The change in average summer, winter, and annual temperatures of 19 Minnesota stations was investigated. Records were studied for a uniform number of years for comparative purposes as well as for the total period of record. Seven of the stations were believed to qualify as climatic reference stations, 11 had records of intermediate reliability, and one was included solely for its long record dating from 1819.
Based on the linear-regression equation, the climatic reference stations showed an average increase in summer (1900 to 1958), winter (1900 to 1958), and annual (1900 to 1957) temperatures of 1.4, 3.7, and 1.3F, respectively. These rises were equivalent to 2.4, 6.3, and 2.2F per century. Stations of intermediate value generally did not show such large increases. Slopes of the temperature regression lines were generally significantly greater than zero for the reference stations and for a few of the others. Records for longer periods showed similar results.
The St. Paul record, originating in October 1819, indicated that the central date of the smoothing period in which summer, winter, and annual temperatures reached a minimum was in 1863, 1854, and 1865, respectively. The latter date appeared to be anomalous compared to east coast records. Recent summer, winter, and annual maxima were obtained in 1934, 1943, and 1933–34, respectively. Winter maximum temperatures at some other Minnesota stations occurred in the early 1930's.
1 To be submitted as University of Minnesota, Agricultural Experiment Station Paper No. 1011, Miscellaneous Journal Series.