More than a dozen flights were delayed Wednesday morning when the Portland International Jetport’s main runway was closed because of freezing rain.

Paul Bradbury, the airport director, said the main runway, which runs roughly east-west, was closed at 5:30 a.m. He said overnight snow turned to freezing rain about 4 a.m. and ice built up on the runway.

Airports don’t use salt brine to treat ice on the runways, Bradbury said, because it’s too corrosive and can damage airplanes. They have to use different chemicals than what highway maintenance crews use, he said, and they sometimes are not as effective.

“We love fluffy snow, but coastal icing is difficult,” he said.

The nature of the storm made the freezing rain more difficult to deal with, Bradbury said, because crews kept plowing the runways after the last flight came in after midnight to keep it clear. That meant the freezing rain fell directly on the runway.

“That allowed the freezing rain to really set,” he said.

The first outbound flight is typically at 5:30 a.m., Bradbury said, but that departure, a Southwest Airlines flight to Baltimore, was delayed until after 8 a.m., when the runway was reopened. A dozen other flights were also delayed Wednesday as the airport and airlines worked to get back on schedule.

Arriving flights were largely unaffected because they don’t get to Maine until later in the morning. A JetBlue flight from New York actually landed nearly a half-hour ahead of schedule at 9:28 a.m. A United Airlines flight due in at 9:19 a.m. arrived an hour and 20 minutes late, because its departure from Newark, New Jersey, was delayed.