Rime Ice: Meaning, Origin & Winter Safety Guide
Quick Definition (Meteorology & Weather Alerts)
A rough, milky-white crust of ice needles and plumes that builds on the windward side of trees, signs, and towers when wind-driven freezing fog droplets freeze on contact.
Also called: hard rime, soft rime, freezing fog ice
If you only do three things
- Treat freezing fog and rime ice as an active black-ice alert on roadways.
- Use low-beam headlights (never high beams) when driving through freezing fog.
- Check windshield wipers for rime buildup so blades don't streak or freeze mid-drive.
Origin, meteorology & what "Rime Ice" means
Derived from the Old English 'hrim' (frost), rime ice forms when supercooled liquid cloud or fog droplets are blown by the wind against a sub-freezing surface.
Because the droplets freeze almost instantaneously where they hit, rime ice grows into the wind — creating horizontal white banners on mountain peaks, bridge cables, and highway signs that point in the direction the wind is blowing from.
Real-world winter safety & what to do
When rime ice is forming in freezing fog at valley level, highway bridges and road surfaces are simultaneously coating in invisible black ice.
In high elevations or coastal ridges, thick hard rime can add heavy wind-load weight to power lines, radio antennas, and tree limbs.
Frequently asked questions about Rime Ice
Why does rime ice appear to grow sideways into the wind?
Wind-blown supercooled fog droplets freeze the split second they touch the upwind side of an object, stacking layer upon layer toward the oncoming wind.
Is rime ice dangerous for aircraft and cars?
Yes. Rime icing ruins aerodynamic lift on aircraft wings and coats car windshields, headlights, and bridge decks during freezing fog events.
Related situational guides
- Sliding on Black Ice: Handling slick roads during freezing fog and rime events.
- Before You Drive in Winter: Preparing wipers, washer fluid, and headlights for winter.