it-has-been-cold-as-a-mutha factor
This is the opposite of a wind-chill factor.  When it has been cold for a long time, and then there is a mild winter day, it feels a lot warmer than it is.
Today is February 10.  In Cleveland, it is 50 degrees, but with the it-has-been-cold-as-a-mutha factor it feels like 75.