Break-up as a function of freeze-up the year before (1981–2014). The relationship between median freeze-up date (x-axis) and median breakup date of the following year (y-axis) is shown for 1981 to 2014. The slope of each regression line is significant at the 99% confidence level (p < 0.01). Slope units are days day–1. Characteristics of the datasets and regression lines are displayed in Table 4. DOI: https://doi.org/10.1525/elementa.130.f9