With temperatures over 40 degrees in the forecast, the ice covering your neighborhood lake may not be long for this world.
As the sun shines more directly and the temperatures warm, March through May in Minnesota is generally ice out season, when the ice covering lakes thins, cracks and then disappears, leaving open water.
Just when that happens depends on a lot of different factors: the weather, of course, which changes by latitude and from year to year, but also things like how big a lake is.
How lake ice melts
Minnesota’s lakes spend most of the winter covered in snow, which insulates them from the sun’s rays and reflects that light back up toward the sky, said Pete Boulay, assistant state climatologist.
Once temperatures get above freezing, though, that snow starts to melt, allowing light to penetrate the ice.
“The ice acts like a greenhouse, so the water beneath it begins to warm and that’s how it thaws — it thaws from the bottom up,” he said. “That’s how it gets thinner.”
Meanwhile, the shoreline starts to warm, creating separation between ice and land. As temperatures warm and precipitation turns from snow to rain, that rain can also accelerate the melting of lake ice, Boulay said.
As ice melts, it turns into what are called “candles,” long, vertical ice crystals. Melted water fills in the spaces between crystals, and they break apart. Wind can assist in the deicing process, too, breaking up chunks and blowing them around. When wind blows crystals around, they make a jingling sound as they collide.
Ice out data
The Minnesota Department of Natural Resources has a big dataset on ice out dates, with some data points going back to the 1800s. This dataset has been compiled from many different sources over the years: from newspaper clippings, DNR and MPCA observations and records from citizen scientists.
The records for Lake of the Isles include a set of observations with a note indicating they were written on a cabinet at a house by the lake. Boulay said he drove over to that address and took those dates down after the DNR got a call from a realtor selling the property.
The records can be spotty: they include multiple observations from some lakes, some years, and no observations for lakes other years. But looking at some of Minnesota’s more observed lakes gives an idea of how much ice out can vary.
Lake Pepin, a Southeastern Minnesota lake that’s part of the Mississippi River, has iced out as early as Feb. 29 (2000), and as late as May 20 (1843) according to recorded observations. Its median ice out date is March 29.
Leech Lake, in north-central Minnesota, saw its earliest ice out more than a month after Pepin’s earliest ice out on record, on April 2, in 2012, a particularly warm spring when many lakes recorded their earliest ice outs. Its latest ice out was recorded on May 23, 1950. Its median ice out is April 28.
Measuring ice out
So what, exactly, determines “ice out”?
That depends. Generally speaking, “ice out” means someone could launch a boat and could get around in it without hitting ice, Boulay said. But other, less technical measures have sometimes been used.
“Back in the old days, they’d have ‘dump the clunk,’ basically they put a car out on the ice and have a contest over when it’s going to fall through,” Boulay said.
The specific definition of ice-out can vary based on the lake, too. For Lake Minnetonka, it’s when a boat can travel from Wayzata Bay to Excelsior Bay. On Rainy Lake, the ice is considered out when a boat can travel from Sha Sha Resort to Kettle Falls Hotel unimpeded by ice.
Generally, the DNR tries to use the same observer every year to get a consistent measure of ice out, Boulay said.
Tracking ice out data can help scientists better understand the lakes themselves as well as how broader climate events affect them.
It’s hard to draw conclusions from the Minnesota ice out data alone, but Lesley Knoll, associate director of Itasca Biological Station and Labs, pointed to research that includes Minnesota lakes and finds dwindling days of ice cover.
One study looked at 60 northern hemisphere lakes, several in Minnesota, and found that ice-out was happening an average of 6.8 days earlier per century.
Another study found Bayfield Bay, on Lake Superior (not included in the DNR’s dataset), thawed 18 days earlier per century.
The ice outlook
Unless something changes in the forecast drastically, we’re likely to see generally later ice-outs this year, Boulay said, owing to colder temps.
“We’re not making a lot of headway,” Boulay said.
In the meantime, it’s a good idea to be careful out there. The deadline for anglers in roughly the southern two-thirds of the state to remove fish houses from lakes was March 7. For northern-third lakes, the deadline is March 21.
Once ice starts weakening, it can quickly be unsafe for people to be on, Knoll said.
“You can still have inches of ice and it’s not safe anymore,” she said. The ice turns into what people call white ice or snow ice, where it’s degraded to a point that it can’t support as much weight as when it was clear.
“I’ve seen people go through the ice, even if it was like 7 inches thick, but it was just this white, mushy stuff and it didn’t support their weight,” she said.