Dense fog socked the Chicago area overnight and became more dense thanks to light winds, clearing skies and a moist air mass, according to the National Weather Service.
A dense fog advisory expired at 12 p.m. for all Chicago-area counties in Illinois and Northwest Indiana.
"I called and my flight was on time when I left the house. So, that's disturbing to get here and to come an hour and 10 minutes early before, and now it's canceled," said O'Hare traveler Debbie Gallowitch.
One group of travelers was flying to Iowa, but their flight was scrubbed due to the fog. So, American Airlines arranged a bus instead.
"I'm not a happy camper. Won't travel again for Thanksgiving. Won't come again for Thanksgiving. I told my daughter we'll have our turkey in the summer time , but I'm never coming ever, ever again on Thanksgiving," traveler Dennis Pizzo said.
Just before 8 a.m. Wednesday --- on one of the busiest travel days of the year -- O'Hare and Midway saw more than 500 flights delayed or canceled. Travelers should check their airlines' websites before heading to the airport.
"We don't expect this to cause any kind of major ripple effect with delays throughout the day or even around the country. So, I think travelers who are heading out later today will be just fine," said Orbitz travel expert Jeanenne Tornatore.
Because of the weather, the FAA issued a temporary ground stop slowing flights into O'Hare causing 45 to 50 minute delays, but by afternoon the fog was starting to lift -- though not in time for people like Audrey Scranton, whose flight to Cedar Rapids was canceled.
"At first, I felt like crying because I have been traveling for the past two or three days, and I just want to get home after being away for four months," Scranton said.
Airport officials say domestic passengers should plan to arrive at least two hours before their flights are scheduled to take off.
Both major airports in Chicago are going to be busy over the next five days. According to the Department of Aviation, 1.8 million people will move through Midway and O'Hare over the long weekend.
Drivers also are being advised to allow extra time. AAA says 43.6 million Americans will be traveling 50 miles or more this holiday weekend. That's a slight increase from last year, and majority of travelers -- 90 percent-- are driving.
"Wednesday and Sunday are the biggest travel days of the week. So, if you can avoid the peak rush hours, maybe leave a little bit later or leave a little bit earlier than you had planned to avoid the congestion, then by all means to do so," said Beth Mosher of AAA.
Amtrak was busy Wednesday, as well, with a lot of hustle and bustle at Chicago's Union Station. Traditionally, the day before Thanksgiving has been Amtrak's busiest, but in recent years, people have been leaving for their holiday destinations earlier in the week.
Extra train cars have been added on many routes.
(The Sun-Times Media Wire contributed to this report.).