Flights bound for Chicago's O'Hare International Airport were delayed an average of nearly four hours Tuesday morning because of dense fog.
The National Weather Service said fog and mist reduced visibility at the airport to about two miles as a rainstorm moved through the area.
According to the flight tracking website FlightAware.com, flights scheduled to arrive at O'Hare were being delayed at their points of origin by an average of three hours and 49 minutes.
Around 170 flights to and from the airport were canceled.
The tracking site reported no delays for inbound or outbound flights at Chicago's Midway International Airport.