Heat ADVISORY for most of southeast Wisconsin until 7pm tonight

The hairy side of winter


It is a cold morning with light wind, perhaps a little fog, and temperatures definitely chilly. Below freezing chilly. You look out the window and it appears like the landscape is covered with a frosting. A dusting of white no matter where you look.

This is frost. Actually more like frost on steroids. The technical name is hoar frost. The word hoar is derived from the Old English word for “hair”. And upon closer inspection, a morning with hoar frost looks like everything is growing white hair.

Hoar frost "growing" on a cold winter morning with light wind.

How does this happen? The ingredients are simple: supercooled water droplets and a freezing surface. “Supercooled” means water exists in the liquid state even when the temperature is below freezing (32°). When the wind is light and the air is saturated, we have fog. If the temperature is 32° or colder, we have freezing fog. That means tiny water droplets that make up fog freeze on contact with roads, sidewalks, tree branches, road sides, etc.

Something else is going on as well. It is called “deposition”. Moisture in the air is deposited directly on sub-freezing surfaces without going through the liquid phase. That’s right. Water vapor directly to ice. So the hoar frost “grows” out like small stands of hair as the vapor deposits and freezes.

Frosted landscape on a morning with hoar frost.

And while hoar frost looks pretty, remember that it can make all surfaces icy and slippery. Walk carefully on roads and sidewalks. But use the camera because the scenery is spectacular.

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