Lake Mendota Ice, Winter by Winter

Every Winter

Download the data (CSV) · 499 rows · checked 2026-10-10

What We Found

• 101Average days of ice, 1855-56 to 2025-26
• 76Average, last 10 winters
• 161Most days, 1880-81
• 21Fewest days, 2001-02

• Mendota's ice season is 42 days shorter than in the early record. It averaged 118 days in 1855-1884, 85 days in the 1991-2020 winters, and 76 in the last 10.(SCO)
• It freezes later and opens earlier. The median freeze-over moved from Dec 13 in 1855-1884 to Jan 1 in the last 10 winters; the median ice-out moved from April 13 to March 21.
• Record winters: 1880-81 had 161 days of ice, frozen Nov 23, 1880 to May 3, 1881. 2001-02 had 21, across two short frozen spells. 2023-24 was the second shortest at 44 days.
• Some winters freeze twice. In 8 winters Mendota froze, opened and froze again. Days of ice count only the frozen days, so they can be fewer than the span from first freeze to final thaw.
• Last winter, 2025-26: Mendota froze Jan 1, 2026 and opened March 13, 2026, 71 days. Monona had 89 days.(SCO)
• Monona averages 103 days over its record since 1851-52; its shortest was 44 days in 2023-24.(SCO)

The Last 10 Winters

Average Days of Ice on Mendota, by Era

• 1855-84118 days
• 1885-14104 days
• 1915-44102 days
• 1945-74104 days
• 1975-0491 days
• 2005-2582 days

How the Dates Are Called

The State Climatology Office keeps the record, which goes back to the mid-1800s and draws on observers including the UW's Washburn Observatory.(State Climatology Office) The rules came down by tradition, so the office notes there's some judgment involved.(State Climatology Office)

• Mendota: because of its size and shape, a second test applies: whether you could row a boat between Picnic Point and Maple Bluff.(State Climatology Office)
• Monona and Wingra: a general "50 percent covered" rule, checked from several spots around each lake.(State Climatology Office)
• A full day: a date isn't fixed until the lake has stayed closed at least a full day.(State Climatology Office)

The record doesn't say whether ice is safe to walk on. For the lakes' water levels and flooding, see Dane County flooding and lake levels.

Every Winter on Mendota

Winters with a complete record. Tap a heading to sort. The CSV also has Wingra and the incomplete winters.

How We Did This

• Data: the State Climatology Office's freeze and thaw tables for Mendota, Monona and Wingra, read 2026-10-09.(Mendota)(Monona)(Wingra)
• One row per winter. When a lake froze more than once, we kept the first freeze-over, the final thaw and the office's total days of ice. Winters missing a date or a total are left out of our averages (Mendota 1852-53 to 1854-55). Wingra's record has many winters without a total, so we report it only in the CSV.
• Averages and medians are ours, computed from those tables. The office's own summary puts Mendota's average at 102 days, its longest at 161 days (1880-81) and its shortest at 21 days (2001-02).(SCO)

Questions

Is Lake Mendota frozen?

The State Climatology Office announces freeze-over once ice stays a full day; its page is the place to check. Last winter (2025-26) Mendota froze Jan 1, 2026 and opened March 13, 2026. In the last 10 winters the median freeze-over date was Jan 1.

When does Lake Mendota freeze over?

Over the full record the median freeze-over date is Dec 21; for 1991-2020 it was Dec 28. Ice-out has a median of April 3 over the record and March 27 for 1991-2020.

Is the ice safe to walk on?

This record says when the lake as a whole closed and opened, not whether the ice is safe anywhere.

How is freeze-over decided?

For Mendota, a key test is whether a boat could be rowed between Picnic Point and Maple Bluff. Monona and Wingra use a general 50% covered rule. A date isn't set until the lake has stayed closed at least a full day.

Related Guides

• Madison Snowfall History
• Flooding and Lake Levels
• Frost Dates and Frost Depth
• Ice Dams
• Weather and Hazards