CHICAGO (AP) — Dense fog caused flight delays and some cancellations at Chicago's O’Hare and Midway international airports early on Christmas Eve — one of the busiest travel days of the holiday season.

The Federal Aviation Administration briefly grounded flights at both airports until about 8 a.m. Tuesday and eased those restrictions as the fog cleared.

The National Weather Service said “dense, freezing fog” developed overnight throughout the city and its surrounding suburbs, leading to travel difficulties in the air and on the ground. The weather service reported visibility at Chicago's airports was less than a quarter of a mile (400 meters) in some spots.

“These are just about the worst conditions you can have for flying,” weather service meteorologist Kevin Donofrio said. “When visibility gets down to a quarter-mile, that’s when it gets really dangerous for planes to land.”

By midmorning, nearly 50 flights had been canceled at O’Hare, and more than 70 flights canceled at Midway, split evenly between arrivals and departures, according to the Chicago Department of Aviation. Flight delays were about half an hour at O’Hare and less at Midway.

The weather service advised drivers to slow down, turn on low beams and increase following distance while the freezing fog hangs over expressways.