A spokeswoman for the Chicago Department of Aviation says foggy conditions forced airlines to cancel about 200 flights at O'Hare International Airport Thursday morning.
Airlines at Midway International Airport are reporting 80 cancellations there due to the fog.
Dense fog also made it hard for some motorists to read road signs.
With warm temperatures and more snowmelt anticipated Friday, foggy conditions are expected again.
The Associated Press contributed to this report.