Variation of the Koppen C/D climate boundary in the central United States during the 20th century

The spatial and temporal variation of the C/D Koppen climate boundary in th e central United States is examined for the period 1900 to 1999. Mean Janua ry temperature data from the U.S. Historical Climatology Network for 67 sit es located between 37 degrees N and 41.5 degrees N latitude and 90 degrees W and 100 degrees W longitude are utilized. The variation of the boundary b etween the C and D climates (i.e., 26.6 degrees F isoline) is illustrated f or the entire 100-year study period and four quarter-century periods, as we ll as for individual decades (1900s, 1910s, .... 1980s, 1990s). For the qua rter-century climatic periods, the latter two (1950 to 1974, 1975 to 1999) had C/D boundaries farther south (implying "colder" winters) compared to th e positions for the first two quarter-century periods. The most anomalous f eature for the decadal maps is the distinct southerly location of the C/D b oundary for the recent decade of the 1970s. Although the C/D boundaries for the decades of the 1980s and 1990s generally are located slightly north of the 100-year overall mean location (implying "warmer" than average winter conditions), several earlier decades (e.g., 1900s, 1920s, 1930s) had even m ore northerly positions. Therefore, this study does not provide evidence of a trend toward wintertime warming and a northerly migration of the UD clim ate boundary within the central United States.