When to open your pool: why the water lags the air
The first hot week is not the first swim week
A pool holds tens of thousands of gallons of water, and water warms slowly. The model sets each month's water temperature partly from the month before, so the pool trails the air by several weeks in spring and holds its heat into early autumn. That is why the first 78°F afternoon is a poor signal to open the pool; check when the water itself gets there.
Air versus water, in real cities
| City | Air highs reach 78°F | Covered pool reaches 78°F | Uncovered pool reaches 78°F |
|---|---|---|---|
| New York, NY | Jun | Jul | Aug |
| Houston, TX | Apr | May | Jun |
| Phoenix, AZ | Apr | May | May |
| Philadelphia, PA | Jun | Jun | Jul |
| San Antonio, TX | Apr | May | Jun |
| Dallas, TX | May | May | Jun |
From NOAA 1991–2020 normals and the model's estimated water temperature. A dash means it never gets there in a normal year.
What to do with this
If you want to open earlier, put the cover on as soon as you open and keep it on whenever nobody is swimming; that is the cheapest way to pull the first swimmable month forward. Opening the pool a few weeks before the water is ready also gives the filter time to clear it. Your city's page shows the month-by-month estimate.
Related: Does a cover alone get you there? · Are solar panels enough?.