What is ICON-D2?

ICON-D2 is the German Weather Service's (DWD) high-resolution regional weather model — the same numerical forecast that powers official DWD aviation products. ClearToFly visualizes the raw ICON-D2 data directly, so VFR pilots see it without the usual smoothing or coarsening.

The one-line summary

Why the resolution matters

Global weather models (like the ECMWF IFS at ~28 km, or GFS at ~25 km) can't resolve the terrain that shapes VFR weather. A 2.1 km grid can see the Schwarzwald crest, an Alpine valley, or the Ruhr Basin as distinct features. That means:

• Föhn walls and terrain-driven cloud show up in the ceiling forecast, not as an averaged blur.
• Convective initiation along ridges and lake breezes is captured, not smoothed away.
• Valley fog in the Rhine or the Danube is a distinct feature, not a regional average.

Why the update cadence matters

ICON-D2 has two update modes:

• ICON-D2 RUC ("Rapid Update Cycle") — a fresh forecast every hour, with 15-minute time steps, out to 14 hours. This is what fuels the near-term nowcast.
• ICON-D2 — the full model run every 3 hours, out to 48 hours, at 1-hour time steps.

ClearToFly stitches these together automatically: hours 0–14 use the freshest hourly RUC data, and hours 15–48 use the most recent 3-hourly ICON-D2. You always see the newest available data at every forecast hour.

Which parameters does ClearToFly show?

ICON-D2 outputs dozens of atmospheric variables at multiple levels. ClearToFly surfaces the ones a VFR pilot actually decides on:

• Cloud ceiling (CEILING field — lowest level where cloud cover > 4/8)
• Horizontal visibility (diagnostic combining humidity, precipitation, aerosol)
• Significant weather (WMO WW codes)
• Simulated radar reflectivity (dBZ)
• Precipitation rate (all types combined)
• CAPE (convective fuel)
• 2 m temperature, 2 m dewpoint, freezing level
• Wind at 10 m, 850 hPa (~5,000 ft), 700 hPa (~10,000 ft), 500 hPa (~18,000 ft)
• Vertical cloud cover on 28 levels (for meteograms and cross-sections)
• Boundary-layer parameters (for the Thermals product)

What ICON-D2 is not

ICON-D2 is a forecast — not a live observation. Actual conditions can differ, especially in fog, thunderstorms, and rapidly-changing frontal weather. For go/no-go decisions close to flight time, always cross-check ICON-D2 against current METARs, live radar, and — for German airspace — the official DWD FlugWetter briefing.

Where does the data come from?

ICON-D2 GRIB files are published by DWD's Open Data portal eight times daily. ClearToFly's pipeline detects each new run within minutes, downloads the raw GRIB files, and generates the maps, meteograms and cross-sections you see here. No third-party API sits between the model and the pilot.