lake clouds winter steam fog

Those dramatic walls of fog Toronto keeps waking up to have a simple explanation

Monday's historic blizzard may be the biggest weather-related topic people are talking about this week, but early risers have been noticing another winter phenomenon on the especially frigid days that followed the intense blast of snow.

Waking up and seeing the temperature outside is in the negative double digits is alarming enough, but anyone noticing a low, thick, almost apocalyptic-looking layer of fog rolling in as the sun rises on a cold day might be wondering just what this unique weather condition is all about.

Called sea smoke, steam fog, or frost smoke, depending on who you ask, these walls of cloud materialize when winds push frigid air over warmer water, mixing with the layer of warmer air that sits atop the surface and forming billowing clouds of vapour as the warmer air drops below its dew point.

When these clouds form over the surface of Lake Ontario, they can take on an especially menacing quality, but their turbulent-looking plumes are really much closer to the steam that rises from your morning coffee than the storm-producing cumulonimbus they can sometimes resemble.

These conditions are actually pretty regular in arctic climates, but they only appear on the coldest days in more temperate zones like Toronto, when differences between air and water temperature can be more than 20 C apart.

In some cases, steam fog brings with an even rarer accompanying phenomenon known as "steam devils," as was the case on Friday morning.

Steam devils are vortices — similar in appearance to waterspouts — that can apparently reach heights of half a kilometre, though the dozens of cyclones seen on the Lake Ontario horizon Friday morning were orders of magnitude smaller. At these sizes, steam devils typically go by the less impressive name of steam whirls.

Even when someone does manage to capture these whirling vortices over Lake Ontario, it rarely lasts long, as steam devils don't often sustain for more than three or four minutes before vanishing.

So next time you see one of these worrying-looking clouds rolling in off the lake, ease your mind and enjoy the rare show.

Lead photo by

Phil Marion

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