Frost coating is a type of weather phenomenon that occurs when the air temperature drops below freezing and moisture in the air condenses into tiny ice crystals. The two most common types of frost coating are HOAR and RIME. HOAR is a white, feathery frost that forms on cold surfaces, while RIME is a milky-white, granular frost that forms on objects exposed to freezing fog. Both HOAR and RIME are composed of tiny ice crystals that form on surfaces when the air temperature is below freezing.