Real-life case: the hailstorm of 15 August
- The situation: a violent hailstorm on 15 August — pool open, garden turned upside down, water unrecognisable in 20 minutes.
- Triple damage: massive debris, diluted water (weakened sanitiser), pH and alkalinity shifted by the rain.
- The key reflex: clean then TEST before dosing anything — the post-storm state is unpredictable.
- Water back to immaculate in 36 h, without a shock: the numbers didn't call for one.
15 August, 5:40pm. The sky turns violet in ten minutes; at 6pm, hailstones like marbles strafe the terrace, the rain falls horizontally, and the parasol crosses the garden. 6:20pm: silence. And in the middle of the soaked scenery, the pool — unrecognisable. Surface covered in shredded leaves, soil and petals, water up to the copings, a grey haze spreading. Twenty minutes of storm, and a whole summer’s water looked lost. Spoiler: it wasn’t, not remotely.
The damage inventory (real and imagined)
First useful reflex: inspect the equipment before the water. Hail = cover, shutter and waterline to check — here, nothing broken. Pool-side, the storm had done its classic triple job: a massive influx (leaves, soil, dust — an algae buffet), serious dilution (the equivalent of hundreds of litres of untreated, slightly acidic rainwater), and a level above the skimmers, which were no longer skimming.
What looked catastrophic — the colour, the haze — was actually just mechanics: suspended particles. The real stake was invisible: what had become of the chemistry?
The clean-up, in the right order
6:45pm — drain down and skim. Level brought back to mid-skimmer, net working non-stop for twenty minutes, baskets emptied twice. Anything left to steep will feed the algae: out it goes.
7:30pm — continuous filtration started, and walls brushed where the soil had settled.
8pm — the test, and not before. This is the moment the beginner rushes to pour “a shock to be on the safe side”. The photo test said otherwise: sanitiser diluted but still at the low end of the range, pH pulled down by the rain, alkalinity dropped. The app’s verdict: no shock — a calculated sanitiser top-up, a gentle alkalinity correction, and the filtration would do the rest. Dosing blind would have been, at best, a waste.
The next day. Morning re-test: numbers stabilised, haze clearly receding. The app kept the extended filtration in place and scheduled a re-test at 48 h — the window where “shaken” water decides whether to flip or not. 36 hours after the hail, the water was crystal clear and the swim indicator green. No shock, no miracle product: a serious clean, two calculated corrections, and filtration.
What this case teaches
A storm isn’t treated on reflex, it’s treated on evidence: clean, filter, test — then dose what the numbers ask for, nothing else. Half the “post-storm” shocks poured in a panic achieve nothing (and add stabiliser for nothing when they’re stabilised chlorine). The other half are underdosed because poured before knowing. The app puts the order of operations back the right way up: know first, act second — and an automatic 48 h reminder to confirm the episode is closed.
Frequently asked questions
Can hail damage a pool?
The water takes it without a problem; the sensitive points are the equipment: roller cover, bar cover, liner around the waterline if the hailstones are big. After the storm, a visual inspection of the equipment on top of the water treatment.
Is a shock treatment always needed after a storm?
No — it's the classic panic reflex, and it's often unnecessary. The test decides: sanitiser still in range and water clearing fast = a simple top-up. The shock is justified if the chemistry has collapsed or the water is starting to turn.
Why does the water rise or change after a big storm?
Dozens of millimetres of rain = hundreds of litres of new, untreated, slightly acidic water: sanitiser diluted, pH and alkalinity pulled down, level drowning the skimmers. Hence the trio: level, cleaning, test.