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Definition:
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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 0C 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 m in diameter) supercooled water droplets in the form of drizzle or rainfall on aircraft with a temperature near or below 0C; 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.
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