A thick fog arrived on the afternoon of Friday, Dec. 10, 1965, leaving Chicago blanketed in its wake. The disgusting murk made vision a nightmare. Chicago's downtown skyscrapers were invisible from the city's streets, while hundreds of planes were grounded at O'Hare and Midway.
Weather of that type is rare during the winter, but the mix of a warm Southern breeze and chilly Midwestern temperatures caused the dense air to hang. The next day, visibility improved, but the gray sky stayed. Flights were cleared to come and go, including a cross-country airliner carrying the San Francisco 49ers into Chicago for Sunday's NFL Western Division matchup against the Bears.