Rime vs Clear Ice: Two Kinds of Airframe Icing, Decoded
Rime vs clear ice is a distinction pilots report and forecasters predict, and it comes down to one thing: how big the water droplets are when they hit you.
Both are supercooled water freezing on contact with an airframe. What changes is how fast it freezes, and that changes everything about how the ice behaves.
What rime vs clear ice each are
Rime forms from small droplets. They freeze the instant they touch the surface, before they have time to spread.
Each frozen droplet keeps its own outline, so air is trapped between them. That gives rime its look: white, opaque, rough, and light. It builds forward from leading edges into a blunt form that spoils the aerofoil.
Clear ice forms from large droplets. They hit, spread along the surface, and freeze as they run.
Because the water flows before it sets, there is no trapped air. The result is transparent, dense and smooth, and it bonds hard to the airframe. It is often described as glaze for that reason.
Why droplet size decides it
The size of the droplets is set by the cloud, and the cloud type gives it away.
Stratiform cloud has gentle lift and small droplets. That produces rime, usually light to moderate, spread over a wide area and a fair thickness of cloud.
Cumuliform cloud has strong updraughts that hold droplets aloft while they grow. Those large droplets produce clear ice, in a smaller area, at a much higher rate.
Temperature narrows it further. Clear ice is most common between zero and about minus ten Celsius, where large supercooled droplets survive. Below roughly minus fifteen, most cloud water has already frozen into crystals, and ice crystals do not stick to a cold airframe.
How each behaves on the aircraft
This is where the comparison stops being academic.
Rime is brittle. It cracks and sheds reasonably well under boots or heat, and because it is opaque you can usually see it building on a wing or a strut. It accumulates on the leading edges and mostly stays there.
Clear ice is the harder problem. It is heavier for the same thickness. It bonds tightly, so deicing boots can flex under it rather than break it off.
It also runs back. Large droplets that do not freeze immediately flow aft before they set, forming ice behind the heated or protected area. That runback ice sits where nothing can remove it.
And it is hard to see. A layer of clear ice on a wing can look like nothing more than wet paint from the cabin.
How pilots report it
Icing reaches you through PIREPs, in the icing field.
A PIREP marks that field with /IC. A typical entry reads /IC LGT RIME 060-080, meaning light rime ice between 6000 and 8000 feet. The type words used
are RIME, CLR for clear, and MIXED for both together.
Intensity runs trace, light, moderate and severe. Severe has a specific meaning: the rate of build is more than the aircraft can handle, so the ice protection cannot cope and an immediate change of altitude is needed.
/IC NEG means the pilot met no ice, which is a useful report in its own right.
A run of negative reports through a layer is evidence.
Mixed icing and what to do with it
In practice a lot of cloud holds a range of droplet sizes, and both types form
together. That is reported as MIXED.
Treat mixed icing as clear ice for planning. The clear part is the one that bonds hardest and runs back, so it sets the difficulty. The rime around it does not make it easier.
The general escape is the same for all three. Change altitude to leave the supercooled layer, and know which way to go before you need to. Climbing into colder air works when the tops are close. Descending into air above freezing works when the terrain and the freezing level allow it.
A PIREP that gives cloud tops alongside the icing is worth more than one that gives the intensity alone, because it tells you how far the climb is.
Verdict on rime vs clear ice
Read rime vs clear ice as a signal about droplet size, and read droplet size as a signal about how much trouble the ice will be.
Rime is the common one and the more visible one. Clear ice is less common, forms in convective cloud and warmer subzero air, and is heavier, stickier and harder to see. If a PIREP says clear or mixed, give it more weight than the intensity word alone suggests.
Frequently asked questions
What is the difference between rime and clear ice?
Rime forms from small droplets that freeze on contact, trapping air and making it opaque and brittle. Clear ice forms from large droplets that spread before freezing, making it dense, transparent and much harder to shed.
Which is more dangerous, rime or clear ice?
Clear ice, in most cases. It is heavier, it bonds harder, it can flow back behind the protected surfaces, and it is difficult to see. Rime is more common but usually easier to detect and remove.
What temperature does clear ice form at?
Most often between zero and about minus ten Celsius, where large supercooled droplets survive. Rime dominates lower down the scale, roughly minus ten to minus twenty, where droplets are smaller and freeze on impact.
How is icing reported in a PIREP?
In the icing field, marked with a slash and IC. The report gives an intensity such as light, moderate or severe, and usually a type such as RIME, CLR or MIXED, plus the altitude it was met at.
What is mixed icing?
Both types forming together, when the cloud holds a range of droplet sizes. It is common in practice and is reported as MIXED. Treat it as behaving like clear ice, because the clear component is what bonds hardest.