Demystifying those dazzling disks
Last Thursday, I shared a letter I received from David Hutt, a Scouter with the 36th Halifax Scout troop. The Scouts had been on a hike in Long Lake Provincial Park. A few hundred metres in, the group came upon a small waterfall; that's where they noticed something unusual happening at the bottom of the pool: "froth from the falls was gathering in the eddies, with foam forming into strangely uniform disk, about 20 to 30 centimetres in diameter."
They were curious about what they saw and reached out, hoping I could solve the mystery.
The description was perfect, and the photos were quite telling — we were dealing with pancake ice.
Pancake ice is the name given to free-floating and mainly circular pieces of ice that form when surface slush accumulates into floating pads.
Pancake ice typically forms when the temperature is between -4 and 0 C; cold enough to form ice, but not cold enough to create a sheet of ice, at least not immediately. Pancake ice can begin as a thin ice layer, known as grease ice or slush, on the water surface which accumulates into circular disks.
The lily pad, or raised-edge appearance of pancake ice, can form when each disk bumps up against one another, or when slush splashes onto and then freezes on the slab's edge. With time and diminishing winds, the individual pancake ice slabs can freeze together to form a solid ice sheet and a masterpiece.
And if that's not cool enough, here's a little more science:
Ice circles tend to rotate even when they form in water that is not moving. The ice circle lowers the temperature of the water around it, which causes the water to become denser than the slightly warmer water around it. The dense water then sinks and creates its own circular motion, causing the ice circle to rotate.
I was so glad that the 36th Halifax Scouts were able to take their learning outside; the bad news is that the meeting will be their last for a while. With new COVID-19 restrictions in place, they have had to suspend in-person activity for the time being.
When those restrictions are relaxed, my wish is that we all spend a little more time outside, questioning things.