An orange balloon escaped from someone's hand at the climax of "The Star-Spangled Banner" just after 8 p.m. yesterday on the West Lawn of the U.S. Capitol. It was last seen soaring high over the Capitol dome in the direction of the Library of Congress, driven by a brisk breeze that was also driving dark clouds and seemed to be threatening a thunderstorm.

But serious rain stayed away from the Mall, and the music provided all the atmospheric drama. Conductor James Conlon and the National Symphony Orchestra snapped out Leonard Bernstein's "Candide" Overture with an energy that seemed to smash into the cloud cover.

The skies turned blue again while Roberta Flack was singing "The First Time Ever I Saw Your Face" and "Killing Me Softly," then shifted into a classic, pink-tinged sunset while Marvin Hamlisch was playing some of the Scott Joplin music he used in the soundtrack of "The Sting."

In spite of serious threats, the closest thing to a tempest during the concert was tenor Jon Vickers' performance of "Wintersturme wichen," a sort of Wagnerian answer to "Stormy Weather."

Earlier, the National Weather Service had forecast a 30 percent probability of rain and, in fact, the crowd had about 30 percent of a rainfall -- seven minutes of fat, lazy raindrops about 7:20 that made little impression on people already soaked with perspiration.

It did produce what you might call a 30 percent rainbow -- a pale arc of colors that seemed to stretch up over the Capitol dome.

On stage, the musical keynote was vigor. This was, after all, a concert for people who had been reclining on the grass for hours, soaking up sunshine and beer and looking forward to a storm of fireworks after -- or, as it turned out, during -- "The Stars and Stripes Forever."

Conlon, who now spends most of his time in Europe conducting music that is somewhat older and less brash, showed that his heart is still American in a muscular, raucous rendition of Gershwin's "An American in Paris," full of the sound of old-fashioned taxi horns and older-fashioned jazz.

The most traditionally classical item on the program was ambiguously American music: the last movement of Dvorak's "New World" Symphony, composed in the United States by a European who had been appointed the conductor of the New York Philharmonic. There was an appropriate reverse-English effect in having it conducted by an American who now spends 90 percent of his time in Europe conducting operas and symphony orchestras.

The crowd began arriving hours before the music was scheduled to begin and filled almost all the vast green expanse from Independence Avenue to Constitution Avenue -- including a lot of places where the orchestra could barely be seen or heard. By 6:45, all the good places to watch the 8 p.m. concert were gone, but this did not bother the spectators who kept on pouring in.

On the fringes of the lawn, children scrambled up trees, adults chased Frisbees, a procession of young men marched by chanting a hymn and portable radios provided preconcert music. But except for the extreme fringes, where there was no chance of seeing Flack, Conlon or the NSO, the Capitol steps and the lawn in front of them were packed solid.

Not all balloons were as lucky as the one liberated during "The Star-Spangled Banner." Throughout the concert, one red balloon undulated precariously in the early evening breeze, 10 feet above the people's heads, near the geometrical center of the West Lawn of the Capitol.

About 100 feet away, two balloons, a red one and a white one, bobbed up and down in tandem. Elsewhere, a bright red banner, a tall staff with ribbons streaming from it, even a picnic table topped by a multi-colored umbrella, provided reference points in the sea of humanity sprawling on the grass. It would not have helped much to tell a friend, "Meet us under the American flag." There were too many of them.

Behind the bandstand by the reflecting pool, the sound was better than in some parts of the lawn and dozens of young music lovers also showed their interest in the art of sculpture, swarming over the equestrian statues to find good spots for watching fireworks.

Those who stayed at home and caught it on television may have skipped a few raindrops, and the sound from their TV sets was probably as good as they could have heard on the lawn. But being a part of a crowd like that -- orderly, good-natured and thoroughly enjoying its togetherness -- is probably worth a bit of dampness and a 12-block walk to a parking space.