To the government warrents and museums on the Mall, through downtown to the bookshops and theaters of the red-light district, past the slum projects and the hospital atop the hill to the shady neighborhoods beyond, autumn return to the city yesterday, disguised in a costume of summer.

Temperatures stayed in the mid-90s yesterday. The humidity hovered near 50 percent. Leaves remained green and limp. But among the people of 14th Street, the residents, merchants and their assorted hangers-on, there was a vague sense of something new in the air, a feeling that the unbearably steamy summer of 1980 was finally, mercifully nearing an end.

"Maybe soon I'll be able to get outside again. Go for those walks in Rock Creek Park. Fishing, who knows?" said Millie, a brown-eyed model who greeted a reporter yesterday at the Paradise Escort and Model Service in her underwear.

"This can't go on forever. You know what it's like to be stuck inside all day?"

It's even worse for those hardy souls stuck outside all day, like William Field, a sunburned city maintenance worker who has been sweeping gutters and cleaning sidewalks on 14th, from Massachusetts Avenue to Pennsylvania Avenue, for the last 12 years.

"Let me tell ya about fall," said he. "My foreman says I got the most important job in the division because 14th is the busiest street in town. In the fall it rains, you know, the work isn't as hard.

"In the summer, the nastiest animals around come here. Yeah," he said, "yeah, I'm glad fall's here."

To National Weather Service forecasters, fall has been here for three weeks already.Don Gillman, a long-range forecaster, said autumn, to meteorologists, began Sept. 1. To the rest of the world, autumn falls when the astronomers say it will. When the sun hangs directly over the equator. Which was yesterday.

"It's just easier for us, as forecasters, to say fall goes from September through the end of November," said Gillman, who added that the 30-day forecast from mid-September through mid-October calls for temperatures, like the summer past, to average above normal. Forecasters believe temperatures for the period will range between one and two degrees above the average of 65.6 degrees.

Rainfall, meanwhile, will amount to somewhat less than the median of 2.56 inches, according to forecasters.

But none of that seemed to bother people like G.A. Kline, a security guard at the District Building who thought a moment, called summer "a plan in the a--," and then daydreamed about touch football and fishing in West Virginia.

"It really keeps me going," he said "thinking about fall, leaves changing color. It's fresh. Clean. Problem with summer is that everybody sweats. It's a dirty season."

Slowly, imperceptibly, qualities of autumn appeared in numerous forms and shapes yesterday on 14th Street. The straw Panama hats of summer have been replaced in haberdashery windows by sturdy felt. Rugged cowboy boots stand alongside leather slippers, replacing flip-flops and canvas loafers.

At Luther Place Memorial Church at Thomas Circle, which runs a shelter for homeless women nearby, officials and volunteers prepared to reopen for the cooler seasons, when the destitute and the cold return.

On concrete schoolyards in upper Northwest, beyond the complex of buildings at Walter Reed Army Medical Center, youngsters in shorts and tennis shoes shouted, cheered and dashed after rubber footballs, willing autumn to be.

Fall was in the heart, if not the air. But that should change a bit today.

Yesterday's high temperature at Washington National Airport was 93 degrees. yThe National Weather Service forecast for Washngton and vicinity today calls for variable cloudiness and breezy weather, with high temperatures between 82 and 86 degrees.

The invisible transition has happened. A season arrives. A season departs.