How long is the swim season in Northeast Ohio?

NOAA publishes 30-year climate normals for weather stations across the region. These are the 1991 to 2020 averages for Akron-Canton Airport in Green, which sits near the middle of our service area:

Degrees are Fahrenheit. Cleveland's airport station runs within about a degree on daytime highs through the summer, with nights a degree or two warmer near the lake. The pattern is simple: three warm months, June through August, with May and September as shoulder months where days can be pleasant but nights drop into the 50s.

A pool holds thousands of gallons of water, and water takes a lot of energy to warm up. In May, a string of 75 degree days does not make the pool 75 degrees, because cool nights and a cold start pull it back each evening. The pool catches up with the season a few weeks late. The same thing works in your favor at the other end: in early September the water is still holding heat from August even as nights cool.

The biggest thing pulling heat out of a pool is evaporation. The U.S. Department of Energy calls it by far the largest source of energy loss from a pool and notes that every pound of 80 degree water that evaporates takes 1,048 Btu of heat with it. Cool, breezy nights in May and September speed that loss up. So does leaving the pool uncovered.

This is why the rough rule of Memorial Day to Labor Day holds up so well for an unheated pool here. Before late May the water has not caught up. After early September the nights start winning.

Frost dates bracket the working season for the pool itself: when you can safely open and run equipment, and when you need to be closed. NOAA's normals give the median dates of the last spring and first fall temperature of 32 degrees:

• Akron-Canton Airport: last spring freeze April 24, first fall freeze October 26.
• Ravenna: April 21 and October 29.
• Cleveland Hopkins: April 20 and November 3, helped by Lake Erie.
• Hiram, in northern Portage County: May 1 and October 20.
• Chardon, in the Geauga County snowbelt: May 5 and October 18.

Those are medians, so in about half of years the dates fall later in spring or earlier in fall. They explain why most owners open in May and close by early October. Opening a covered pool before the last frost is common, but heaters and plumbing need to be protected if a hard freeze returns.

Frost dates also shape the building calendar for new pools. Crews can dig and set shells well into fall, but concrete decks and patios need to be placed before hard freezes arrive, and spring work waits for the ground to firm up after the thaw.

Location matters more than people expect across a region this size. Chardon's normals show a July average of 70.0 degrees with a high of 81.0, compared with 73.9 and 84.3 at Akron-Canton. June and August follow the same pattern. That is a few degrees cooler every day of summer, on top of a later spring and an earlier fall. Pools in the higher parts of Geauga County and inland Lake County have the shortest seasons in our service area and gain the most from a heater and cover.

The lakeshore is the other extreme at the ends of the season. Cleveland's first fall freeze arrives about a week later than Akron-Canton's because Lake Erie holds heat into autumn. Ravenna and much of Portage County sit close to the Akron-Canton numbers, with slightly cooler nights: Ravenna's July low averages 60.1 degrees against 63.4 at the airport.

Sun is free heat, and Northeast Ohio does not get a lot of it. NOAA's comparative climate data show Cleveland receiving 64 percent of its possible sunshine in June and 66 percent in July, dropping to 59 percent in September and 48 percent in October. Akron-Canton averages about 198 cloudy days a year.

Summer is the sunny window, which is part of why the unheated season is so concentrated. The Department of Energy notes that an outdoor pool absorbs 75 to 85 percent of the solar energy that hits its surface. A pool that gets full afternoon sun gains a noticeable amount of that energy on a clear day. A pool shaded by the house or a tree line after 3 p.m. loses it, and in May and September that can be the difference between a swim and a pass.

Some, but less than people fear. NOAA's normals for Akron-Canton put June at 4.43 inches of precipitation, the wettest month of the year, with July at 4.14 and August at 3.61. Much of that falls in short summer thunderstorms rather than all-day rain, so a wet week usually still leaves swimmable afternoons. Get everyone out of the water at the first sound of thunder and wait until the storm has clearly passed.

Rain matters more for the water than for the swimmer. A heavy storm adds water, dilutes the chlorine, lowers the pH and washes in dirt and pollen, so plan to test and adjust after a big one. On a site with slow-draining clay soil, rain also runs across the yard toward the lowest point, which is one reason we grade the deck and surrounding ground so runoff moves away from the pool instead of into it. A pool that stays clean after storms is a pool you can swim in the next day.

Quite a bit, and the tools stack. A cover used every night is the first step, because it stops most of the evaporation loss. The Department of Energy says covering a pool when it is not in use is the single most effective way to cut heating costs, with savings of 50 to 70 percent possible. A heater is the second. A gas heater warms the water quickly and holds temperature in any weather. A heat pump costs less to run, but the Department of Energy notes it works efficiently only while the outside air stays above about 45 to 50 degrees, which covers most of May and September here but fewer October nights.

With a heater and a cover used faithfully, most owners in the Akron and Canton area can swim from roughly mid-May to early October. In the snowbelt, expect a little less at each end. An ICF concrete pool, with insulated walls that hold heat better, helps further. Our guide on extending the swim season goes through each option.

• April: cover stays on. Plan the opening once the worst freezes are past, and book early if you use a service.
• Early to mid May: open the pool, balance the water and let the filter run. Heated pools can start swimming on warm days.
• Memorial Day to Labor Day: the main season. Unheated pools are comfortable most days by mid June.
• September: still good swimming in a covered, heated pool. Unheated pools cool quickly after mid month.
• Late September to mid October: close and winterize before the first hard freeze, earlier in the snowbelt.

That calendar is a starting point, not a rule. A hot May or a warm October happens often enough that owners with a heater and cover end up using the pool on days a neighbor with an unheated pool would not.

It depends how you measure. Fourteen weeks of summer is a short season on paper, but it is also the fourteen weeks when families are home, kids are out of school and evenings are long. A pool that gets used most days in that window gets used more than many people's second car. Stretch it to twenty weeks with a heater and cover and the math shifts further.

What we would steer anyone away from is a pool designed as if Ohio were Florida. Plan for the heat you will add and the cover you will use every night, place the pool in the sunniest part of the yard and pick a size you can afford to heat. That is how you get the most weeks out of the water here.