The widespread dense fog is expected to gradually improve Sunday afternoon after visibility was less than a quarter mile in some areas of Chicago earlier in the day, according to the National Weather Service.

       Weather service officials issued an advisory on the fog early Sunday suggesting drivers slow down, increase driving distance and use low beams while on the roads.

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       Several Southwest Airlines flights departing from Midway International Airport were canceled due to the fog, affecting Christmas Eve travel.

       In a statement, Southwest Airlines said the airline cancelled around 58 flights for Sunday, which is less than 1% of total flights system-wide.

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       “We’re working with our Customers whose travel is impacted by fog at Chicago Midway that began Saturday night and prevented inbound aircraft from landing, forcing some diversions and subsequent flight cancelations,” Southwest Airlines officials said.

       “With visibility remaining below required operational minimums throughout the night and continuing this morning, we’ve modified our planned start for today (Sunday) at Chicago Midway,” the company said. “We have all-hands on deck as our Employees are working quickly to take care of our Customers and accommodate them on alternative flights.”

       Midway is the airline’s fourth busiest airport operation, with more than 200 departures a day scheduled over the holiday weekend.

       At O’Hare International Airport, 159 flights were delayed Sunday and 11 were canceled, according to flight-tracking website Flight Aware. Afternoon conditions at both O’Hare and Midway are expected to be misty and foggy at 51 degrees.