Variability and Trends in Temperature Threshold Exceedences and Frost Dates in the U.S

The Intergovernmental Panel on Climate Change report (IPCC 1995) suggests that if the climate changes due to human influences, more extremes of temperature and precipitation can be expected. In this paper trends in temperature extremes in the USA are examined. There have been relatively few studies of temperature extremes in the USA. DeGaetano (1996) found increases in the number of days where the temperature exceeded 32C (90F) in the northeastern USA. Furthermore, Cooter and LeDuc (1994) found that the frost-free season in the northeastern USA has increased by almost 11 days since 1950. One of the major problems with conducting an analysis of long-term changes in temperature extremes is a lack of digital data prior to 1948. Currently, there are a limited number of Cooperative stations with digital daily data for the early part of the 20th century. Recently the NCDC has started a project to digitize all data in the archive that is not in digital form. One of the first priorities has been to digitize daily data for stations in the United States Historical Climatology Network (USHCN). The first results from this effort has effectively doubled the number of stations with long-term (century long) daily data, going from 187 to 310 stations. These data are used to examine long-term variability and trends in temperature extremes and number of days exceeding various thresholds in the U.S. First results indicate that over the entire U.S. there has been a decrease in the number of days below freezing, with the change concentrated in winter and spring. Results for changes in the first fall freeze date, and last spring freeze date are consistent with the changes in days below freezing, with no change in the first fall freeze date, but a trend to earlier last spring freezes, with a subsequent increase in the frost-free season.