As the smaller of Madison's two biggest lakes, Lake Monona typically thaws sooner than its larger sibling, which is why it is now melting and turning into one big waterpark and will not support its large cast of ice fishermen much longer.
In contrast, just to the north on the other side of the isthmus, Lake Mendota is likely to continue providing a stable platform for this curious male bonding ritual a little longer. How much time do these guys have left on their big ice raft? The answer varies considerably from year to year. The median opening date for Lake Mendota is April 5, but last year it opened on March 27. In the past, it has opened as early as February 27 (1998), and as late as May 6 (1857).
Asking when the lake opens also raises a larger philosophical question: What do we mean by a lake being "open" and how do we measure it? The Wisconsin State Climatology Office provides some answers on their website, which explains that it's important to maintain consistency with observational methods that would have been available to 19th century observers, so the records can be meaningfully compared. Surprisingly, the methodology bears some relationship to the earliest date a case of beer can be transported from Picnic Point to Maple Bluff by boat.
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 Dept., now known as the Dept. 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.I find it reassurring that the underpinnings of the scientific method are so thoroughly rigorous.