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A newly available data set of daily temperature observations was used to study the temporal variability of the frost‐free season, based on an inclusive 0°C threshold, for 1895–2000 in the conterminous United States. A national average time series of the length of the frost‐free season is characterized by 3 distinct regimes. The period prior to 1930 was notable for decreasing frost‐free season length from 1895 to a minimum around 1910, followed by a marked increase in length of about 1 week from 1910 to 1930. During 1930–1980, frost‐free season length was near the period average with relatively little decadal‐scale variability. Since 1980, frost‐free season length has increased by about 1 week. The national average increase in frost‐free season length from the beginning to the end of the 20th Century is about 2 weeks. Frost‐free season length has increased much more in the western U.S. than in the eastern U.S.
Kunkel et al. (2004) studied this question.