The Science Behind Hoar Frost
Posted on September 15, 2012
Hoar Frost is defined by the National Weather Service 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. The deposition of hoar frost is similar to the process by which dew is formed, except that the temperature of the frosted object must be below freezing. It forms when air with a dew point below freezing is brought to saturation by cooling."
It can form on objects when the temperature of the object is colder than the surrounding air. The "hoar" term has to do with the white hair-like look of the frost. Accuweather says hoar frost can even be a source of avalanches when it forms on the surface of snow. It can also form indoors around freezers. Take a look: