The winter of 1997-1998 was one of the warmest on record. Winter is often defined by climatologists as the months of December, January, and February ("meteorological winter"). The Twin Cities experienced unusually mild temperatures in each of these months. The average temperature for December, 1997 was 26.9 degrees F, which is 9.0 degrees above normal. January's average temperature was a mild 19.1, above the norm by 7.3 degrees. The month of February was extraordinarily warm, averaging 31.9 degrees, exceeding the normal by 14.0 degrees. The temperature for the 1997-1998 meteorological winter (December - February) averaged 26.0 degrees, which makes this Winter the second warmest of the modern record, and the third warmest of the overall record.
The warmest December - February temperatures in the Twin Cities record
Another measure of the warmth of this winter season is the number of days that the temperature matched or exceeded the historical average ("normal"). Preliminary Twin Cities data indicate that 83 days were at or above normal for the December 1997 - February 1998 season. This is the greatest number of at or above normal winter days found in the modern (1891 - present) record.
The list below presents the winter seasons with the greatest number of days at or above normal. Daily data from these years were compared to the normals generated for the period 1961 through 1990.
Twin Cities winter days => normal season days 1989-90 64 1994-95 64 1959-60 65 1952-53 66 1931-32 66 1920-21 69 1991-92 69 1943-44 70 1982-83 72 1918-19 72 1986-87 77 1930-31 79 1997-98 83
It is of interest to note that the record LOW number of days above normal is 26 days which occured in 1935-36, followed closely by 27 days in 1977-78 and 1916-17.
The 1997-1998 data offered above are preliminary and subject to change. However, it is reasonable to expect the final data summary to indicate that the winter of 1997-1998 will rank with the warmest winters in Twin Cities history. An up-to-date daily summary of Twin Cities climate data is available from this Web site.
Warm often means Cloudy
Warm winter weather is often associated with cloudiness. Clouds absorb radiation emitted from the earth's surface during the long winter nights, and return this energy to the surface. This phenomenon acts as a warming blanket. Thus, overnight temperatures do not drop nearly as far on cloudy nights. This was certainly the case during the warm winter of 1997-1998. Many all-time "maximum low" temperature records were set. Thus, in spite of the fact that few all-time maximum high temperature records were set, this Winter was among the warmest ever.
Dave Ruschy, of the University of Minnesota - Department of Soil, Water, and Climate, maintains a climate observatory that measures incoming solar radiation, an excellent indicator of cloudiness. The average daily incoming radiation measured at this site for this winter (December, 1997 - February, 1998) was 123.2 Langleys, the lowest winter value in the 35 year record. This compares with a long-term average of 168.6 Langleys.
Another measure of cloudiness is the statistic, "Percent Possible Sunshine". This measurement was made by the National Weather Service from 1915 until 1996. These values depict the amount of time that direct sunlight that was observed during the day, compared with the total possible sunlit hours. The solar radiation measurements made by the U of M correlate highly with Percent Possible Sunshine values. Therefore, estimates of Percent Possible Sunshine from solar radiation data were created for this winter season. Using these estimates, we find that the average daily percent possible sunshine was 34% for December, 1997 - February, 1998. This makes the winter of 1997 - 1998 second only to 1953 (32%) as the cloudiest winter on record. The long-term average daily Percent Possible Sunshine for December - February is 50%.
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Last modified: February 15, 2002