Coating of frost is a phenomenon that occurs when the temperature drops below freezing and water vapor in the air condenses into tiny ice crystals. The two most common terms used to describe this phenomenon are HOAR and RIME. HOAR is a type of frost that forms when the air is saturated with moisture and the temperature is below freezing. RIME is a type of frost that forms when the air is dry and the temperature is below freezing. Both HOAR and RIME are white, crystalline deposits that form on surfaces exposed to the cold.