origin: (1250) Middle English; Old Norse hor= white/grey coating, to loose color with age + frost= to freeze.
When condensation freezes upon nature and objects creating a delicate & prickly-looking layer of ice. A dictionary compiled at the time of the word's origin described hoarfrost as: "expressing the resemblance of white feathers of frost to an old man's beard." A.k.a. hoar frost (two words), radiation frost (low wind conditions), pruina (a white coating), jack frost (an elf), and white frost.
The effect is created when water jumps directly from a gas state to a solid state (skipping liquid); this happens when the surface that air comes in contact with is much colder than the air itself. Fog is a particular good vehicle for this and the extra moisture is sure to produce ample opportunity for crystalline spikes. When an entire forest is covered in hoarfrost, it creates the impression of a winter wonderland; an ethereal otherworld.
Have you ever seen hoarfrost?