Hoarfrost is a deposit of interlocking ice crystals formed by sublimation on objects directly exposed to the cold air. The deposition is similar to the process which forms dew, but the object that collects the frost must be below freezing. It forms when air that has a dew point lower than freezing (32 oF) is brought to saturation by cooling. In addition to forming on exposed items, it can also form in unheated buildings and vehicles, caves, and on snow surfaces. Hoarfrost is fluffier than rime. Rime is light deposit of ice crystals. On the other hand, a heavier coating of ice would glaze. Heavy deposits of glaze are the basic components of an ice storm.
Mary Knapp, Weather Data Library