(CNN) Dense fog in the Chicago area is causing significant delays and cancellations for Chicago's O'Hare and Midway airports.

The Federal Aviation Administration issued a ground stop for flights at Chicago's O'Hare International Airport because of fog on Tuesday morning.

All flights heading for O'Hare were being held at their origin until 9:30 a.m. CST due to the fog, according to FlightAware . So far, more than 130 flights out of O'Hare and more than 150 flights into O'Hare have been delayed, according to FlightAware

The dense fog caused about 70 flights into and out of Midway International Airport to be canceled, the airport said