Up until this weekend, I don’t ever recall hearing the term ‘hoarfrost’. The meteorological definition is :
A deposit of interlocking ice crystals (hoar crystals) formed by direct sublimation on objects, usually those of small diameter freely exposed to the air, such as tree branches, plant stems and leaf edges, wires, poles, etc., which surface is sufficiently cooled, mostly by nocturnal radiation, to cause the direct sublimation of the water vapor contained in the ambient air. — source
Yeah, complicated. In short, it’s frost that coats trees and looks remarkably like hair. The word ‘hoar’ comes from Old English meaning ‘hair’, specifically in regards to appearing like the white hair found on an old man’s beard. You can read up on that here.
In any case, the hoarfrost was everywhere this morning. It sure gets around.