Warm enough to swim? Philadelphia month by month
| Month | Daily high | Daily average | Verdict |
|---|---|---|---|
| January | 42°F | 35°F | Off-season |
| February | 45°F | 37°F | Off-season |
| March | 54°F | 45°F | Off-season |
| April | 66°F | 56°F | Heater territory |
| May | 76°F | 66°F | Brisk — fine in a warm spell |
| June | 85°F | 75°F | Prime swimming |
| July | 89°F | 80°F | Prime swimming |
| August | 87°F | 78°F | Prime swimming |
| September | 80°F | 72°F | Comfortable |
| October | 68°F | 60°F | Heater territory |
| November | 56°F | 49°F | Off-season |
| December | 47°F | 40°F | Off-season |
Verdicts follow the daily average: 68°F+ is comfortable for most people, 75°F+ is prime, and below 62°F you're in heater territory. An unheated pool's water tends to sit near the recent daily average, a touch above it in spring and below in fall.
Reading the Philadelphia pool year
The Philadelphia season splits into three acts. Opening (May 3 – May 22): clear the cover, rebalance, and shock — our dosing calculator turns your first test kit reading into exact amounts. Midseason: weekly testing and small corrections, with chlorine demand peaking around July. Closing (Sep 24 – Oct 9): wait for water to hold below ~60°F, balance, then cover — algae is nearly dormant by then, which is what makes spring easy.
Frequently asked questions
When should I open my pool in Philadelphia?
Most Philadelphia owners are happy opening in the May 3 – May 22 stretch — that's when daily average air temperature climbs through the low-to-mid 60s°F, which puts the water near 70°F within a couple of weeks. Opening earlier is fine (algae actually prefers a late opening: a warm, covered pool with no chlorine is a petri dish); swimming just starts brisk.
When should I close my pool in Philadelphia?
Around Sep 24 – Oct 9, as daily averages fall back through the low 60s°F. Waiting until water holds below ~60°F is smart — algae is nearly dormant there, so the pool goes into winter clean. Closing too early in warm weather is the classic recipe for a swampy spring opening.
How long is the swim season in Philadelphia?
About 142 days without a heater — from roughly May 12 to Oct 2. A heat pump or gas heater typically adds 3–6 weeks on each end; a solar cover alone adds a week or two by holding overnight gains.
Where do these Philadelphia dates come from?
From 1991–2020 NOAA temperature records at the Phila Franklin Inst station, 1 km from central Philadelphia. The working assumption: pools become pleasant as daily average air temperature passes 62–68°F, since water lags air by a week or two. A heater, cover, screened enclosure, or an unusually sunny yard all shift the real dates.