Freeze, Then Thaw, Then Freeze? Lake Ice A Result of More Than Just Cold Weather

On the morning of Wednesday, December 19th, I got off the bus at University Avenue and began my walk north on Park Street – under the pedestrian bridge proclaiming “University of Wisconsin – Madison,” past Bascom Hill and toward the shores of Lake Mendota. It was a pleasant stroll, as the overnight lows had barely hit freezing, bottoming out at 32 Fahrenheit, and temps that morning were already pushing 40.

As I approached the end of Park Street, though, something seemed off. The surface of the lake looked flat and calm, despite the gentle breeze at my back. And my eyes weren’t deceiving me, the surface of Lake Mendota was flat and calm. It was also frozen. 

When I got into the office, I learned that the Wisconsin State Climatology Office had made the official announcement days earlier. Sometime during the early morning hours of December 15th, a thin layer of ice had formed across the middle of the lake – from Picnic Point to Maple Bluff. Mendota’s official freeze date was added to the 166-year record of “ice on” dates.

A thin layer of ice crusts Lake Mendota’s surface on December 19th. Photo: A. Hinterthuer

For most people living in Madison, the idea that a lake can freeze over during a week of highs in the 40’s is hard to grasp. But it goes to show that daytime air temperature isn’t the only factor playing a role in the process. 

During the week Lake Mendota froze, we had partly to mostly sunny days with above average December temperatures. But, when the sun set, those temps routinely fell into the low 20s. 

And, since the water is the substance actually doing the freezing, we have to take a look at its temperature as well. 

When we pulled our buoy out of the lake in November, water temperatures were already well on their way toward the freezing point of 32 Fahrenheit. Even though the sun had spent a few cloudless days shining down on Mendota in the days leading up to December 15th, it wasn’t doing much to raise the water temperature. In the summer, these calm, sunny days result in a lot of heat building up in the surface of the lake. But, in winter, (as everyone in the upper Midwest knows) the sun’s rays aren’t quite so potent. The sun rises later and sets earlier and, when it does get up in the sky, it’s rays aren’t hitting us as directly as they do during summer and we end up getting a lot less heat from our closest star.

If you want a fuller explanation of this phenomenon here’s a cool video from the California Academy of the Sciences. 

Of course, the sun does this every winter (it’s why we have winter) and water temps hover around freezing most years during December. Those are pretty standard variables in the lake freezing process. The wild card this time around, as it often seems to be, was the wind.

Or the lack thereof. 

On the evening of Friday, December 14th the wind suddenly died down. Completely. By the time the evening commute was over, there was no recorded wind at all and temperatures had dropped to low 20’s. As the still surface of Mendota sat exposed to the colder air, the top of the lake began cooling toward freezing and a condition known as “ice fog” developed.

By the time the fog cleared the next morning, we had a frozen lake.

Anatomy of a lake freeze – no wind on Dec. 14th allowed Lake Mendota to ice over. 

But, where the (absence of) wind giveth, the wind can also take away. 

When a lake is fully frozen over, of course, wind can howl across it, doing little to disturb its winter cover. But give that wind the tiniest crack and it can transform the lake. In fact, I heard this happening as I left the office on Thursday, December 20th.

Sometime during the day – thanks again to those above average temperatures and a light rain, the thin ice out in the middle of the lake warmed just enough to give way to a small band of open water. During the afternoon, a northerly breeze developed, blowing straight at Hasler Lab. And as that wind picked up steam, it began to also pick at the exposed edge of the ice. 

I couldn’t see it happening, but I heard the distinct musical tinkling of ice as small plates of it broke off and began pushing toward Lake Mendota’s south shore. 

On the morning of Friday, December 21st, I got off the bus at University Avenue and began my walk north on Park Street – under the pedestrian bridge proclaiming “University of Wisconsin – Madison,” past Bascom Hill and toward the shores of Lake Mendota. It was an unpleasant stroll, as a 14 mile-per-hour wind blasted me full-on and temperatures began to plummet to afternoon highs well below freezing. 

As I approached the end of Park Street, everything had changed. The surface of the lake seethed and swelled – the tinkling of broken ice mixed with a howling wind and crashing waves. 

Six days after it officially froze, Lake Mendota was again open water. 

Now we enter 2019 with a 10-day forecast of weather similar to mid-December when Lake Mendota froze and thawed. Maybe on the next still night, we’ll get Act Two. And maybe this time the ice will stick around. 

3 thoughts on “Freeze, Then Thaw, Then Freeze? Lake Ice A Result of More Than Just Cold Weather”

  1. The average number of years with an ice on duration >99 days is about 15 in each of the 25 year periods prior to the 1976-2000 period, when the number fell to 8 years with ice on duration >99. There have been 5 years in the past 18 with an ice on duration >99. Will there be 3 more years in the next 7 with ice on days >99?

    1. Also, if Mendota ices over in December, there is a 56% chance the duration will be >99 days.
      If it ices over in January there is only an 11% chance that the duration will be >99. Makes sense.
      In the 20 year period from 1950 to 1969 the lake iced over in January only 10% of the years.
      In the last 20 years the lake iced over 50% of the years in January. Climate change is real!

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