Pancake ice on the River Dee in Scotland

Pancake ice on the River Dee
Photo by Jamie Urquhart, biologist from here

If the Queen of England were in residence at the moment at Balmoral Castle, Scotland, her summer residence, and if she walked downstream a few miles, she'd see the stunning cluster of pancake ice on the River Dee!

How does such ice form? To start with, we probably need to review a phenomenon known as "frazil ice."  Water normally freezes at 273.15 K (32 F), but can be supercooled down to almost 231 K if there are no nuclei for the ice crystals (that is, the water needs to be very pure). Frazil ice forms in turbulent, slightly supercooled water. It consists of small discs of ice 1-4 millimeters in diameter and 1-100 microns in thickness.  It is estimated that sometimes there can be one million ice crystals in a cubic meter of water. As the crystals grow, they will stick to objects in the water and tend to accumulate on the upstream side of objects.  This can cause ice dams and serious flooding.  

Frazil ice in rivers can be a serious problem if there are hydroelectric facilities because it can block turbine intakes, or can freeze open gates. It's also hard on the fish! In the ocean, frazil ice forms around the edges and within open water within ice packs.  Here it has become of concern because of oil and gas development in the Arctic. A review article on this by Sellye Martin can be found in Annual Reviews of Fluid Mechanics, v. 13, pp. 379-397, 1981.)

According to the CNN article referenced in the figure caption, there have been some cold nights in Scotland. They speculate that the disks form at night (and are round because they form in swirling eddies), soften in the daytime so that the rims get pushed up by collisions, and then grow further the next night, etc. Pancake ice is a well understood phenomenon on the oceans.