The advisory is in effect for Boone, Cook, DuPage, Lake, Kane, Kankakee, Kendall, LaSalle, McHenry, Livingston and Will counties until 4 a.m. Tuesday.
The fog is reducing visibility to .25 miles or less and creating hazardous driving conditions due to low visibility.
A ground stop for arriving flights was in effect earlier in the evening at O'Hare International Airport. As of 9 p.m. the FAA said the airport was experiencing general departure delays 16 and 30 minutes due to the low ceilings caused by the fog.
Drivers should slow down, use headlights and leave plenty of distance ahead of them in the fog.