050 Meteorology topic guide
Airframe Icing: Clear, Rime and Mixed
Water in a cloud does not have to be frozen just because the air around it is below zero. Small droplets can remain liquid, supercooled, down to temperatures of roughly minus 40 degrees Celsius, and it is the disturbance of an aircraft surface striking them that finally gives them a reason to freeze. What that ice looks like, and how dangerous it becomes, depends almost entirely on how large the supercooled droplets were and how cold the surface is at the moment of impact.
Large droplets spread out before they finish freezing and build clear ice, the shape-changing, structurally significant kind. Small droplets freeze almost on contact and build rime, rough but comparatively benign. Most real encounters produce some mixture of the two, and the worst case, freezing rain, sits in a category of its own. This page works through the physics behind each type, the temperature bands where they form, and where in frontal and convective cloud they are most likely to be found.
Droplet size, temperature and the three ice types
Rime ice forms from small supercooled droplets that freeze almost instantly on impact, trapping air inside and producing an opaque, rough, brittle deposit that closely follows the shape of the leading edge it forms on. It is typically found roughly between minus 15 and minus 40 degrees Celsius, most often in stratiform cloud where the droplet population is small and fairly uniform.
Clear ice forms from large supercooled droplets that do not freeze immediately on impact. Instead they spread across the surface and can run back before finally solidifying, producing a smooth, dense, transparent ice that can change the shape of the aerofoil and is harder to shed once it has built up. It is typically found roughly between 0 and minus 10 to minus 15 degrees Celsius, most often in cumuliform cloud or in freezing rain, where the droplets involved are larger.
Mixed ice is a combination of both, typically found roughly between minus 10 and minus 20 degrees Celsius, where both a small-droplet and a large-droplet population can be present at the same time. It often looks like rough rime carrying streaks of clear ice, and can disrupt airflow as badly as pure clear ice while adding the extra roughness that rime contributes.
- Rime: small droplets, roughly minus 15 to minus 40 degrees Celsius, opaque and brittle, follows the surface shape
- Clear: large droplets, roughly 0 to minus 10/15 degrees Celsius, smooth and dense, changes the aerofoil shape
- Mixed: both droplet populations together, typically minus 10 to minus 20 degrees Celsius, combines the weight of clear ice with the roughness of rime
Why clear ice is the structural threat
Clear ice is heavy for its volume, it can run back beyond the leading edge before freezing, sometimes past the area that ice protection systems are designed to cover, and its smooth, adherent bond makes it hard to shed once formed. All three of those properties combine to disrupt airflow, add weight and drag, and erode the stall margin more seriously than the same thickness of rime would.
Freezing rain, and the closely related case of supercooled large droplets, involves droplets even bigger than the ones that typically build ordinary clear ice. The classic setup is rain that formed in a warmer layer aloft falling into a shallow subfreezing layer near the surface without fully freezing on the way down. This is one of the most hazardous icing environments in the syllabus, precisely because it can ice surfaces well aft of where standard ice protection is effective.
Where icing concentrates in frontal and convective weather
Warm frontal icing is commonly found in the nimbostratus deck ahead of the surface front, and the classic freezing rain setup, warm air above freezing sitting over a shallow cold layer at the surface, is closely tied to that same frontal structure. Cumulonimbus cloud carries the strongest updraughts and the broadest range of droplet sizes, so icing inside a CB tends to be severe and can present as any of the three types depending on exactly where the aircraft happens to be in the cell, while layer cloud tends to hold more consistently to one type.
Worked example
Worked example: identifying an icing type
An aircraft flying at minus 6 degrees Celsius encounters continuous moderate rain that has fallen from a warmer layer above. Ice begins building smoothly along, and beyond, the leading edge. Which type of icing is this, and why?
- ARime ice, because the temperature is close to freezing.
- BClear ice from freezing rain
- CMixed ice, because both rain and cloud droplets are present.
- DIce crystal icing, because the aircraft is inside precipitation.
Show the answer and walkthrough
Correct answer: B
- A. Rime needs small droplets that freeze almost instantly, giving a rough, opaque deposit confined to the leading edge, not the smooth ice extending beyond it that is described here.
- B. Correct. Large droplets from rain falling through a subfreezing layer do not freeze on contact, so they spread and run back before solidifying, producing smooth ice beyond the protected area, consistent with the minus 6 degree temperature.
- C. The scenario describes one droplet population, rain-sized droplets, at one temperature, not the alternating small and large droplet mixture that defines mixed ice.
- D. Ice crystal icing involves already-frozen crystals, typically affecting engine internals rather than building visible ice on the airframe from supercooled liquid rain.
Step by step
- Identify the droplet source: rain falling from a warmer layer above through a colder layer at the aircraft is the classic freezing rain setup.
- Freezing rain droplets are much larger than ordinary cloud droplets, so they do not freeze instantly on impact.
- Large, slow-freezing droplets spread across the surface and run back before solidifying, producing smooth, clear ice, unlike the small droplets that freeze almost immediately into the opaque texture of rime.
- The temperature of minus 6 degrees Celsius sits inside the roughly 0 to minus 10/15 degree band typically associated with clear ice, reinforcing the identification.
- Ice extending beyond the protected leading edge is a specific warning sign of freezing rain, since the larger droplets take longer to freeze and can run back past the area ice protection normally covers.
Common mistakes
Assuming colder always means worse icing
The coldest cloud, well below about minus 40 degrees Celsius, often holds too little liquid water to ice at all, since the droplets have already frozen into crystals. Assuming risk simply keeps rising with cold loses the mark on stems that test this ceiling.
Treating rime as harmless because it is common
Rime still disrupts airflow and adds drag, and heavy accretion over a long segment in stratiform cloud is a genuine hazard even though it is lighter and more brittle than clear ice.
Forgetting that ice protection has a certified droplet range
Supercooled large droplet conditions can carry ice aft of the surfaces that standard ice protection was designed to clear, a distinction the exam tests directly whenever a stem names freezing rain or freezing drizzle.
Related topic guides
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Last reviewed July 2026