The past month has been one of the foggiest I can remember here at FortWhyte. A side effect of milder-than-average overnight temperatures and daytime sun, we have experienced many nights with ground-level fog.
We've always been lucky enough to experience a fairly large number of mornings when the world has been coated with white hoar frost.
Hoar frost at FortWhyte Farms
(photo by J. Mulaire)
We've been especially lucky because every morning we've had hoar frost, we seem to have light south winds, as well. The relatively slow wind speed has not broken the crystals of frost off trees, buildings, and plants, allowing us more brilliant white-frost-against-blue-sky mornings.
(Photo by I. Carter)Hoar frost occurs when the air temperature drops. Cold air can hold less water than warm air - as the air cools, water no longer bound up as water vapour condenses. In the summer, we call this dew, and we see it on grass, trees, tents, etc. In the winter, the grass, trees, tents, etc. are at a temperature colder than freezing. As the water condenses, it freezes into thin, long crystals, "frosting" the landscape like a cake.
(Photo by I. Carter)
We've had a number of days in the last few weeks where the air has warmed up, snow has melted, and started to evaporate. At nightfall, the air is warm and full of water. After sunset, the air cools, and fog forms. On some nights, objects cool faster than the air, creating ideal conditions for hoar frost.
Hoar frost is one of the perks of living in a winter climate. I think knowing the natural principles at work make the phenomenon just a little bit more breathtaking.
(Photo by I. Carter)
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