An exact count, unfortunately, was not forthcoming. After a regal fanfare played on trumpets and timpani, a gentle rain shower started during the opening remarks by Midge Woolsey, a radio host at WQXR-FM, and Christopher W. London, the president of the Naumburg Orchestral Concerts board. Both optimistically predicted that the rain would move through quickly.

Instead, umbrellas popped up like mushrooms on a lawn as the rain grew steadier during an appropriately brisk, breezy account of Mendelssohn’s Overture to “A Midsummer Night’s Dream.” Most audience members remained in place, sharing shelter with their neighbors.

“We love you for staying,” the conductor Eric Jacobsen brightly proclaimed from the podium. He announced a program change: Dvorak’s “Silent Woods,” meant to feature the cellist Jan Vogler, would be saved for the second half of the concert: insurance against a shower still expected to pass.

The rain did in fact stop. Mr. Jacobsen announced that the orchestra would play Schubert’s Symphony No. 8 (“Unfinished”) without amplification, so that the microphones and loudspeakers could dry. The old band shell served its original purpose, focusing and projecting the ensemble’s appealing sound during a brisk, unsentimental account of the first movement.

When a heavy downpour suddenly started, many listeners held their ground. Ultimately Mr. Jacobsen cut the performance short; rain was blowing into the band shell. “Schubert’s ‘Unfinished’ Symphony is still unfinished,” he said as violinists sought cover and audience members took flight. After perhaps 15 minutes of consideration, Mr. London called the event off.

It was disappointing that the Knights could not finish their promising performance, and that Mr. Vogler could not perform (though audience members who had turned up early heard him play through “Silent Woods” with the orchestra during a sound check). The Knights will return to the Naumburg Bandshell with another soloist, the violinist Vera Beths, and a new program on Aug. 3, weather permitting, of course.