Three weeks late but as unforgiving as ever, the real summer finally marched its oppressive forces into Washington yesterday, extracting from the populace an unconditional surrender to sweaty misery.

The dense, polluted air, the noontime temperatures above the 90-degree mark, the furious late afternoon rainstorm - all the elements of tropical Washington made their ungracious entrance on a day - Friday the 13th - that could hardly have been more appropriate.

"There's not much to say," said National Weather Service forecaster Brian Smith, who is paid to say things about the weather, "except that summer is here."

Of this summer's arrival, however, there is something more to say.

The sticky season came to the Nation's Capital just two days before a federal edict will leave the sweltering masses with one less defense against overheating - the air conditioner.

Beginning Monday, all nonresidential buildings here and around the country will be permitted to cool their air to no more than 78 degrees.

Several businesses in the Washington area attempted a practice run at that yesterday, and the results were not always applauded.

"You just can't work," said Tookie Pedersen, a teller at the Virginia National Bank office in Arlington, where the thermostat was set at 78 degrees.

"It's so damn hot it makes you aggravated. They're saving on the energy, but they're losing on the efficiency of their employes."

But judging from local utility company reports, most buildings in the Washington area enjoyed one last full-bore fling with their precious cooling systems yesterday as the temperature soared to 92 degrees. The peak megawatt usage was the highest of the summer.

For those unable to find air-conditioned refuge, water was one of the few alternatives. It was available in three forms over the course of th day - the swimming pool, the shower and the brief afternoon rain.

"I stay in a tub somewhere, if I can," said a construction worker on Lincoln Road NE as he emerged from a manhole. "I like to get in a nice cool place, to drink cold beer, take a cool shower, wait till the sun goes down and go to the park."

For Derick Deal, an eight-year-old, relief came at the Kenilworth Parkside Pool. In a statement that few would argue with, Deal said: "It feels good to my body splashing in the cool water."

There are some people, of course, who actually enjoy the heat. Everette Forbes, a construction worker, is among that minority. "I think this hot weather is wonderful," said Forbes. "It's the work of the Lord. What man can't control is the summer and winter."

Clesta Rhodes wished she had such powers yesterday, as she and her two young children cycled from their Sixth Street home in Arlinton to their grandmother's house several blocks away.

"I promised my daughter we'd come to see Grandma at lunchtime," said Rhodes, wiping the sweat from her brow as she pedaled along. "I should have my head examined for being out in this heat."

For the unemployed in the housing projects, there is not all that much to do on a hot summer day. But Julian Haley, 38, set up a blanket under a shady tree outside his Alexandria apartment and spent the afternoon shooting craps with some friends.

As Haley rolled the dice, one of his partners, Melvin Carroll, said he wished "there was a law that everyone walked around nude" on days like this. "We want a nudist camp for northern Virginia," he said.

Haley looked down at his fistful of dollars and scanned the group for more players. Finding none, he leaned back against a wood fence and said: "It didn't get hot like this till Skylab came down."

Actually, according to the weather experts, it did not get hot like this until the jet stream mover over.

"The jet stream finally got to where it should be at this time of year, running west to east just above the Canadian border," said a National Weather Service forecaster.

"For the last month, it hasn't been there and all the cool air has been able to slip through. When it runs west to east up there, it forms a ridge and stops the cooler air masses from comiing down here." CAPTION: Picture 1, At sunrise in Falls Church, the hot weather and high humidity became palpably visible as Washington's summer arrived three weeks late. By Larry Morris - The Washington Post; Picture 2, WayneJones tries to cool off on the lawn of the Washington Marina, as humidity enveloped the city. By Margaret Thomas - The Washington Post; Picture 3, no caption, By Larry Morris - The Washington Post; Picture 4, Terry Dunn, a tourist from Ohio, cools his feet at a hydrant on the Mall. By Margaret Thomas - The Washington Post