Rime ice is a rough, milky-white ice deposit that forms when supercooled water droplets freeze instantly on contact with an aircraft's surface.
How It Works#
Clouds and freezing precipitation contain supercooled water droplets: liquid water that stays liquid even below 0°C (32°F). When an aircraft flies through these droplets, they strike the airframe and freeze on impact. The freezing happens so fast that trapped air has no time to escape. That trapped air gives rime ice its characteristic opaque, white, and brittle appearance.
Rime ice typically forms in:
- Stratiform clouds (flat, layered clouds with small, uniform droplets)
- Light freezing drizzle
- Temperatures between -10°C and -20°C (14°F to -4°F)
At colder temperatures, droplets are smaller and freeze faster. This is exactly the condition that produces rime rather than the clearer, denser glaze ice (also called clear ice), which forms at warmer subzero temperatures.
Rime ice tends to build up on the leading edges of wings, tail surfaces, propellers, and antennas. Because it freezes in place immediately, it often forms irregular, pointed shapes that project forward into the airstream.
Example in Aviation#
A Cessna 172 climbs through a solid overcast on an IFR (Instrument Flight Rules) flight in January. Outside air temperature at cruise altitude reads -15°C. The pilot notices the leading edge of the wing accumulating a white, rough coating. This is rime ice forming in the stratiform clouds. The ice is opaque and looks almost chalky. The pilot activates carb heat and requests a lower altitude from ATC (Air Traffic Control) to exit the icing layer.
Why It Matters#
Rime ice is a serious aerodynamic hazard. Even a thin, rough layer on a wing's leading edge disrupts the smooth airflow that generates lift. It increases drag and can sharply reduce the stall speed margin, making the aircraft stall at a higher airspeed than expected. Pilots who rely only on visual cues may not notice the degradation until it is too late.
Understanding rime ice also helps pilots make better go/no-go decisions. Recognizing the conditions that produce it, stratiform clouds, temperatures in the -10°C to -20°C range, and small supercooled droplets, lets a pilot anticipate icing before it forms.
Key Takeaways#
- Rime ice forms when supercooled droplets freeze instantly on contact with an aircraft surface.
- It appears white and opaque due to trapped air inside the ice structure.
- It builds on leading edges and disrupts airflow, reducing lift and increasing drag.
- Rime ice most commonly forms in stratiform clouds at temperatures between -10°C and -20°C.
- Pilots should exit icing conditions promptly and use available anti-ice or de-ice equipment.