Convection (CAPE) — how to read it
The convection map shows Convective Available Potential Energy (CAPE) in J/kg — the thermodynamic fuel for thunderstorms. It answers "is the atmosphere ready to make storms if triggered?".
How to read J/kg
- Below 100 J/kg: stable atmosphere. Isolated showers at most, no organised convection.
- 100 to 500 J/kg: isolated summer showers possible; expect the odd Cu build-up but no thunderstorms.
- 500 to 1,000 J/kg: ordinary summer thunderstorms. Airmass storms, usually pop-up, decaying by evening.
- 1,000 to 2,000 J/kg: strong storms. Hail risk, gusty outflows. Route around, not through.
- Above 2,000 J/kg: severe / supercell environment. Rare in Central Europe but real; plan for a ground day.
The trap: CAPE alone doesn't fire storms
For the timing question ("when today do the storms fire?"), open the meteogram at your airfield and look at the CAPE/CIN panel: it shows both curves hour by hour. A day where CAPE rises past 1,500 J/kg while CIN drops below 50 J/kg is a day storms are almost certain to fire. See the meteogram guide.
Combining products for a convective day
- Morning: check the CAPE forecast for the afternoon — is fuel expected?
- Around midday: check simulated radar for the expected firing time and cell distribution.
- Along the route: a cross-section tells you the vertical depth of the cells and whether you can top or under-fly them (usually you cannot).
- Close to flight time: switch to the Storms (KONRAD3D) map for live DWD observation of the cells actually firing, plus a 60-min extrapolated track for each. Compare with live radar and satellite.
Seasonal calibration
The same J/kg value doesn't mean the same thing in every season. Central European convection climatology is dominated by the summer half-year, but marginal-CAPE spring days can still produce nasty storms — because the shear is high, the freezing level is low (large hail with a shorter fall to survive the melt), and daylight is short enough that CAPE doesn't have much time to decay before firing.
- May / June: 800 J/kg with 15 kt of shear is enough for organised cells — the classic pre-summer setup.
- July / August: pop-up airmass storms usually need 1,000+ J/kg to organise. Below that, showers only.
- September: re-enter the "spring rules" regime — cold-air aloft on top of a still-warm surface.
- Winter: CAPE > 300 J/kg is unusual but happens; when it does, expect snow squalls, not thunderstorms. Watch significant weather instead.
The "cap breaks" playbook
On a capped day (CAPE high, CIN also high), you have a decision timing problem. If the cap breaks at 15:00 you can plan around it; if it breaks at 18:00 you can't. Three signals in ClearToFly tell you what to expect:
- CAPE/CIN meteogram at your destination — track hour-by-hour whether CIN is decaying faster than CAPE is building.
- Simulated-radar animation — the model's own guess at firing time. If cells appear in the animation at 14:00, that's your working hypothesis.
- Cross-section along your route — a rising thermal top (soaring layer) approaching the freezing level is the same physical setup as a rising boundary layer approaching the LCL. If it's still 3,000 ft below the LCL an hour before your ETA, you're probably safe. If it's within 500 ft, you're not.
Model caveats
Convection is inherently poorly resolved even at 2.1 km. ICON-D2 represents thunderstorm mechanics parametrically, not by explicit turbulence. Expect the CAPE field to be roughly right, but the timing and exact location of specific cells to shift by 1–2 hours and 10–30 km when compared to reality. That's why the combined-product playbook exists — never plan on a single field alone.