Wikipedia says the ice crystals form on cold clear nights, but I've always seen hoar frost after a moist fog blankets an area well under freezing temperatures. "Air hoar" is frost crystals on the surface of things - baling twine, horse hair, even horse coats, eyelashes, manes and tails. "Surface hoar" is fernlike ice crystals deposited directly on frozen surfaces (like, say, the ice already in your water troughs).
The word Hoar, says Wikipedia, "comes from an Old English adjective for showing signs of old age, and is used in this context in reference to the frost which makes trees and bushes look like white hair."
We'll call it Santa's beard here.
Merry Christmas all!