Pool season · California
Castro Valley Pool Season: the Year on One Bar
Last updated: July 25, 2026 · dates · water temp
Castro Valley'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.
Castro Valley, CA — 0 swim days a yearEach dot on the bar is one week of Castro Valley's typical year, colored by what the model says that week is for: gray weeks are off-season, hollow teal marks the opening window (April 28–May 19), solid teal is swim season, and hollow amber is the closing window ending November 7. The black marker is today — its position against the colors is the entire executive summary.
The Castro Valley pool calendar
Opening — target May 5
Target May 5; the practical window runs April 28–May 19, and the 7-day mean crosses the 61°F threshold around May 19. Cold-water starts are the cheap ones.
Closing — deadline November 7
Hold the cover until October 28, then winterize by November 7, a week ahead of the December 25 freeze normal. Late and cold beats early and warm.
Estimated water temperature by month
Monthly averages of the site's water model run on Castro Valley'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 |
|---|---|---|---|---|---|---|---|---|---|---|---|
| 51° | 52° | 55° | 57° | 60° | 63° | 66° | 67° | 68° | 66° | 60° | 53° |
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 Carson's pool season and Chico's — neighboring normals stations can shift the windows by a week or more. The full ranking lives on the California season index.
Castro Valley pool season FAQ
When should I open my pool in Castro Valley, CA?
Open by May 5 — the model places Castro Valley's 61°F crossing about two weeks later, and every warm day you wait pushes the water toward the 65°F algae zone. The Castro Valley opening guide has the 12-step sequence.
When should I close my pool in Castro Valley?
Close by November 7: cool water keeps its winter chemistry, but freeze damage is the hard stop. The Castro Valley closing guide sequences the work backward from that date.
How long is the swim season in Castro Valley?
Castro Valley averages 0 days a year with normal highs of 80°F or better — about 34 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 Castro Valley?
An unheated pool near Castro Valley typically peaks around 68°F in high summer and bottoms out near 51°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 Castro Valley hits the closing window
Data: NOAA 1991–2020 climate normals via Hayward Air Terminal (4.7 mi); live outlook by Open-Meteo. Attribution: sources · assumptions: methodology.