per day (8 hours total) whether it’s fine or raining.
Why does the pool turn green after it rains?
The rain has diluted the pool chemicals particularly the chlorine level. Low chlorine allows algae to grow in the pool. During warm weather, algae can grow very fast and make a pool turn green overnight. It can completely take over a pool within a week.
Is it safe to swim in pool after heavy rain?
It’s perfectly safe to swim in a pool after light rain as the rainfall will have little effect on the pool chemistry.
Always check the pool chemistry after heavy rain. Sometimes the water is fine and you can swim. If the water chemistry has changed you need to treat it before swimming in the pool. Normally a dose of shock fixes the problem. However, don’t swim after shocking the pool until the chlorine level reduces to the normal level.
