Winter range in temperature is defined as the difference between the hottest and coldest temperature during the December 1 through February 28 (29) time period each year.  Climatologists define the seasons on a monthly basis so Winter is defined as December 1 through February 28 (29), and NOT  December 21 - March 21. 

The larger the number on the graph, the larger the range or difference in temperature during that season. 

There are two time series graphs, with the 2nd one showing a recent trend line.