Scrooge did not recognize the fog surrounding him. J. G. Duesing writes,
When Scrooge is first greeted by the caroling of “God rest ye merry, gentlemen,” he responds such that “the singer fled in terror, leaving the keyhole to the fog.” Dickens’s fog is not dismal or dark, Chesterton says, but rather something that draws in and, in the case of Scrooge, corners. Fog “makes the world small …”
He describes how this plays to the particular comfort of Christmas and the light that has pierced the fog of the world.