If cold water sinks, why doesn't Rice Lake freeze from the bottom up? It nearly could have. The USGS puts it this way: if water were densest at its freezing point, "the very cold water at the surface of lakes would sink," and a lake could freeze from the bottom up.
If cold water sinks, why doesn't Rice Lake freeze from the bottom up?
It nearly could have. The USGS puts it this way: if water were densest at its freezing point, "the very cold water at the surface of lakes would sink," and a lake could freeze from the bottom up. (USGS Water Science School)
Water doesn't work like that, because of one odd habit. Freshwater is densest at about 4°C (39°F), not at 0°C (32°F). Cool it below 4°C and it gets lighter again. Freeze it and it gets lighter still, which is why the ice ends up on top and the fish end up with a roof over their heads all winter. (University of Wisconsin Sea Grant, pages 6-7)
The 4°C Quirk
Density is just how much mass fits in a given space. Fill two identical jugs with water at different temperatures and the denser one weighs a little more.
The USGS density table peaks at 4°C (39.2°F). Water at that temperature is denser than water at 0°C, so as a lake cools in the fall, the 4°C water settles to the bottom and the colder, lighter water stays up top. That surface layer is the part that gets cold enough to freeze, while the water underneath stays liquid. (USGS)
That's the piece the old "cold water sinks" line leaves out. It's true right up until the water gets close to freezing, and then it flips.
Why Ice Floats
Freezing makes the flip bigger. When water turns to ice, the USGS explains, the molecules line up in a regular lattice instead of tumbling around loosely the way they do in liquid. The lattice holds them farther apart, so ice takes up more room for the same mass. Its density drops by about 9 percent. (USGS)
That's also why about 10 percent of an ice cube, or an iceberg, sits above the waterline. The ice cube in your drink and the ice sheet off our dock are doing the same thing. A lake just adds depth, wind, snow and a few months of weather on top.
How the Ice Actually Builds
Density decides where the ice goes. Losing heat is what makes it.
Minnesota's Department of Natural Resources has a good plain-language description. In late fall the lake gives up heat to the air, and then on a day or night when the wind isn't blowing, ice forms. It keeps thickening as long as the lake keeps losing heat. (How lake ice melts, Minnesota DNR)
Snow changes the deal. Through most of January and February, the DNR says, snow on the ice both reflects sunlight and insulates the lake, and with a thick layer the lake neither gains nor loses much heat. Meanwhile the bottom mud is actually warming the water slightly, from heat it stored over the summer. (Minnesota DNR)
So one cold night isn't freeze-up. If you want to follow a lake's freeze, you'd watch it over weeks, not check the thermometer at the cottage once.
And Then It Melts From the Bottom
Here's the part most people have never heard. Around March, as the air warms and the sun gets stronger, the snow on top melts and light starts getting through the ice. The DNR describes the ice working like greenhouse glass: the water underneath warms, and the ice starts melting from the bottom. (Minnesota DNR)
When it's down to roughly 10 to 30 cm (4 to 12 inches), the ice turns into long vertical crystals called "candles." Candled ice lets light straight through, so it looks dark. Meltwater fills in between the crystals, they start coming apart, and then a wind comes up and breaks the whole sheet. The DNR says the candles often blow to one side of the lake and pile up on shore, tinkling against each other. (Minnesota DNR)
If you've heard that sound at ice-out, now you know what it is.
What We See at HBW
From the shore at Gores Landing you see the top of this story every winter: the lake skims over, the ice thickens, the snow comes, and in spring it all goes out again. What you don't see is how hard that ice can push.
Rice Lake ice has a history. In the 1850s a railway bridge crossed the lake from Harwood by way of Tic Island, and on January 1, 1855, just days after it opened, ice shoved its spans hard enough to slide one four feet onto the embankment and bend iron rails double. Ice made the bridge unsafe and finished it off in 1860-61. (Cobourg and District Historical Society; Ontario Heritage Trust) The full story is in the train that crossed Rice Lake.
The bridge is gone, but the old railway bed and trestle remains near Tic Island are still down there, in places just inches below the surface once the ice is out. There's no fixed depth you can count on. So when ice-out comes and the boats go back in, that's the stretch where we tell people to slow down and watch the sounder. Our Rice Lake boating guide has more on running the lake safely.
An Ice Cover Isn't a Green Light
Knowing how ice forms doesn't tell you whether a patch of it will hold you. The Minnesota DNR's answer to "when is ice safe?" is "There really is no sure answer." (Ice safety, Minnesota DNR)
A few things from that page every Rice Lake ice visitor should know:
- You can't judge ice strength by how it looks, how old it is, how thick it is, the temperature, or whether there's snow on it.
- Ice seldom freezes evenly. It can be 30 cm (a foot) thick in one spot and only 2 to 5 cm (an inch or two) a few feet away.
- Ice over moving water is often dangerous. Think about where the Otonabee comes into the lake.
- Ice near shore can be weaker than ice farther out.
- New ice is usually stronger than old ice. About 10 cm (4 inches) of clear new ice may hold one person on foot, while 30 cm (a foot) or more of old, partly thawed ice may not.
(Minnesota DNR)
The DNR also says there's no such thing as 100 percent safe ice. The best place to enjoy this particular science lesson is from shore.
Sources
Phone: 905-342-2153
Address: 5369 Harris Boat Works Rd, Gores Landing, ON
Sites: harrisboatworks.ca | mercuryrepower.ca
