Every year around May, pools across Broward County start behaving differently, and most owners put it down to heat. Heat is part of it. The bigger factor is rain, and what it does is not obvious from looking at the water.
A South Florida rainy season delivers a great deal of water into an open pool over a few months. That water is not neutral, it is not clean, and it does not simply raise the level and drain away.
Four things heavy rain does to pool water
1. It dilutes everything you have added
Rain is effectively distilled water. Every inch that falls into the pool waters down the chlorine, the stabiliser, the calcium and the alkalinity along with it. Nothing has gone wrong chemically — there is simply less of everything per gallon than there was yesterday.
Stabiliser matters most here. Cyanuric acid is what stops Florida sun burning your chlorine off within hours. Dilute it enough and chlorine stops lasting, no matter how much you add.
2. It pushes pH and alkalinity down
Rainwater is mildly acidic. A single shower will not move a balanced pool far, but a season of them does, and low pH is not cosmetic — it corrodes the metal in heaters and pump internals, and it makes water aggressive toward plaster and grout. Damage from months of low pH is not reversible by fixing the chemistry afterwards.
3. It washes phosphates into the water
This is the one that catches people out. Runoff carries fertiliser, soil and decaying plant matter off the deck and the lawn and into the pool, and all of it brings phosphate. Phosphate is algae food.
A pool that keeps going green despite a chlorine reading that looks fine is usually a phosphate problem. You are feeding algae faster than the chlorine can kill it. Adding more chlorine treats the symptom; the phosphates are the cause.
4. It raises the water level and stops the skimmer working
An overfull pool is not just untidy. When water rises above the skimmer mouth, the skimmer stops pulling the surface layer — which is exactly where oils, pollen and fine debris sit. The pool can look calm while its main cleaning mechanism is doing nothing at all.
What to do about it
- Check the level after heavy rain. The water should sit around the middle of the skimmer opening. Above that, the skimmer is not skimming.
- Test more often, not less. Rainy season is when weekly testing earns its keep. Chemistry moves further and faster between May and October than at any other time.
- Watch stabiliser, not just chlorine. If chlorine keeps vanishing, low stabiliser after months of dilution is a common cause.
- Treat phosphates deliberately. If the pool greens up while chlorine reads normally, test phosphates rather than adding more chlorine on top.
- Clear the deck. Less organic matter sitting around the pool means less of it washing in with the next storm.
The short version
Rainy season does not break pools. It moves them steadily out of balance, in four directions at once, over several months. Pools that come through it well are the ones corrected weekly while the numbers are still close to right — not the ones rescued in September once the water has already turned.
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