When a polar vortex descended on North America in early January 2014, people were expecting cold temperatures. But very few anticipated quite how cold it was going to get.

Ontario has been rocked by ice quakes. Roads, offices and schools have been shut in almost every state in the US. Low temperature records were broken, and flights were cancelled en masse. This wasn’t your average cold snap.

But one of the effects of the extreme cold, seen over the Great Lakes, was more beautiful than destructive. The bone-chilling Arctic wind passing over their warmer surface created a phenomenon known as steam fog, or sea smoke.

Arctic Vortex causing Lake Michigan to steam near Navy Pier

It’s exactly the same effect that you see when you run a hot bath. The hot water makes the layer of air just above its surface both warm and saturated with moisture. When the colder air passes over, it mixes with this warm layer, and cools it. The cooler air can no longer hold as much water vapour, so it condenses out into tiny water droplets that appear as fog or mist.

This layer usually stays close to the surface of the water, but if there’s a wind then it can be whipped up into columns as high as 20 or 30 metres. It’s not particularly rare — you’ll often see it over ponds and lakes in the wintertime, particularly at higher latitudes. But the stronger the difference, the more intense the steaming effect is.