Pool season · California
Union City Pool Season: the Year on One Bar
Last updated: July 25, 2026 · dates · water temp
Union City's whole pool calendar, condensed: when the water warms, when it cools, and where today falls between those edges. Use the opening guide and closing guide for the step-by-step work; keep this page for the dates.
Union City, CA — 0 swim days a yearReading it takes ten seconds: 52 dots, one per week. Solid teal is Union City's swim season; the hollow teal run before it (April 21 to May 12) is when to open; the hollow amber run after is when to close, with November 2 as the deadline; gray is the covered season. Today's marker shows where you stand in that cycle right now.
The Union City pool calendar
Opening — target April 28
Open by April 28 — a two-week head start on the May 12 crossing keeps startup chemistry small and beats the local service rush.
Closing — deadline November 2
The window opens October 23 once water leaves the algae zone; the model deadline is November 2, a week ahead of the December 22 freeze normal.
Estimated water temperature by month
Monthly averages of the site's water model run on Union City'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 |
|---|---|---|---|---|---|---|---|---|---|---|---|
| 50° | 52° | 55° | 57° | 60° | 63° | 67° | 68° | 68° | 65° | 59° | 52° |
The curve peaks near 68°F — comfortable-water months here usually involve a heater or a solar cover.
Every station tells its own story: compare this curve with Tustin's pool season and Upland's — neighboring normals stations can shift the windows by a week or more. The full ranking lives on the California season index.
Union City pool season FAQ
When should I open my pool in Union City, CA?
Target April 28. That date backs off two weeks from the day Union City's 7-day mean temperature reaches 61°F, so you start up in cold, algae-resistant water. Work the full checklist in the Union City opening guide.
When should I close my pool in Union City?
The closing deadline is November 2 — late enough that the water has cooled out of the algae zone, early enough to beat Union City's first-freeze normal by about a week. See the Union City closing guide for the winterizing steps.
How long is the swim season in Union City?
Union City averages 0 days a year with normal highs of 80°F or better — about 27 days later than the California median. The California season index ranks all 147 covered cities by the same metric.
How warm does pool water get in Union City?
An unheated pool near Union City typically peaks around 68°F in high summer and bottoms out near 50°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 Union City hits the closing window
Data: NOAA 1991–2020 climate normals via Fremont (4.2 mi); live outlook by Open-Meteo. Attribution: sources · assumptions: methodology.