CHICAGO (CBS) — With a thick blanket of fog shrouding much of the city, Southwest Airlines has cancelled all flights in and out of Midway International Airport until 6 p.m. Thursday.

“Proactive cancelations allow our Customers either more time to reschedule their travel plans or the ability to reroute their trip, utilizing other connection points in our network to complete their travel,” Southwest Airlines spokeswoman Michelle Agnew said in an email.

Southwest said its employees were working to re-book affected customers.

The National Weather Service has issued a dense fog advisory for most of the Chicago area until 4 p.m., with visibility as low as a quarter mile or less at times.

Earlier in the day, the FAA had issued a ground stop for flights headed into Midway.

O’Hare International Airport also was reporting delays averaging 30 minutes on Thursday due to the fog.

Travelers using either airport should check their flight status before heading to O’Hare or Midway.