at 2:14 PM by Mallory Schnell

Hoar Frost Forms in Colorado

Snow may not have fallen in Boulder, Colorado this morning, but prickly needles of ice – known as hoar frost – formed along trees and fences. According to NOAA’s glossary, hoar frost is 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. In order for hoar frost to form, the temperature of the frosted object must be below freezing, and the dew point must be below freezing as well.

Hoar frost is similar to dew in that it forms on clear, calm nights when the air becomes saturated. Sublimation, rather than condensation occurs with hoar frost, however; sublimation is the process of water vapor solidifying without first changing to a liquid stage (or vice versa). The result of these conditions and processes is the formation of prickly, ice needles, known in the meteorological world as hoar frost.