Ice formation can occur on aircraft either on the ground or in flight. Ice accretion in flight may constitute a danger by affecting the aerodynamic characteristics, engine performance, or in other ways. There are four types of airframe icing: 1) rime: a light, white opaque deposit that forms generally at temperatures well below 0°C in clouds of low water content, consisting of small supercooled water droplets; 2) clear ice or glaze: a coating of clear ice that forms in clouds of high water content consisting of large (greater than 40 μm in diameter) supercooled water droplets in the form of drizzle or rainfall on aircraft with a temperature near or below 0°C; 3) mixed ice or cloudy ice: a rough, cloudy deposit that occurs in clouds containing a large range of drop sizes or a mix of ice crystals, cloud droplets, and snowflakes; and 4) hoarfrost: a white crystalline coating of ice that forms in clear air by deposition of water vapor when an aircraft surface is colder than the frost point of the air; this can occur when an aircraft moves rapidly (usually in descent) from very cold air into a region with warm and relatively moist air.