What is ICON-D2?
ICON-D2 is the German Weather Service's (DWD) high-resolution regional weather model — the same numerical forecast that underpins DWD's own aviation products and is used by third-party services like Windy. ClearToFly visualizes the raw ICON-D2 data directly, so VFR pilots see it without smoothing or downsampling.
The one-line summary
Why the resolution matters
Global weather models (like the ECMWF IFS at ~25 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, published by DWD:
- ICON-D2 RUC ("Rapid Update Cycle") — a fresh forecast every hour of the day, with 15-minute time steps, out to +14 hours. This is what fuels the near-term nowcast.
- ICON-D2 — the full model run 8 times a day (00, 03, 06, 09, 12, 15, 18, 21 UTC), 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.
Beyond +48 h — ICON-EU and ECMWF
ICON-D2 stops at +48 hours. For a longer horizon, ClearToFly stitches two coarser models onto the tail:
- Hours 49–120: ICON-EU (~7 km grid, also from DWD). Coarser than ICON-D2 but still finer than any global model, and specific to the European domain. Same schedule as ICON-D2 (00/03/06/09/12/15/18/21 UTC). Used to extend most products to a five-day horizon — ceiling, wind, temperature, precipitation, and the meteograms and synopsis.
- Hours 121–360: ECMWF Open Data (IFS HRES and AIFS, both at 0.25° / ~28 km). Published by ECMWF four times a day (00/06/12/18 UTC). This is a genuinely global model — much coarser than ICON-D2 — but it's the only public source that reaches out to 15 days. ClearToFly uses it for coarse-resolution long-range extensions of a subset of products (temperature, precipitation, ceiling, convection, wind, isobars).
Every step down in resolution loses detail — a 7 km ICON-EU grid can't resolve an Alpine valley the way a 2.1 km ICON-D2 grid can, and a 28 km ECMWF grid can't resolve a small mountain range at all. Use the extended horizon for planning trends (is a warm front approaching by Sunday?), not for takeoff-window decisions.
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. Between the deterministic runs (8 per day) and RUC (24 per day), DWD publishes roughly 32 ICON-D2 events per day that ClearToFly could consume.
We don't process every one. The generation pipeline runs 13 times per day — a deliberate trade-off between recency and cost. Each pipeline run generates all products (maps, meteograms, cross-section cache, synopsis) for the freshest available D2+RUC pair, which typically costs a few minutes of compute on an EC2 spot instance. Running the pipeline against every RUC hour would triple the cost for a marginal improvement in freshness; 13/day catches every D2 publication plus half the RUCs, which puts the "run age" on the site at 30–60 minutes most of the day.
No third-party API sits between DWD and the pilot — ClearToFly downloads the raw GRIB files, decodes them and generates the maps, meteograms and cross-sections you see here directly.