Jon Erdman
Published: October 20, 2015
No, this frost isn't on performance-enhancing drugs. What you see in the photos above is called hoarfrost.
The first photo shows an early-season example of hoarfrost from Oct. 19, 2015 in Corning, New York, when the coldest air so far this season moved through the region. Notice the brilliant fall foliage contrasting with the hoarfrost on the fence.
According to the National Weather Service office in Riverton, Wyoming, the Old English dictionary (c. 1290) defines hoarfrost as "expressing the resemblance of white feathers of frost to an old man's beard."
First, to produce any frost, you need water vapor (gaseous form of water) in the air over cold ground with a surface dew point at least as cold as 32 degrees.
When these water vapor molecules contact a subfreezing surface, such as a blade of grass, they jump directly from the gas state to solid state, a process known as "deposition", leading to a coating of tiny ice crystals.
So what provides the boost for frost to grow into hoarfrost like this?
Generally speaking, you want a much more moist air mass in place. In late fall, winter or early spring, one or more days in a row of freezing fog (fog with air temperatures of 32 degrees or colder) is a perfect scenario.
With more moisture in the air, the interlocking crystal patterns of frost become more intricate and much larger, building up to a greater depth on tree branches, signs, fences, anything. This is hoarfrost.
If there is a light wind, the hoarfrost can accumulate on the downwind side of objects.
Perhaps the single best example I've seen of hoarfrost occurred in January 2013 in Washington state. Weather.com/photos contributor garnertoo, who sent us some incredible photos during that event in the slideshow above, said six day's worth of sub-freezing temperatures, along with a light breeze, produced the impressive hoarfrost.
A hoarfrost event in central Wyoming in December 2014 produced such large accumulations of hoarfrost that NWS staff knocked it off a fence with a football.
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