Pool closing · Illinois
When to Close Your Pool in Alton, IL: Deadline, Window & Checklist
Last updated: July 15, 2026 · Model window year: 2026 · dates · checklist · FAQ
Plan to close your Alton pool by October 18. The local 7-day mean temperature drops out of the algae-risk zone around October 8, and NOAA normals put the first 32°F freeze near November 5 — winterize between those dates and the water goes under the cover cold, clean, and easy to reopen. Below: today's water estimate, the full closing window, and a step-by-step winterizing checklist.
Alton closing dates at a glance
| Closing window | October 8 – October 18 |
|---|---|
| Close by (deadline) | October 18 |
| First freeze, 50% probability | November 5 |
| Open by (recommended) | April 17 |
| Opening window | April 10 – May 1 |
| 61°F crossing (7-day mean) | May 1 |
| Swim-season length (80°F+ days) | 106 days |
| NOAA normals station | Alton - Melvin Price L&d · 2.5 mi · 405 ft |
Alton's 106-day warm season leaves comfortable margins on both ends — the windows above aim you at the cheap-chemistry versions of each transition.
Put dates aside and follow the water: the Alton curve says roughly 54°F by mid-April, 73°F by mid-June, 77°F in mid-August, then back down through 61°F in mid-October. The summer ceiling sits near 78°F, and every window above is just a line drawn on this curve.
The 12-step Alton winterizing checklist
The order matters more than the date: balanced water first, verified-dry lines before anything else freezes-proofs, and the cover only after everything below it is done. Work the list inside the window above.
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Balance the water a few days ahead
Three or four days before closing, adjust alkalinity and pH into label ranges. Balanced water is gentler on the liner, plaster, and equipment through the long covered months ahead.
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Deep-clean the pool
Skim, brush walls and steps, and vacuum carefully. Any leaves or algae you seal under the cover become spring's chemistry problem, so closing day cleanliness pays twice.
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Service the filter one last time
Send the filter into winter clean: backwash the sand or DE, rinse and dry the cartridges indoors. Media stored dirty over winter hardens into a spring problem no backwash fixes.
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Apply winter chemicals per label
Winter chemicals go in before shutdown, not after: label-dosed, circulated for a few hours, distributed evenly. A floater dropped on still water protects one corner.
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Lower the water level
Take the level down only as far as the cover's manual says — usually just below the skimmer for solid covers, higher for many mesh systems. An empty pool is never the goal; shells crack and shift without water's weight.
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Blow out the lines and plug returns
Air through every line — skimmer, returns, cleaner — until each blows dry mist, plugging returns while the air still pushes. Nothing else on this list protects as much plumbing per minute.
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Protect the skimmer
Seat a skimmer guard or bottle in the throat — ice that forms there needs a sacrifice, and a two-dollar bottle beats a plumbing repair under the deck.
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Add pool antifreeze where blowouts fall short
Antifreeze is the insurance policy for doubtful lines, not a replacement for the blowout: pool-grade product, label dosing, and only where air couldn't finish the job.
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Drain the equipment
Every vessel on the pad — pump, filter, heater, chlorinator — gets its drain plugs pulled. Drop all the plugs in the pump basket; that's the one place everyone looks first in spring.
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Set the air pillow and cover
Center an inflated air pillow, then fit the cover and secure it with water bags, cable, or straps as designed. The pillow gives ice a place to push besides your walls.
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Note this year's dates
Jot down when Alton's water actually cooled and when you closed. A two-line note beats memory next October 8 — your own yard runs a few days off any model, including ours.
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Store chemicals properly
Seal opened containers, keep oxidizers and acids separated, and store everything cool, dry, and locked away from kids and pets — exactly as each label describes.
What to buy before the rush
Every item below sells out somewhere in Illinois every October. Stocking the short list before the rush costs nothing extra and saves the mid-project store run — the chemicals guide explains what each category actually does.
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Pool antifreeze
Pool-rated glycol for the lines air couldn't clear.
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Winter closing kit
The under-the-cover chemistry, measured for your gallons.
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Air pillow
Centers under the cover so ice pushes inward, not outward.
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Winter cover
The one purchase every other closing step depends on.
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Cover pump
Keeps rain and melt off a solid cover all season.
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Expansion plugs + skimmer guard
Seals blown-out lines and sacrifices itself to skimmer ice.
How Alton compares locally
Alton closes in the earliest quarter of Illinois's calendar. Neighbors run close: St. Louis (19 mi away) models its deadline at October 23 (about a week later vs Alton's October 18), while O'Fallon (32 mi) shows October 14. The spring mirror of this page is the Alton opening guide, and the season overview draws both windows on a single bar.
Local means local: Alton's dates come from Alton - Melvin Price L&d, the nearest station with complete daily temperature normals — 2.5 miles south, about 405 feet up. Between that station and a Madison County backyard there's always a degree or two of microclimate; the windows are built wide enough to carry it.
Field notes for Alton owners
Cover pumps die in the cold — plan for it
A cover pump left running into a hard freeze can lock in ice and burn out. On freezing forecasts, pull it, let the storm pass, and put it back for the melt. Automatic models with freeze protection earn their price in exactly one forgotten weekend.
Salt cells overwinter indoors
Remove the cell at closing, inspect and clean per the manufacturer, and store it inside with the unions capped. A dummy cell or spacer keeps the plumbing sealed. Cells left in outdoor plumbing through freezes are a common — and completely avoidable — spring casualty.
The warm spell after you closed
A 78°F week in October doesn't mean reopening. Water under an opaque cover warms far less than air suggests, and a closed, balanced pool tolerates a warm stretch fine. Check the cover pump has somewhere to send rain, enjoy the weather, and leave the plumbing sealed.
Alton pool closing FAQ
What temperature should water be to close a pool?
Below roughly 65°F, and trending down. Water closed warm keeps feeding algae under the cover for weeks; water closed in the 50s goes dormant almost immediately. Alton's cool-down lands near October 8 in the normals, which is why the window opens there.
Can you close a pool too early?
Yes — it's the most common closing mistake. Seal 70°F water under a cover and algae keep growing in the dark all autumn; the spring opening turns green and expensive. In Alton, hold off until the cool-down near October 8 before covering.
Do I need antifreeze in pool lines?
It depends entirely on your confidence in the blowout. Lines that blew fully dry need nothing; anything uncertain — low runs, water features, a stubborn cleaner line — gets pool-grade antifreeze at the label's rate per foot. With Alton's freeze clock starting near November 5, uncertainty is the thing to eliminate.
How far should I drain my pool for winter?
Follow the cover's instructions first: solid covers usually want water a few inches below the skimmer; some mesh setups run higher with the skimmer sealed. The hard rule is never empty — hydrostatic pressure can lift or crack an empty pool, a far worse outcome than any freeze.
What happens if you don't winterize a pool?
The repair list writes itself in order of cost: heater heat exchanger, pump housing, filter tank, then every fitting the ice reached — discovered one leak at a time in spring. Around Alton the exposure begins near November 5, and each skipped checklist step above is one more candidate for that list.
When is the last safe date to close in Alton?
The model draws the line at October 18 for Alton. It isn't arbitrary: the cool-down plus ten days (the freeze normal, November 5, leaves room to spare), and the whole closing sequence needs a working weekend of margin. The one exception that overrides any date — a hard freeze inside the 10-day forecast, which the widget above flags as urgent.
Email me when Alton hits the closing window
Data: NOAA 1991–2020 climate normals via Alton - Melvin Price L&d (2.5 mi); live outlook by Open-Meteo. Full attribution on the sources page. Model assumptions and error bars on methodology.