It was the first Fog Bowl - December 10, 1961. The Eagles played the New York Giants in a huge game at Franklin Field. There were 60,671 people in the stands and for much of the day, they had a hard time following the action. I know because I was there.
The fog wasn't quite as dense as the fog in Chicago that turned the Eagles-Bears 1988 playoff game into a whiteout, but it was pretty thick and, unlike the Chicago fog, it was there from the start of the game. Our family's seats were in Section EE behind the end zone at Franklin Field and we could not see Weightman Hall, the athletic administration building, at the opposite end of the field.