Hoar frost forms like dew where very humid air cools enough to produce condensation. So when there’s been a warm day during a cold spell that melts a lot of snow and the night is set to be cold and calm, the next morning might bring a chance for it. Sometimes you’ll see this on the top of the snow as surface hoar.

Rime looks more like barbs of ice on the windward side of an object. Rime forms when fog rolls in overnight and leaves combs or barbs of ice on everything.