Data include dates of freeze-up, thaw, and ice duration of three primary lakes in the Madison area (Lake Mendota, Lake Monona, and Lake Wingra). Data are obtained from the State Climatologist. For Monona and Wingra, the freeze date (ice on) is defined as the first date on which the water body is observed to be completely ice covered, and the breakup date (ice off) is the date of the last breakup observed before the open water phase. Ice duration is the number of days that a water body is completely covered with ice and excludes any period during which the lake thawed in mid-winter before freezing again. For Mendota, duration is calculated in the same manner, but ice on and ice off dates are assigned as follows: The lake is deemed frozen if it has solid ice from Picnic Point to Maple Bluff and total ice cover is greater than 50 %. The lake is deemed open if it is ice-free from Picnic Point to Maple Bluff and total ice cover is less than 50%. Sampling Frequency: annually. Number of sites: 3.