The Washington area suffered through the third day of a sweltering heat wave yesterday. But then you knew that.

Schools again closed early, people scurried back to the beaches and most everyone made more than polite conversation about the weather. "It's hot, it's really hot," said one woman, a perfect stranger, stating the obvious to fellow elevator passengers and getting sympathetic nods all around.

Temperatures soared to 96 degrees at National Airport for the second day in a row, two degrees short of the record for that date set in 1954. It was 95 degrees at Dulles Airport.

The weather was no friend to those with respiratory ailments and allergies. And it didn't do much for electronic weather indicators, either. The one at Dominion Federal Savings in Alexandria, apparently delirious with heatstroke, registered 101 degrees.

The National Weather Service, keeping tabs on this unusual post-Labor Day hot spell, had some good news and some bad news about what to expect in the days ahead. The good news is that it is probably NOT going to hit 100 degrees today as previously forecast -- just the mid-90s. The bad news is that the region's heat wave will last through Tuesday, and probably beyond.

"East of the mountains, we're under the sunshine and we're feeling it," said forecaster Larry Wenzel.

The rest of the East Coast and the Plains states were feeling it, too. At least seven cities from Norfolk to New York broke or tied temperature records this week and carriage horses were ordered off the streets in Manhattan. In Oklahoma, officials issued a statewide heart alert for the elderly and ailing.

Even President Reagan took note of the wilting weather.

Speaking to Republican women state legislators at the White House, Reagan said the muggy heat reminded him of a church service in his home town during the dog days of summer.

The preacher, according to Reagan, promised "the shortest sermon we ever heard. He said just seven words: 'If you think it's hot now, wait.' "

A few hardy Washingtonians seemed to take perverse pleasure in mocking the elements. There were the sweat-dripping bicyclists, weaving in and out of traffic while air-cooled motorcycles overheated and sidelined their riders. There was the elderly woman in McPherson Square, gamely knitting a blue wool sweater.

"It made me hot just to look at her," one observer grumbled.

The August-in-September turn of events here was a blessing, sort of, for Zia Saidi, a food vendor at the corner of 18th and L streets NW. Though he sizzled inside his metal stand where the gas oven kept the hot dogs warm, the 27-year-old native of Afghanistan was selling a lot of sodas to panting passers-by.

"Business is better when it's hot," he said.

At Gantos clothing store down the block, however, desk manager Nancy Martin couldn't say the same thing.

"People don't want to try on clothes when it's too hot," she said. Fall wardrobes have been in the store's windows since early August, but most of her customers don't care: "Four people came in yesterday to buy swimming suits -- we were out."

Those who do have swimming suits can use them this weekend at Cameron Run Regional Park in Alexandria. Public pools everywhere else in the area shut down after Labor Day, but the Northern Virginia Park Authority decided to reopen Cameron Run, with its wave pool and three water slides, by popular demand. The pool will be open today and tomorrow from 11 a.m. to 7 p.m.

The weather kept many people indoors, including a retired schoolteacher in Rockville who has not been out of her apartment building since Monday because the heat makes her dizzy. The bus that usually takes senior citizens to lunch and other activities is not air conditioned, and the woman said she is getting her exercise by walking the corridors of her high-rise building.

But the heat didn't stop Warrenton London Jr., 40, and his brother, Anthony, 26, from their rounds as window washers.

They were out there yesterday, dangling by ropes along the side of a downtown office building and furiously splashing soapy water on a wall of glass panes.

"We're used to it, we've been out here all summer," the elder London said. "But we'd rather wash windows in the winter."

Despite near peak usage by customers, there were no reports of electrical power failures.

While some parents grumbled about a lack of notice when their children were sent home from school early, most seemed to agree with Fairfax County school bus driver Lidia Teluk, who said, "I think they'd be more upset if they got heatstroke."

At Laurel-based radio station WLMD-AM 90, program director Pat O'Leary treated his listeners to Nat King Cole singing "The Christmas Song" and to "Cool Water" by the Sons of the Pioneers.

"Psychologically, we think it brings people back to a cooler time," he said.

Folks in search of cooler times headed for the beach yesterday in a surprise surge of post-Labor Day tourism. Ann Faunteleroy, executive director of Ocean City's Chamber of Commerce, reported watching "droves" of motorists drive past her Rte. 50 office. She noticed something else, too.

"Normally, you go down to the beach and it's full of sunbathers," she said. "This time, the people are there, but they're all in the water."