Recent studies have shown that in most middle latitude areas the number of frost-free days, the date of the last spring-freeze and the date of first fall-freeze have been changing systematically. Increase in the concentration of greenhouse gases has been attributed as one of the causes responsible for these changes. Coupled atmosphere-ocean processes, such as the El Niņo-Southern Oscillation (ENSO), Pacific Decadal Oscillation (PDO) and Atlantic Multidecadal Oscillation (AMO) phenomena, might also influence frost occurrence. To further our understanding of the factors affecting the long-term variability of the frost-free season in the contiguous United States, we have examined the minimum temperature data from the U. S. Historical Climatology Network (USHCN). First, time series of the number of frost-free days, the date of the last spring-freeze and the date of the first fall-freeze in a calendar year, constructed from the daily minimum temperature data, were analyzed to determine whether the deterministic trend (or linear trend) or stochastic trend adequately describes the long-term variability exhibited in them. After ascertaining the nature of the temporal variability, time series were subjected to the principal component analysis to determine the spatial pattern associated with the long-term variability of the frost-free season. Since a component of the long-term variability in these time series is related to corresponding fluctuations in the global surface air temperature, the principal component analysis was repeated after controlling for the effect of the later. Results of these analyses will be presented at the conference.