Pool season · Colorado
Thornton Pool Season: the Year on One Bar
Last updated: July 25, 2026 · dates · water temp
One bar, one year, one Thornton 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.
Thornton, CO — 109 swim days a yearEach dot on the bar is one week of Thornton'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 2–May 23), solid teal is swim season, and hollow amber is the closing window ending October 4. The black marker is today — its position against the colors is the entire executive summary.
The Thornton pool calendar
Opening — target May 9
Target May 9; the practical window runs May 2–May 23, and the 7-day mean crosses the 61°F threshold around May 23. Cold-water starts are the cheap ones.
Closing — deadline October 4
Hold the cover until September 25, then winterize by October 4, a week ahead of the October 11 freeze normal. Late and cold beats early and warm.
Estimated water temperature by month
Monthly averages of the site's water model run on Thornton'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 |
|---|---|---|---|---|---|---|---|---|---|---|---|
| 33° | 34° | 39° | 47° | 54° | 65° | 73° | 75° | 70° | 58° | 46° | 36° |
The curve peaks near 75°F — comfortable-water months here usually involve a heater or a solar cover.
Every station tells its own story: compare this curve with Pueblo's pool season and Westminster's — neighboring normals stations can shift the windows by a week or more. The full ranking lives on the Colorado season index.
Thornton pool season FAQ
When should I open my pool in Thornton, CO?
Open by May 9 — the model places Thornton's 61°F crossing about two weeks later, and every warm day you wait pushes the water toward the 65°F algae zone. The Thornton opening guide has the 12-step sequence.
When should I close my pool in Thornton?
Close by October 4: cool water keeps its winter chemistry, but freeze damage is the hard stop. The Thornton closing guide sequences the work backward from that date.
How long is the swim season in Thornton?
Thornton averages 109 days a year with normal highs of 80°F or better — about 4 days earlier than the Colorado median. The Colorado season index ranks all 19 covered cities by the same metric.
How warm does pool water get in Thornton?
An unheated pool near Thornton typically peaks around 75°F in high summer and bottoms out near 33°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 Thornton hits the closing window
Data: NOAA 1991–2020 climate normals via Northglenn (4.0 mi); live outlook by Open-Meteo. Attribution: sources · assumptions: methodology.