The Wisconsin State Climatology Office maintains a list of ice-over and ice-out dates for three Madison-area lakes — Mendota, Monona, and Wingra. These records, extending back into the mid-19th century, are based upon the observations made by various people, including those at Washburn Observatory on the University of Wisconsin–Madison campus.
How Freezing and Thawing Dates are Determined
The rules of opening and closing determination have been handed down by oral tradition, so there is some question how scientific the dates have been arrived at over the years. Our vantage point on the 13th floor of the Atmospheric and Spaces Sciences building on campus (1225 W. Dayton St.) does provide a fair view of the three lakes. However, closer inspection of the lakes from various vantage points is made.
Lakes Monona and Wingra have a general “50 percent covered” rule, but often a mere half-iced lake does not look convincingly closed (or open, as the case in the spring), so a bit of admitted subjectivity is involved. Thus the vantage point of the foot of South Few Street is the prime central location on Lake Monona for asking, “does this lake look ice-covered or open?” but consideration from other points is taken as well, such as from the causeway, Monona Terrace, and the northeastern end.
Lake Wingra is most often observed from Vilas Park and sometimes from within the UW Arboretum. In addition, the date is not fixed until the lake has remained closed at least a full day, so until the ice cover persists into a second day, neither a date of closure nor the beginning of ice cover duration is called.
Determining the opening and closing dates for Lake Mendota is more of a challenge because the length and shape of the lake would require a sufficiently high vantage point that was not readily available to 19th century observers. Partly because Lake Mendota has a more irregular shoreline, an important secondary criterion applies for that lake: whether one can row a boat between Picnic Point and Maple Bluff.
This rule arose from the era of E.A. Birge and Chancey Juday (according to Reid Bryson, founder of the UW meteorology department, now known as the Department of Atmospheric and Oceanic Sciences), because they frequently were out on the lake in a rowboat, and the ice along that line determined if they could transport a case of beer over to their friends in Maple Bluff.