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Temporal Trends in the Lengths of Eastern and Western U.S. Frost-Free Seasons
Kunkel, K.E., Easterling, D.R., Hubbard, K. and Redmond, K. 2004. Temporal variations in frost-free season in the United States: 1895-2000. Geophysical Research Letters 31: 10.1029/2003GL018624.

What was done
Using recently digitized data from 794 stations of the U.S. National Weather Service's Cooperative Observer Network, the authors analyzed characteristics of the frost-free seasons of the eastern and western United States over the period 1895-2000, splitting the country at 100W longitude.

What was learned
Over the course of the 20th century, the mean length of the frost-free season over the contiguous United States increased by approximately two weeks. This increase occurred primarily over two separate time periods:1910-1930 and 1980-2000. Changes in the spring of the year were more significant than changes in the fall, while changes in the west were greater than changes in the east.

What it means
The authors note that the two periods of major frost-free season extension are "broadly consistent with the timing of periods of increases in U.S. mean annual temperature." However, as noted by Knappenberger et al. (2001), a greater portion of the observed warming of the past century has been expressed in minimum as opposed to maximum (and, therefore, mean) temperatures; and alluding to this fact, Kunkel et al. note that "a lowering of the range in diurnal temperature is typical of air masses that carry more moisture (Hubbard et al., 2003)." This observation leads them to conclude that "it is possible that the shorter growing seasons in the early 1900s were characterized by drier air masses ? while the longer growing seasons in recent years may be associated with air masses that are moister in comparison," concluding, however, that "we can only speculate at the moment."

Hubbard, K.G., Mahmood, R. and Carlson, C. 2003. Estimating daily dew point temperature for the Northern Great Plains using maximum and minimum temperature. Agronomy Journal 95: 323-328.

Knappenberger, P.C., Michaels, P.J. and Davis, R.E. 2001. Nature of observed temperature changes across the United States during the 20th century. Climate Research 17: 45-53.

Reviewed 3 March 2004