Magic on ice

Assuming our cold snap goes away and stays gone, something almost magical will soon happen to your lake’s ice. It’s called candling, and it reveals a property of ice that’s hidden from us most of the time. It’s fascinating, but it also leads to a significant hazard for those venturing out for late-season ice fishing or other adventures.

As the thaw sets in, lake ice changes from what we know as a strong, monolithic structure to a matrix of crystals, arranged (if imagined from above) as hexagons, like the cells in a bee’s honeycomb, though by no means as perfect. These crystals align vertically, from the top of the ice to the bottom; they are shaped somewhat like candles.

In its candled state, ice is often called “rotten.” You can see how weak it is in this video. A man (wearing a life vest, over shallow water) walks on candled ice and repeatedly breaks through, even though the ice is 13.5 inches thick and if intact would support a 9,000-pound vehicle with a 3:1 margin of safety.

How and why does this happen? The best explanation I got came from Dan Heim, an old friend, an Arizona resident, and author of the Sky Lights blog about astronomy, meteorology, and earth science.

As ice forms, he tells me, the mostly hexagonal crystals grow from the surface down. In the dead of winter, the crystals are strongly fused (frozen) together so that the ice appears monolithic.

“Ice expands as it warms, up to the point where it melts,” says Dan. “As the thaw approaches, the ice goes through many cooling and warming cycles, and that’s where the stress to form cracks begins to build. When you look at images of candled ice, you see that most but not all candles are hexagonal. Because of impurities in the water, the fractures are sometimes non-hexagonal. As things warm up, the ice preferentially cracks along the crystal boundaries.

“Once the cracks form, that’s where additional melting happens, further separating the candles. The load-bearing capacity of the ice, which is proportional to the square of its thickness, starts dropping as soon as the microscopic cracks form.” And dropping quite fast, one might add. So for safety’s sake, stay off of candled ice.

Another thing about candled ice: It can be almost musical. If you were to find a thick sheet of such ice driven by wind up onto shore, and if you were to tap at it, candled crystals would tumble off, making a soothing sound a bit like a set of wind chimes. Magical indeed!