Dark clouds loomed over many travelers in the Midwest on the eve of Thanksgiving as a dense blanket of fog forced airlines to cancel more than 140 flights and delay nearly 600 more in and out of Chicago, Detroit and St. Louis.

At least 92 flights out of Chicago O’Hare were canceled Wednesday, while another 398 were delayed, according to industry tracker FlightStats.com. The site reported visibility at the airport Wednesday morning as zero.

Chicago Midway, meanwhile, saw 20 cancellations and 142 delays, while Lambert-St. Louis and Detroit Metropolitan Wayne each had about a dozen cancellations and many more delays.

A dense fog advisory issued by the National Weather Service stretched from southern Missouri to northern Michigan and Wisconsin, including Chicago, St. Louis and Milwaukee. The foul weather not only affected travel in the sky but also on the ground, with the National Weather Service claiming visibility was well under a quarter of a mile in many locations.

“This dense fog is affecting travel conditions on roadways, with multiple accidents being reported,” the agency said, adding that it expected the fog to gradually improve by the afternoon.

Delays on land and in the sky came on the eve of Thanksgiving in the U.S. and fell on one of the busiest travel days of the year. Chicago’s main airport, O’Hare, is a hub for both American Airlines and United Continental, the world’s largest airline. According to the Chicago Department of Aviation, nearly 1.8 million people are expected to travel through the city’s airports during the eight-day holiday travel period, which runs from Nov. 20 to Nov. 27.

If the fog subsides, travel should return to normal by Wednesday evening.