Travelers at O'Hare International Airport saw about 35 minute delays around 6:45 a.m. At Midway Airport, it was closer to 40 minutes.
The fog delayed schools in parts of central and northern Indiana for dozens of school districts. Most schools that issued delays pushed back their openings by two hours.
The weather service says the combination of unseasonably moist air and light winds have caused fog that can reduce visibility to a quarter mile or less in some locations. The dense fog advisory was in effect until 8 a.m. for most parts of the Chicago area. It remains in effect for parts of southern Wisconsin.
Breezy and cloudy conditions with a chance of an isolated thunderstorm Monday morning are expected Monday. This afternoon, temperatures could reach a record high in the mid-60s.
Winds could reach 15 to 25 mph, with a possibility of gusts around 35 mph.