The Old English dictionary defines hoarfrost as “expressing the resemblance of white feathers of frost to an old man’s beard.” Since this definition can leave much to be desired for most people of our current era, we’ve decided to explain this icy enigma with science.
In order to create any type of frost, you need water vapor. This is a gaseous state of water that forms in the air above cold ground featuring a surface dew point of at least as low as 32 degrees. As the water vapor molecules make contact with a subfreezing surface (like a blade of grass), they will hop directly from the gas state to a solidified state in a process referred to as “deposition.” Consequently, deposition leads to the coating of mini ice crystals.
But what offers the extra push for frost to form into hoarfrost like the pictures featured here?
For the most part, you need greater air moisture mass. Throughout late fall, winter or even early spring, just one or more days in a row of freezing fog provides an ideal scenario. The greater moisture in the air causes interlocking crystal patterns of frost to gain intricacy and size. In turn, they build up a greater depth on tree limbs, signs, fences or anything you could think of. That’s the essence of hoarfrost.
When the weather offers a light wind, the hoarfrost may accumulate on the downwind side of items. One of the prime examples of hoarfrost happened in January of 2013 in Washington state. After six days’ worth of below freezing temps with light breezes, the area had an impressive and rather memorable hoarfrost. In December of the following year, Wyoming produced such great accumulations of the phenomena that NWS staff members made a video of themselves knocking it off of a fence in slow motion.