Significant weather — how to read it
The significant weather map shows the dominant weather phenomenon per grid cell as a WMO WW code — fog, drizzle, rain, snow, freezing precipitation, thunderstorms, hail. It's finer than the precipitation map because it distinguishes hazard type.
What the colors and emoji mean
- Fog / mist — grey shading. Visibility problem, ceiling irrelevant. Cross-check with the visibility map.
- Drizzle — light blue. Usually low ceiling, poor visibility, but manageable.
- Rain — blue → green shades by intensity. Standard VFR concern; check ceiling.
- Snow — light violet. Winter operations: runway conditions, icing risk on climb.
- Freezing rain / drizzle — pink / magenta. Airframe-icing emergency. Do not depart, do not descend into it.
- Ice pellets — a specific pink. Indicates a warm layer aloft over a subfreezing surface; freezing rain often coexists nearby.
- Thunderstorm — red/orange with a ⛈️ marker. Combine with CAPE and simulated radar. Close to flight time, cross-check with the observation-based Storms (KONRAD3D) map.
- Thunderstorm with hail — dark red / purple. Serious; hail cores can extend well outside the visible cloud.
What one frame actually represents
Two ClearToFly-specific details about how these frames are constructed:
- Each frame is a summary of the preceding hour, not an instantaneous snapshot. ICON-D2 diagnoses the WW (weather) code as the “dominant significant weather that occurred over the past hour” ending at the frame's timestamp. So the frame labelled “14:00” is really “what happened between 13:00 and 14:00”. When you play the animation, treat each frame as a one-hour window sliding forward, not a single moment.
- Sig-wx is D2-only, hourly, and starts at hour +1. The WW field isn't published in the RUC hourly-update stream, so the 15-minute sub-hourly resolution you get on simulated radar or on precipitation in the 0–14 h window is not available for significant weather — it stays hourly. And because each frame summarises the preceding hour, there is no meaningful “hour 0” frame; the map starts at +1 h.
Hover / tap for the code
Each cell surfaces its WMO WW code on hover (desktop) or tap (mobile). The code disambiguates similar-looking colors — e.g. WW 71 (light continuous snow) vs. WW 85 (light snow shower). Codes 60-69 are rain, 70-79 are solid precipitation, 80-89 are shower type, 90-99 are thunderstorm codes.
The freezing-precipitation trap
Freezing rain is the single most dangerous weather this map surfaces for GA aircraft. A pink patch on your route inside the 0°C to +5°C layer means an airframe that will pick up ice faster than your de-ice can shed it — assuming you have de-ice at all. Treat any pink patch as a hard no-go unless you have a specific plan to stay above or below the warm-nose layer.
The mechanism — why it happens
Freezing rain (WMO WW codes 66/67) exists in a specific thermal profile: warm layer aloft, cold layer near the surface. Snow falls from the top of the cloud, melts to rain as it crosses the +2 °C to +5 °C warm nose, then re-cools in a subfreezing layer close to the ground — but stays liquid because there's no nucleation trigger. On impact with your airframe or the runway, it flashes to ice immediately.
The three ingredients — warm layer aloft, subfreezing near surface, liquid precipitation — are visible across three ClearToFly products. The signature is unmistakable once you know the pattern:
- ICON-D2 temperature map shows a surface layer at −2 to +2 °C.
- Cross-section along your route shows the freezing level dipping to (or below) the terrain, with cloud above the 0 °C isotherm.
- Significant-weather map shows pink over the affected cells.
Real-world example
Two hours before departure the pink patch may look benign — light precipitation, temperatures near freezing. Two hours after, the airframe has picked up half an inch of glaze ice. This is why the map's type field matters more than the rate field.
Ice pellets — the warning signal
A related WMO code is ice pellets (sleet, WW 79 / 87 / 88). Ice pellets form when the warm nose is thin enough that the rain partially refreezes into pellets before landing. On ClearToFly they show as a specific pink variant next to freezing rain in the legend.
Ice pellets almost always coexist with freezing rain nearby. If you see the ice-pellet marker on your route, assume freezing rain is within 20-40 km — start looking for it explicitly.