Step 1: Lower Water Level to Mid-Skimmer
When rainwater fills your pool above the top edge of the skimmer faceplate, the skimmer weir door can no longer flap, completely stopping surface skimming action. Floating leaves and insects will waterlog and sink to the pool floor instead of being collected:
- Sand or DE Filters: Turn off the pump, set the multiport valve to Waste (not Backwash), and run the pump until water drops to the middle of the skimmer opening.
- Cartridge Filters: Open the manual spigot drain valve on the return plumbing line, or hook up a submersible utility pump to discharge excess water to a street curb or storm drain.
- Verify Water Loss: Use our pool water loss calculator to calculate how many gallons are removed.
Step 2: Clean Out Physical Organic Debris
Before adding chemicals, mechanically remove all debris washed into the pool. Empty saturated leaf skimmer baskets and pump strainer pots. Use a deep-pocket leaf rake to scoop mulch, leaves, and worms off the pool floor. Debris left on the bottom consumes free chlorine at an astonishing rate, leaving none behind to kill suspended bacteria.
Step 3: Test and Rebalance Water Parameters
Once water circulation is restored, test your chemistry with a reliable test kit. Heavy storms typically cause four predictable imbalances:
- Drop in pH and Alkalinity: Acidic rain depletes carbonate alkalinity. Add sodium bicarbonate to raise total alkalinity to 90 to 110 ppm using our pool alkalinity calculator, which naturally stabilizes pH.
- Stabilizer Dilution: Adding 4 inches of rain to a 4-foot average depth pool dilutes cyanuric acid by roughly 8 percent. Retest CYA and top off if below 30 ppm using our pool stabilizer calculator.
- Salt Dilution (Salt Pools): Rainwater adds zero salt. Check your digital salt reading and add evaporated pool salt bags if salinity falls below 3,000 ppm using our pool salt calculator.
- Elevated Combined Chlorine: Organic wash-in creates chloramines. Shock immediately with liquid chlorine to achieve breakpoint chlorination.
Storm Rain Impacts on Pool Chemistry Parameters
| Parameter | Direct Impact from Storm | Corrective Action Needed |
|---|---|---|
| Pool Water Level | Rises 2 to 6 inches above skimmer | Drain excess water to waste line |
| Water pH | Drops significantly (Acidic rain) | Add sodium carbonate (soda ash) |
| Total Alkalinity | Depleted by acidic rain mixing | Add sodium bicarbonate (baking soda) |
| Free Chlorine | Plummets to 0 ppm from debris load | Dose liquid chlorine to shock level |
| Cyanuric Acid (CYA) | Diluted by volume increase | Retest and add stabilizer if < 30 ppm |
| Salinity (Salt Pools) | Diluted by fresh rainwater | Add pool salt to restore 3,200 ppm |
pool after heavy rain FAQs
Does rain really ruin pool water chemistry?
Yes. Rain droplets absorb atmospheric pollutants, sulfur dioxide, and nitric oxide, falling with an acidic pH between 5.0 and 5.6. In addition, windblown roof and deck runoff carries dirt, fertilizer nitrates, and organic phosphates directly into the water.
Can I leave excess rain water in the pool?
No. When water is above the top of the skimmer mouth, skimming stops entirely. Floating dirt and oils cannot enter the filter and sink to the pool floor, accelerating algae growth and staining plaster.
Should I run my pool pump during a heavy thunderstorm?
Yes, unless lightning threatens electrical equipment or flash flooding submerges the pump motor. Running the pump circulates chlorine and prevents organic debris from settling into plaster pores.
Why is my pool water brown or green after a rainstorm?
Brown water indicates garden soil or mulch washed into the pool, introducing iron tannins and silt. Green water indicates dormant algae spores multiplied as soon as rainwater stripped chlorine down to zero. Shock the pool immediately.
More pool guides
- How to Clear a Green Pool Fast: The Complete SLAM Shock Method
Eradicate green algae permanently with the proven SLAM method: Shock, Level, And Maintain.
- Pool Freeze Protection: Emergency Procedures and Equipment Care
Emergency freeze protection procedures for pumps, heaters, and pipes during arctic blasts.
- Cloudy Pool Water: Causes, Diagnosis Flowchart, and Quick Fixes
Diagnose the root cause of cloudy pool water and restore crystal-clear clarity fast.