Why pool water turns green after rain

Does rain itself turn a pool green?

No. Rain is close to distilled water with some dissolved carbon dioxide in it, and nothing in it grows algae. What rain does is take away your defenses and hand the pool a meal on the way past.

Three things happen at once. Volume goes up, so cyanuric acid and free chlorine both get diluted by whatever fraction of the pool arrived as rain. Runoff carries lawn fertilizer, soil, leaf litter and roof grit into the water. And in a real North Texas storm the power blinks, the pump schedule resets or never restarts, and the water stops moving.

Any one of those alone is survivable. All three landing on the same afternoon in July is how a clear pool becomes a green pool by Wednesday.

Why does the green show up two days after the storm, not during it?

Because algae is a population, and populations take time to become visible. A single spore in the water is invisible and always has been. What you see on Wednesday is the result of repeated doubling that started Monday afternoon when the free chlorine hit zero.

That delay is the reason people blame the wrong thing. The storm gets forgiven because the water looked fine at sunset, and the green gets blamed on the chlorine brand, the salt cell, the heat or the neighbors kids. The actual failure was a chlorine reading nobody took on Monday night.

It is also why testing after rain matters more than testing on a schedule. The schedule test tells you what your pool does normally. The post storm test tells you whether the pool is about to become a $450 to $1,250+ recovery.

What does a storm actually add to the water?

Phosphate, first. Lawn fertilizer is largely phosphate by design, and a heavy rain over a fertilized yard sheets straight into the pool. Phosphate does not cause algae on its own, but it removes one of the limits on how fast a bloom can grow once chlorine is gone.

Organic load, second. Leaves, catkins, mulch, insects and grass clippings all arrive together, and every one of them is something chlorine has to oxidize before it can defend the water. A pool that swallowed a yard worth of debris is spending its sanitizer on cleanup rather than on protection.

Tannins, third, and this is a local specialty. Post oak and live oak leaf litter steeps in standing water and stains it the color of weak tea. Tannin water is brown rather than green, it is not algae, and shocking it hard will often clear it while telling you nothing about your chlorine problem.

Sediment, last. Clay soil runoff drops fine silt that sits on the floor and puffs up when you walk through it. That is a vacuum job, not a chemistry job.

What should you do in the first twenty four hours after a storm?

Confirm the pump is running. Not the timer, not the app, the actual pump. Walk out and look at the return eye and see moving water. A large share of post storm green pools are pools where a breaker tripped and nobody noticed for three days.

Then skim and empty the baskets before you touch a chemical. Debris left floating sinks within a day or two, and sunk oak leaves stain plaster and keep feeding the water. Ten minutes with a net on Monday is worth more than a bag of shock on Thursday.

Then test, and test for real. Free chlorine and pH at minimum, stabilizer if the pool took on a lot of volume. If free chlorine is under about 2 ppm and the water is warm, dose it back up that night rather than waiting for the weekend. Chlorine added at dusk survives; chlorine added at noon in July does not.

Last, lower the water back to mid skimmer if the storm overfilled it. A pool running above the skimmer mouth is a pool that is not skimming, and the surface is where the storm left most of its work.

How do you keep the next storm from doing this?

Keep stabilizer in range going into storm season. A pool holding 30 to 50 ppm cyanuric acid loses some to dilution and still has a working reserve. A pool sitting at 15 ppm loses its sun protection entirely in one heavy rain and burns off its chlorine by lunch the next day.

Fix the drainage that points at the pool. If a downspout, a swale or a slope sends yard water into the shell, that is a landscaping fix that pays for itself in chlorine. It is not glamorous and it works better than anything in a bottle.

Get a habit rather than a product. Test after every heavy rain, top the chlorine up the same evening, run the pump longer for a day. Owners who do those three things ride out storm season with a slightly cloudy Tuesday. Owners who wait for the next scheduled visit meet us at the deep end with a net.