Airlines delayed almost 1,280 flights and canceled about 220 others after heavy fog blanketed Chicago airports at the start of the Thanksgiving holiday travel rush, industry researcher reported. The slowdown persisted even as the morning fog lifted, according to the Chicago Department of Aviation. Chicago is a linchpin in the U.S. air-traffic network, especially for east-west flights, and the aviation department predicts almost 1.8 million travelers will use the two airports in the eight-day Thanksgiving travel period ending Nov. 27. O'Hare is the second-busiest U.S. airport, after Atlanta's Hartsfield, and is a United and American hub.