Ceiling — what the colors mean

The ceiling map shows the forecast height of the lowest broken or overcast cloud layer above ground level, from the DWD ICON-D2 model. It answers a single question: is the sky low enough to spoil VFR here?

What the field actually is

ICON-D2 exports a diagnostic CEILING field: the lowest model level at which total cloud cover exceeds 4/8. That threshold matches the aviation definition of "broken" — a value below 1,000 ft means you can expect a solid layer close to the surface, not just a few scattered patches.

How to read the colors (GAFOR-like scheme)

The colours are inspired by German GAFOR categories but are not a strict GAFOR encoding — GAFOR has additional rules (visibility, area coverage, timing) that a per-cell ceiling forecast can't capture on its own. Use the colours as a fast visual approximation:

• Dark red — below 500 ft AGL: essentially LIFR. Not VFR.
• Red — 500 to 1,000 ft AGL: GAFOR territory around Mike or X-Ray. Not VFR.
• Orange / yellow — 1,000 to 5,000 ft AGL: GAFOR Charlie range. Legal VFR but marginal; combine with terrain along your route before committing.
• Green — 5,000 to 12,000 ft AGL: comfortable VFR under most conditions.
• Blue — above 12,000 ft AGL: broadly unrestricted for the altitudes GA typically flies at.

AGL vs. MSL

The ceiling number on this map is height above ground, not height above sea level. That's the safer convention for the pilot: “below 1,000 ft AGL over the Alps” is a serious problem because it's near the surface, and the map surfaces it that way immediately. An MSL-based ceiling would look reassuring in the mountains (“7,000 ft ceiling!”) even when the cloud is within a few hundred feet of the ridge.

Two implications:

• The same colour means the same operational thing everywhere. Red near the surface is red on the plain and red in the Alps.
• The absolute altitude of the layer varies — a cloud base at 3,000 ft AGL over a 500-ft plain sits at ~3,500 ft MSL, but over a 3,000-ft ridge it sits at ~6,000 ft MSL.

When you need to reason about the absolute altitude of the layer along a route — e.g. to pick a cruise altitude — cross-check with the cross-section, which draws the layer against a terrain profile.

When ceiling is not enough

Ceiling is only half the VFR minima. Combine it with:

• Visibility — GAFOR Charlie can be triggered by visibility alone.
• Significant weather — fog and freezing rain change the picture fast.
• Meteogram at your specific airfield — see the meteogram guide.

Model caveats

ICON-D2's CEILING field can miss thin stratocumulus decks that are solid enough to see but too thin for the model's coverage threshold. In persistent stratus/fog situations, cross-check with current METARs — a model saying 4,000 ft while every station reports OVC008 is telling you the model is late resolving the layer.