Hoarfrost, also known as white FROST, is a type of FROST that forms when the air is cold and the ground is below freezing. It is composed of tiny ice crystals that form on objects such as trees, grass, and other surfaces. RIME is a type of hoarfrost that forms when the air is saturated with water vapor and the temperature is below freezing. It is characterized by a white, milky appearance and is often found on the edges of objects.