If you have a few weather or science-obsessed Facebook friends, you’ve probably seen videos of so-called “ice circles” circulating online in the last 10 years or so. What are ice circles? They’re those slowly rotating, almost perfectly circular, sometimes giant disks of ice that look like frozen lily pads. Scientists have been studying their mysterious rotations since at least the late 19th century—check out this 1895 mention in Scientific American—but it took until 2016 for a team of Belgian physicists to figure them out.
Even after their breakthrough discovery in March 2016, there’s still some misinformation out there about ice circles, published in supposedly reputable places. This list of ice circle facts will hopefully put that to rest. How do ice circles form, for real? Read on to find out.
Until March 2016, the most common scientific explanation for the rotation of ice circles was that they were being spun around while melting by something called eddies. An eddy is a circular movement of water, against the grain of the main current, that forms when water flows, causing a baby whirlpool. Peer-reviewed research published in the journal Weather in 1997, for example, concluded that “the direction of rotation is that of the whirlpool that is necessary for the phenomenon to occur.” While eddies likely contribute somewhat to the spin, newer research reveals that eddies are not, in fact, a necessary part of the phenomenon.
What evidence is there that eddies aren’t necessary to the process? For one, small ice circles and big ice circles under the same conditions both rotate at just about the same speed, which wouldn’t be true if little whirlpools were responsible. The most damning evidence against the eddy theory, however, is that the ice circles still rotate even when the water is still. So if eddies don’t make the ice circles spin, what does?
First, let’s take a closer look at these things. How big can they get? They range in size from 1 to 200 meters across, meaning there are some out there in the wild that you can ride like a merry-go-round, like the guy in the video above (but it’s not recommended!). Researchers in 1987 in Sweden, for example, learned a lot more about these ice circles by stepping out onto one that measured about 50 meters across.
While some ice circles are solid enough to stand on and appear to be one solid chunk of ice, many of them are actually a lot more fragile. Allen Schlag, a National Weather Service hydrologist, told the Associated Press in 2013 that one he observed in North Dakota was more like a mosaic: "If you were to throw a grapefruit-size rock on it, it would go through. It's not a solid piece of ice - it's a collection of ice cubes." The ice around the edges of ice circles can not only be fragile, they’re often frazil, which is what scientists call the “loose, randomly oriented needle-shaped ice crystals” in water that resembles slush.