Pool season · Colorado
Castle Rock Pool Season: the Year on One Bar
Last updated: July 25, 2026 · dates · water temp
One bar, one year, one Castle Rock 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.
Castle Rock, CO — 85 swim days a yearEach dot on the bar is one week of Castle Rock's typical year, colored by what the model says that week is for: gray weeks are off-season, hollow teal marks the opening window (May 12–June 2), solid teal is swim season, and hollow amber is the closing window ending September 25. The black marker is today — its position against the colors is the entire executive summary.
The Castle Rock pool calendar
Opening — target May 19
Target May 19; the practical window runs May 12–June 2, and the 7-day mean crosses the 61°F threshold around June 2. Cold-water starts are the cheap ones.
Closing — deadline September 25
Hold the cover until September 17, then winterize by September 25, a week ahead of the October 2 freeze normal. Late and cold beats early and warm.
Estimated water temperature by month
Monthly averages of the site's water model run on Castle Rock'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° | 37° | 43° | 50° | 61° | 69° | 71° | 66° | 55° | 44° | 35° |
The curve peaks near 71°F — comfortable-water months here usually involve a heater or a solar cover.
Every station tells its own story: compare this curve with Broomfield's pool season and Centennial's — neighboring normals stations can shift the windows by a week or more. The full ranking lives on the Colorado season index.
Castle Rock pool season FAQ
When should I open my pool in Castle Rock, CO?
Target May 19. That date backs off two weeks from the day Castle Rock's 7-day mean temperature reaches 61°F, so you start up in cold, algae-resistant water. Work the full checklist in the Castle Rock opening guide.
When should I close my pool in Castle Rock?
The closing deadline is September 25 — late enough that the water has cooled out of the algae zone, early enough to beat Castle Rock's first-freeze normal by about a week. See the Castle Rock closing guide for the winterizing steps.
How long is the swim season in Castle Rock?
Castle Rock averages 85 days a year with normal highs of 80°F or better — about 6 days later than the Colorado median. The Colorado season index ranks all 19 covered cities by the same metric.
How warm does pool water get in Castle Rock?
An unheated pool near Castle Rock typically peaks around 71°F in high summer and bottoms out near 31°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 Castle Rock hits the closing window
Data: NOAA 1991–2020 climate normals via Castle Rock (3.7 mi); live outlook by Open-Meteo. Attribution: sources · assumptions: methodology.