Definition
A rough, milky, opaque type of structural ice formed by the rapid freezing of small, supercooled water droplets on impact with an aircraft surface. The fast freezing traps air between the droplets, giving rime its brittle, granular appearance. It typically forms in stratiform clouds at colder temperatures (generally between -15°C and -20°C, though it can occur in a wider range).
Plain English
A type of ice that builds up on an aircraft as a rough, white, crusty layer. It forms when tiny, very cold water droplets in cloud freeze almost instantly the moment they hit the airplane.
Context Anchor
Seen in structural icing discussions, especially when comparing types of ice that can form on wings, antennas, windshields, and other exposed aircraft surfaces.
Derivation
From Old English 'hrim,' meaning frost or hoarfrost. The word has long described the rough, white frost that forms on cold surfaces in fog or freezing mist. The aviation meaning carries the same image directly into flight: rough, white ice that grows on a cold surface in moist air.
Why Pilots Care
Rime ice adds weight, increases drag, and disrupts airflow over wings and control surfaces, reducing performance and potentially leading to loss of control if allowed to accumulate.
Analogy
Rime can look like the white, rough frost that builds up in a freezer, but on an aircraft it affects the shape and smoothness of surfaces that need clean airflow.
Grounding Statement
Picture flying through a cold cloud and seeing a rough white coating start to collect on the leading edge of the wing.
Intuition Check
Rime does not mean any ice on an aircraft. In this context, it means the rough, cloudy-looking ice that usually forms when small cold droplets freeze quickly on contact.
Example Sentence 1
Climbing through the cloud layer at 6,000 feet, the pilot noticed rime building up on the wing's leading edge and activated the deicing boots.
Example Sentence 2
The preflight briefing warned of rime ice potential, so the crew activated the deicing boots before entering the clouds.