How to read a cross-section

A cross-section is a vertical slice of the atmosphere along your planned route — the sky rotated 90° so you can see the layers you'll actually fly through. For a VFR go/no-go, it beats a text GAFOR by a wide margin.

The axes

• Horizontal axis: distance along the route, waypoint to waypoint. Ticks show intermediate legs.
• Vertical axis: altitude in feet, surface to FL100 (10,000 ft). Terrain is drawn as a filled profile — you're flying above the shaded ground.

The cloud heatmap

The blue-to-grey shading is a cloud-cover heatmap: darker = denser cloud at that altitude and position along the route. It's derived from 28 vertical layers of the ICON-D2 model, so you can see where the layers actually are — scattered Cu at 4,000 ft with clear air above, versus a solid overcast up to 6,000 ft, versus a lenticular over the ridge at 8,000 ft — all show as visibly different shapes.

The three curve overlays

Three curves sit on top of the cloud heatmap:

• Ceiling (red dashed) — the height of the lowest broken/overcast layer along the route at each point. The single number a text forecast would give you, drawn as a continuous curve.
• 0 °C isotherm (cyan solid) — the freezing level. Anywhere the cloud heatmap intersects or sits above this curve is a potential icing zone (see next section).
• Thermal Top (orange solid) — the top of the convective boundary layer / mixing height. Below this altitude expect bumpy air on warm afternoons; above it usually smoother. Glider pilots read this as the practical ceiling of usable thermals.

FIS handover markers

Small pink boxes labelled FIS: 132.650 (etc.) sit along the top of the profile. They mark the points along your route where the Flight Information Service frequency changes — the frequency shown is the one you switch to when you cross that vertical line. It's a small time-saver in briefing, and one less thing to look up mid-flight.

Freezing level and icing

The 0 °C isotherm is the single most important line for GA icing risk. Below it, you can generally fly through cloud (visibility limits aside) without picking up structural ice. Above it, cloud is a hard no-go for non-de-iced aircraft: even seconds inside can pick up enough clear ice to matter. VMC clear of cloud above the freezing level is fine; a climb through cloud that crosses the isotherm is not.

Wind barbs

Wind is drawn as barbs at 8 altitudes across the profile: 1,000, 2,000, 3,000, 4,000, 5,000, 6,000, 8,000 and 10,000 ft AGL. Direction is the barb angle; speed is the barb count (5 kt per short line, 10 kt per long line, 50 kt per triangle — the standard aviation convention you know from METAR wind-plot charts). This is how you spot wind shear between layers — a change of 60° or more in direction, or 30 kt or more in speed, over 3,000 ft of vertical distance is worth respecting.

The map (top left) and the Sig. weather overlay

The map on the top left of the cross-section page draws your route as a line across the terrain. It has its own Map overlay radio-button control with two states — None and Sig. weather.

Switch to Sig. weather and the map overlays the same significant-weather colour raster used elsewhere on the site, animated through the same 48-hour forecast window as the cross-section below. The two views stay time-synced: as you drag the hour slider, both the vertical profile and the horizontal map advance together.

This is the "map + cross-section" pairing that lets you answer both where and how high weather is along the route without swapping between pages. If a shower cell is drifting across your track at hour 14, you see it on the map; the cross-section tells you how tall it is and whether you can under- or over-fly it.

AI Route Brief

On the right of the cross-section page there's an AI Route Brief form: your email, an optional aircraft type (Cessna 172, CTLS, …), a privacy-policy checkbox, and a Send AI Brief button.

What it does: the server takes your route, pulls the same ICON-D2 / ICON-EU data that drives the cross-section, feeds it plus the aircraft performance profile into an LLM, and generates a written go/no-go briefing tailored to your flight. The output is a formatted PDF sent to the email address you entered — usually within a minute or two.

The brief covers:

• Overview of expected conditions along the route at the selected departure hour.
• Ceiling / visibility / wind assessment per leg, referenced against the aircraft's demonstrated limits when provided.
• Icing and thunderstorm risk assessment, including cross-references between the freezing level, cloud tops and any CAPE / sig-wx signals.
• A recommended cruise altitude per leg (matched to the head/tailwind table on the same page).
• A plain-English recommendation with the caveats a human briefer would add.

Treat the AI brief as a second opinion: it's built from the same model data you're looking at, so it can only be as right as ICON-D2 is. It's most useful when you're not yet fluent at reading the cross-section yourself, or when you want a written record for later. Always cross-check the current METAR/TAF and an authorised weather briefing before flight.

See the AI Synopsis learn page for a more detailed explanation of how the AI briefings are generated and how to interpret them.

The head/tailwind analysis table

Below the cross-section, a table shows the along-track wind component per leg per altitude. Headwind is red, tailwind is green. The optimal cruise altitude per leg is highlighted — it accounts for wind plus the cloud/terrain constraints from the cross-section above.

Reading the EDAZ → EDDL example

Look at the profile screenshot above. It's a 244 NM route from EDAZ Schönhagen (Berlin) to EDDL Düsseldorf. What can you decide in 30 seconds?

• Cloud (grey heatmap): broken layers between about 4,000 and 10,000 ft along most of the route, with denser cores in the eastern third and again near the arrival. Below 3,000 ft is mostly clear.
• Ceiling (red dashed line): hovers around 4,000–6,000 ft along much of the route, dropping in the eastern segment where the denser cloud pushes lower. Cross-check against the surface — anywhere ceiling touches within 1,000 ft of the terrain profile is a marginal zone.
• 0 °C isotherm (cyan solid): sits at roughly 10,500 ft. Anything a piston GA aircraft cruising at 3,000–6,000 ft flies through stays comfortably below the icing band.
• Thermal Top (orange solid): climbs across the day. Below this altitude expect thermally bumpy air on a warm afternoon; above it usually smoother. Where Thermal Top approaches the cloud base is where you get the classic Cu-topping-out signature.
• Terrain (brown fill): mostly low-relief Central German plain; the Harz uplands push through the middle of the route (roughly the 80–140 NM band), lifting terrain from ~500 to about 3,000 ft. VFR minimum clearance of 500 ft (open country) puts your floor at ~3,500 ft over the higher ground.
• Pink FIS-frequency labels (top): tick off the FIS-sector handovers along the route. Values like FIS: 132.650 are the frequency to switch to when you cross that vertical line. One less thing to look up mid-flight.

The 30-second synthesis: fly this route between about 3,500 and 6,000 ft. Watch the eastern segment where cloud pushes lower — you may have to descend to stay VMC. Terrain in the middle bounds your floor. No freezing-level concern.

Why this beats a text briefing

A conventional GAFOR forecast tells you "Charlie in area 22 between 09:00 and 12:00". That's a scalar warning over a wide area. A cross-section shows you where along your route the Charlie conditions actually sit, at what altitudes, and whether you can climb, descend, or route around them. The same 3-minute look answers three questions a text briefing forces you to guess at.