Pool season · Illinois
Alton Pool Season: the Year on One Bar
Last updated: July 25, 2026 · dates · water temp
One bar, one year, one Alton pool. The season bar below draws everything the model knows — opening window, prime season, closing window — against today's date, with the working details one click away on the opening and closing pages.
Alton, IL — 106 swim days a yearThe bar compresses Alton's whole pool year into one lane rope: gray floats for the off-season, hollow teal for the April 10–May 1 opening window, solid teal for swim weeks, hollow amber for the closing stretch that ends October 18. Wherever the today marker sits, the nearest color change is your next appointment.
The Alton pool calendar
Opening — target April 17
Open by April 17 — a two-week head start on the May 1 crossing keeps startup chemistry small and beats the local service rush.
Closing — deadline October 18
Hold the cover until October 8, then winterize by October 18, a week ahead of the November 5 freeze normal. Late and cold beats early and warm.
Estimated water temperature by month
Monthly averages of the site's water model run on Alton's own normals — the shape of a typical year, not a forecast. The widget above shows where the current year actually sits.
| Jan | Feb | Mar | Apr | May | Jun | Jul | Aug | Sep | Oct | Nov | Dec |
|---|---|---|---|---|---|---|---|---|---|---|---|
| 32° | 32° | 40° | 51° | 61° | 71° | 77° | 78° | 74° | 64° | 51° | 40° |
Warmest water typically August–August (peaking near 78°F); heaters and covers shift any month up.
Every station tells its own story: compare this curve with Waukegan's pool season and Arlington Heights's — neighboring normals stations can shift the windows by a week or more. The full ranking lives on the Illinois season index.
Alton pool season FAQ
When should I open my pool in Alton, IL?
Target April 17. That date backs off two weeks from the day Alton's 7-day mean temperature reaches 61°F, so you start up in cold, algae-resistant water. Work the full checklist in the Alton opening guide.
When should I close my pool in Alton?
The closing deadline is October 18 — late enough that the water has cooled out of the algae zone, early enough to beat Alton's first-freeze normal by about a week. See the Alton closing guide for the winterizing steps.
How long is the swim season in Alton?
Alton averages 106 days a year with normal highs of 80°F or better — about 18 days earlier than the Illinois median. The Illinois season index ranks all 18 covered cities by the same metric.
How warm does pool water get in Alton?
An unheated pool near Alton typically peaks around 78°F in high summer and bottoms out near 30°F in winter, never quite reaching the 78°F comfort line without a heater or solar cover. The month-by-month curve above shows the whole cycle; the methodology page explains the ±5°F error bars.
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. Attribution: sources · assumptions: methodology.