There was some very peculiar frost on my car's spare tire this morning. It looked like tiny feathers, or petals, standing up at a 45º angle. Kind of like ruffled fish scales? It must have a name. Everything has a name, if you ask the right person. The rest of the car was covered in ordinary flat frost, like this:
A couple of weeks ago I found hoarfrost in the garden:
Frost is formed when water freezes directly from the air onto a surface that has cooled below freezing (it doesn't form dew first). I think hoarfrost and rime form when the water vapor itself is supercooled, which means it's still vapor but is below freezing point, and it freezes instantly to anything it touches.
To me, hoarfrost is needle-like, as opposed to the flat sort. Since even I can tell the difference between two types of frost, there's got to be a name for my feathery frost flakes.
I've found a few interesting web sources on frost: one is at CalTech, another is here, and the last is at Britannica.