Time series of freeze-free season anomalies

Freeze-free season anomalies shown as number of days per year. Length of the freeze-free season, 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.