Fog descends upon Chicago's O'Hare International Airport as holiday travel begins Wednesday, Nov. 21, 2012. Thanksgiving travelers encountered cancellations and delays at Chicago's airports on Wednesday as extremely dense fog settled over the region. About 90 inbound and outbound flights were canceled and more than 400 flights were delayed at O'hare and Midway airports as of around 7:45am. (AP Photo/Charles Rex Arbogast)

Associated Press
Fog descends upon Chicago's O'Hare International Airport as holiday travel begins Wednesday, Nov. 21, 2012. Thanksgiving travelers encountered cancellations and delays at Chicago's airports on Wednesday as extremely dense fog settled over the region. About 90 inbound and outbound flights were canceled and more than 400 flights were delayed at O'hare and Midway airports as of around 7:45am. (AP Photo/Charles Rex Arbogast)
Fog descends upon Chicago's O'Hare International Airport as holiday travel begins Wednesday, Nov. 21, 2012. Thanksgiving travelers encountered cancellations and delays at Chicago's airports on Wednesday as extremely dense fog settled over the region. About 90 inbound and outbound flights were canceled and more than 400 flights were delayed at O'hare and Midway airports as of around 7:45am. (AP Photo/Charles Rex Arbogast)
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