Rime ice

Rime grows when liquid droplets below their normal freezing point strike a surface colder than 0°C and freeze. In fog or cloud, these droplets can remain liquid while suspended in subfreezing air; contact with a cold object triggers freezing. As successive droplets freeze, they build a deposit that often projects into the wind and can develop a feathery or irregular outline. The precise texture depends on droplet size, wind, temperature and how quickly heat can leave the impact site.

Meteorological descriptions commonly distinguish soft rime from hard rime. Soft rime is generally whiter and more fragile, while hard rime is denser and more firmly attached; usage varies among observational guides, and intermediate forms occur. Aviation references often use “rime ice” for a milky, opaque accretion formed when small supercooled droplets freeze rapidly on an aircraft. Air trapped between frozen droplets makes the deposit appear white and rough. If droplets spread before freezing, a denser, clearer glaze-type accretion can form instead.

Rime is not the same as hoarfrost. Rime develops primarily by droplets in fog or cloud colliding with a below-freezing surface. Hoarfrost forms when water vapour deposits as ice crystals on a cold surface, without droplets first striking it. Freezing fog describes the suspended supercooled droplets in the air; rime is one possible surface deposit they produce. A National Weather Service Spokane explainer describes rime as interlocking ice crystals and contrasts its droplet source with the vapour source of frost. In aviation contexts, the term is also used for impact-freezing ice accretion, so descriptions may emphasize either the meteorological setting or the aircraft process.

Rime is most readily observed on objects exposed to moving, supercooled fog or cloud droplets: tree branches, wires, towers, fences and mountain structures. It can grow preferentially on the windward side because the incoming droplets are carried toward that face. Mountain summits are especially exposed to persistent cloud and strong winds, making rime a familiar feature of cold high-elevation weather. Similar accretion on aircraft occurs during flight through clouds containing supercooled liquid water.

On aircraft, rime ice can change the shape and roughness of wings and other components, interfere with airflow, reduce lift and increase drag. A rough coating can also obstruct small openings or sensors. Rime is often less dense and more brittle than glaze, but it is not harmless: the location and shape of the deposit matter, and mixed conditions can produce more complex accretion. On the ground, heavy icing can load branches, cables and exposed structures; detached pieces may also create hazards. These risks explain why atmospheric icing is monitored in aviation and engineering.

Researchers study rime accretion by measuring droplet transport, surface heat transfer and ice growth, and by comparing laboratory or computational results with observed ice shapes. Peer-reviewed numerical studies model how droplets approach and collect on airfoils, then estimate how the accumulating ice changes the surface and airflow. Such models can support understanding of icing physics, but their results depend on assumptions about droplet distributions, temperature, liquid-water content and geometry; they are not substitutes for operational aviation forecasts or certification tests.