Research has led to a good documentation of the frequency and seasonality of sudden warmings: just over half of the winters since 1960 have experienced a major warming event in January or February" [REF].
A 2001 study found that "there is no apparent trend toward fewer extreme cold events in Europe or North America over the 1948-99 period, although a long station history suggests that such events may have been more frequent in the United States during the late 1800s and early 1900s."[REF]