According to the neat WGN/Trib Weather Blog (a compilation of the various weather features and graphics that dominate nearly a full page of the Tribune daily), the mild winters of late haven’t just been a fluke compared to the storied brutality of the late 1970s: the number of sub-zero days in Chicago has fallen from 15.4 days a year in 1975-79 to 4.1 days a year in 1990-99.