Fog was causing major problems at the Chicago and Detroit airports Thursday afternoon. Conditions at Chicago Midway airport were so bad that Southwest Airlines had grounded all of its flights there through 6 p.m. local time.

"We have proactively cancelled flights to and from Chicago (Midway) Airport through 6:00 p.m. local time due to fog," Southwest spokeswoman Michelle Agnew said in a statement e-mailed to USA TODAY's Today in the Sky blog. "Our employees are working diligently to help reaccommodate impacted customers."

Nearly 200 combined departures and arrivals had been canceled there as of 3:30 p.m. ET, according to flight-tracking service FlightAware. That represented about a third of the entire day's schedule at the airport.

Most of those were on Southwest, which accounts for about 95% of the passenger traffic there.

"Chicago (Midway) serves as a large connection point for our customers throughout the system," Agnew added. "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."

Cancellations were less of a problem at Chicago's much-busier O'Hare airport, but delays were piling up as of mid-afternoon. Nearly 300 departing flights and another 300 arrivals were running behind schedule as of 3:30 p.m. ET, according to FlightAware. That meant nearly a quarter of all flights at O'Hare were running late.

The flight disruptions came as the National Weather Service had issued a dense fog advisory for Chicago. At 10:30 a.m. local time, visibility was just 1/8 of a mile at both airports, according to the NWS. The advisory was scheduled to remain in effect until 4 p.m. local time.

Fog and poor visibility were a problem across large parts of the Great Lakes region Thursday.

In Detroit, where a fog advisory also was in effect, more than 250 arrivals and departures were delayed. That accounted for about 25% of all of Thursday's flights at the airport, a major hub for Delta. About 5% of the day's schedule had been canceled.

There also were visibility problems in the Northeast, where cloudy weather and scattered fog were affecting some areas.

Flight delays were running above normal at the New York City's delay-prone Newark and LaGuardia airports. At Newark, more than 10% of the day's flights were behind schedule, FlightAware calculated. At LaGuardia, delays had affected a little less than 10% of the day's schedule.