New Orleans — A thick white blanket of fog enveloped much of the metro New Orleans area Wednesday, forcing the shutdown of ferry service and briefly closing the Lake Pontchartrain Causeway midmorning to allow crews to place traffic cones on the southbound span.

The fog hung in throughout the day because, unlike typical fog that forms in the early morning hours and then burns off, Wednesday’s weather was what is known as sea fog, National Weather Service forecaster Mike Shields said.

Sea fog forms when weather is warmer than usual for the time of year and moisture-laden air moves over water that is colder in temperature, forming the fog, he said. Fog was thickest in areas near water, he said.

The Chalmette ferry suspended service at 9:01 a.m. Wednesday because of dense fog, and the Canal Street ferry stopped service at 11:31 a.m., followed by the Gretna ferry at 12:12 p.m., according to email bulletins from the Louisiana Department of Transportation and Development’s New Orleans Regional Traffic Management Office.

Late Wednesday afternoon, the ferries were still closed. Steve Bernstein, a traffic operator at the TMC said that it did not look like they would resume operating for the remainder of the day.

The weather forecast called for foggy conditions to persist through the night and into Thursday morning, but Shields said light rain in the forecast for Thursday could help break up the fog.