DENVER (KDVR) — Over the weekend, many Denver-area residents woke up to find the world blanketed in what may have looked like snow or dense frost — and for some, that frost may have had large crystals jutting out at every angle.
That’s because Denver’s atmosphere created frost while the air temperature was below the dewpoint, or freezing point in this season, according to Pinpoint Weather Team Meteorologist Liz McGiffin. The calm, still evenings have also led to the frost formation.
Called “hoarfrost,” this weather phenomenon only occurs when the air is cooled to the frost point and the water vapor bypasses the liquid stage as it condenses around items like tree branches, becoming instantly solidified as crystals instead of a soft frost.
It’s similar to how dew is formed, except that air mass for the area must be below freezing for the hoarfrost to form. The unique frost formation is named after the Old English word “hoary,” meaning gray or white, referencing how the frost can appear as a “beard” on objects.
According to a tutorial from meteorologist Jeff Haby and posted by the National Weather Service, frost forms in two ways: Deposits or freezing in place. Hoar frost is the result of depositional freezing, switching the water vapor directly into ice and bypassing the liquid stage.
The National Weather Service defines hoarfrost as “a deposit of interlocking crystals formed by direct sublimation on objects, usually those of small diameter freely exposed to the air, such as tree branches, plants, wires, poles, etc.”
So the next time you spot these large, dendritic ice crystal formations, know that the air that contained the water vapor that created the frost was below freezing, and the water never had a chance to exist as moisture between vapor and ice.
