Time series of freeze-free season anomalies

Time series of freeze-free season anomalies shown as the number of days per year, for the contiguous U.S. (top, green), eastern U.S. (bottom, blue) and western U.S. (bottom, red). Length of the freeze-free season is defined as the period between the last occurrence of 32F in the spring and first occurrence of 32F in the fall. The dashed line is a linear fit. Based on daily COOP data from long-term stations in the National Climatic Data Centers Global Historical Climate Network data set. Only stations with less than 10% missing daily temperature data for the period 1895-2011 are used in this analysis. Freeze events are first identified for each individual station. Then, event dates for each year are averaged for 1x1 degree grid boxes. Finally, a regional average is determined by averaging the values for the individual grid boxes. There is an overall statistically significant upward trend for the contiguous U.S., as well as for both the eastern and western U.S.