Alexandria’s Great Waves Waterpark — where a swimmer relaxed in sunnier 2011 — has seen a much more relaxed season with cool temperatures this summer. (Matt McClain/For The Washington Post )

The tipping point is 85 degrees.

Above that, says Great Waves Waterpark manager Donny Wensinger, the outdoor spot in Alexandria, Va., is packed. Every space in the parking lot is occupied by a car, baking in the summer heat. Parents scramble for coveted deck chairs. Kids wait 10 or 20 minutes in line for water slides, shifting from foot to foot as the pavement burns their bare soles.

But below 85 degrees?

“It’s much more relaxed,” Wensinger says. No lines, no crowds, no deck-chair battles.

This summer has had a lot of those sub-85 degree days. In fact, Dulles International Airport just registered the longest “heart of summer” cool streak on record. From July 24 to this past Saturday, daily average temperatures were all below 76 degrees — not exactly the kind of weather that makes you want to jump into the water.

At Great Waves, which is in Cameron Run Regional Park, the lower temperatures have meant far fewer visitors than normal. Wensinger estimates that attendance is down 10 percent from last year.

Great Waves isn’t the only facility lacking in swimmers. At 82 degrees, Tuesday afternoon’s temperature fell slightly below Wensinger’s unscientific threshold, and pools around the region were accordingly empty. At the Washington Plaza Hotel’s outdoor pool in mid-afternoon, the water’s surface was disturbed only by one woman doing the breaststroke. And across town at Hains Point, a lifeguard said that a dozen or so swimmers had the 50-meter expanse of the East Potomac Pool to themselves.

Despite this low attendance — in fact, because of it — pool staff members are enjoying the cooler weather more than anyone.

“The atmosphere has been a lot nicer,” says Rieder Grunseth, a lifeguard at George Washington University’s Mount Vernon pool. This is his second summer working at the pool, and he has noticed a big change in the number of swimmers. At the peak of last year’s heat wave — when this reporter was a lifeguard at the Mount Vernon pool — that number got as high as 300. These days, the pool has been drawing only 40 to 50 visitors.

And that’s fine with Grunseth. When no one is in the water, the lifeguards “are getting paid to hang out and talk to each other and read,” he says. “You can catch some rays if you want to without baking, the water is nice to swim laps in. . . . It’s been very serene.”

Joseph Byrne, a lifeguard and assistant manager at the Seven Locks Swim and Tennis Club in Bethesda, Md., has also noticed a drop in attendance during this August cool spell. He has used the extra down time to hang out in the guard office and play foosball.

“Its very nice from a lifeguard’s point of view,” he says.

Even Wensinger, at the water park, says that the mild weather has made for a more enjoyable summer: He’s not spending as much time worrying about making sure his staff stays hydrated or monitoring patrons for heat stroke.

As for other potential casualties of this summer’s nicer-than-normal weather — hair salons where people get humidity-fighting blow-outs, ice cream shops, etc? They don’t mind it, either.

At Michael Anthony Salon on Capitol Hill, more customers are asking to have their hair blown out than is typical during the summer, because cooler conditions mean that the treatment will last longer.

Even the ice cream business is benefiting. Susan Soorenko, owner of Moorenko’s Ice Cream in Silver Spring, Md., says business has been better than usual — perhaps the absence of D.C.’s standard heat and humidity means people have been more likely to leave the comfort of their air-conditioned homes.

“I’d like to order up another one just like it for next year,” she says.