One of the signals expected with greenhouse warming is a change in what are now considered extreme temperatures. In this paper one type of extreme is examined for the 1948–99 period, that is, a change in the number of days when the minimum daily temperature dips below freezing or “frost days.” This is approached by looking at two questions: 1) have there been changes in the number of frost days per year, or per season, and 2) are there trends in the dates of the first autumn frost, last spring frost, and length of the frost-free season? Results show that the country as a whole has experienced a slight decrease in the number of days, with the biggest decreases in the winter and spring. Changes in frost dates for autumn show small changes to a later date, but the date of the last-spring freeze shows a distinct move to an earlier date. This results in an increase in the frost-free season. However, there is a distinct spatial pattern to the results that is consistent with the spatial pattern of annual temperature trends for the twentieth century. The geographical pattern shows the western United States with the largest decreases in frost days, and increases in frost-free season length. But the southeast United States, which is one of the few areas of the world showing cooling over the twentieth century, has no significant changes in the number of frost days or the frost-free season.
National Climatic Data Center, Asheville, North Carolina