Early Wednesday, at least 65 flights were canceled at Chicago O'Hare International Airport and 25 flights were canceled at Midway Airport, the Chicago Tribune reported. Officials said at least 300 flights had been delayed at O'Hare and 107 at Midway.
"Both [airports] are reporting visibility near zero," National Weather Service meteorologist Gino Izzi said. "It's going to be an absolutely awful morning, and will probably go into the afternoon at most airports."
Travel delays at the Chicago airports could create a rippling effect elsewhere.
A dense fog advisory was in effect for northern and central Illinois, as well as most of Missouri and Wisconsin, eastern Iowa and northwest Indiana.
"If it does improve, it probably won't be fast," Izzi said. "We'll probably be sitting in the pea soup through most of the morning hours."