Figure 1.

Regression analysis of the date of ice-melt from Lake Mendota over the years of phenological records. (A) The Julian calendar day of ice-melt is plotted against the year. The regression indicates a change in earliness of −0.124 day per year (R² = 0.046). (B) the Julian calendar day of ice-melt is plotted against the average temperature for the month of march. The regression in this case indicates a change in earliness of −2.719 days per degree of March temperature (R² = 0.453). Data on ice break-up are from State Climatology Office, 1999: Lake Mendota Ice Summary, 1853–1999, Electronic database appearing at http://www.uwex.edu/sco/icemend.html, Wisconsin Geological and Natural History Survey, Madison. Data for Madison temperatures are from National Climatic Data Center, Local Climatological Data for Madison, WI, at the Environmental Data and Information Service, National Oceanic and Atmospheric Administration, Asheville, NC.