What does dissolved mean here?

Nadia had carried the permit in her wallet for six weeks before her mother agreed to sit in the passenger seat. The first three Saturdays they circled the empty lot behind the church, turning wide loops until the orange cones stopped mattering. On the fourth Saturday her mother pointed at the road that ran past the almond orchard.

“Four miles,” she said. “You will not meet anyone.”

Nadia met the fog instead. It came off the fields in low gray sheets, the way it does in the Valley in January, and by the second mile the orchard rows had dissolved into a single pale wall. She could see the yellow line and about two car lengths past the hood. Her hands went tight on the wheel.

“Slower,” her mother said. She did not raise her voice. She had not raised it once in four Saturdays, which Nadia found more unnerving than shouting would have been.

Nadia let the speedometer fall to fifteen. The engine noise dropped with it, and for a while the only sounds were the tires on wet asphalt and her mother breathing evenly beside her, as though this were a bus she happened to be riding.

A pair of headlights surfaced ahead, floated past, and vanished. Nadia's shoulders came down half an inch.

“You are doing what you should be doing,” her mother said. “Keep the line on your left. Count the driveways.”

At the fourth driveway Nadia turned onto the gravel, stopped, and put the car in park. Her fingers had left damp marks on the wheel. She waited for the review of everything she had done wrong, the one that had always followed her brother's lessons.

Her mother unbuckled her seat belt, leaned back against the headrest, and said, “Again next Saturday.”