Convection (CAPE / CIN / Shear) — how to read it

The convection map offers a three-layer view of the storm environment — select the layer via the button bar at the top of the map. CAPE (fuel), CIN (the cap that suppresses or releases that fuel), and deep-layer wind shear (storm organisation) answer three sequential questions: is there fuel, will it fire, and if it does — how organised?

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.

CIN — the cap

Convective Inhibition (CIN) is the energy (in negative J/kg) needed to lift a parcel through the stable layer sitting above the boundary layer. A strong cap can prevent storms from firing even when CAPE is enormous — it is the lid on the pressure cooker. CIN is shown on the map for the same horizon as CAPE (0–120 h).

• 0 to −50 J/kg — weak cap: the boundary layer breaks through easily. Storms can fire early and broadly; timing is unpredictable.
• −50 to −100 J/kg — moderate cap: a stronger trigger (daytime heating, orographic lift, convergence line) is needed. Typical firing window is early-to-mid afternoon.
• −100 to −200 J/kg — strong cap: the atmosphere can hold large CAPE in reserve all day, then release explosively once the cap breaks — often late afternoon or along a front. Expect a concentrated, intense outbreak rather than scattered showers.
• Below −200 J/kg — very strong cap: storms are unlikely to fire unless a hard trigger is present (cold front, MCS outflow). High CAPE + very high CIN often stays quiet; check the following day's forecast.

On a capped day, cross-check the meteogram's CAPE/CIN panel hour by hour: track whether CIN is decaying while CAPE builds — that is the most reliable guide to storm timing.

Deep-layer shear — storm organization

Deep-layer shear (0–6 km wind shear, in knots) is the vector difference between the wind at the surface and at 6 km altitude. It controls how organised any storm that fires will be: shear separates the updraft from its own rain-cooled outflow, allowing it to sustain and intensify.

• Below 20 kt — airmass / pulse: short-lived, self-limiting cells. The classic summer-afternoon pop-up. Avoidable with standard storm deviation.
• 20–35 kt — multicell: longer-lived clusters capable of hail and gusty outflows. Can organise into a bow echo. Give cells a wider berth; deviations eat more fuel.
• Above 35 kt — organised / supercell potential: squall lines, rotating supercells, large hail. Above 40 kt the environment is explicitly favourable for supercell rotation. Treat affected zones as no-go until conditions pass.

The shear layer is available to 360 h (15 days) — significantly further than CAPE or CIN — because shear is tied to the large-scale flow pattern the model resolves well at extended range. At Day 3+ use shear to assess whether the synoptic pattern could support organised convection at all; combine with the CAPE layer for day-of timing.

Using CAPE, CIN, and shear together

Read the three layers in sequence — they answer three questions:

1. Is there fuel? — Check CAPE. Below 100 J/kg: the atmosphere is stable, no action needed. Above 500 J/kg: continue.
2. Will it fire? — Check CIN. A strong cap (below −100 J/kg) with high CAPE is a loaded-spring day: quiet until the cap breaks, then explosive. Track the CAPE/CIN meteogram to see when the cap is expected to give way.
3. How bad if it does? — Check shear. Below 20 kt: isolated pop-ups you can route around. Above 35 kt: organised, fast-moving convection that is hard to avoid. On a high-shear day, make the ground-day decision before departure rather than waiting for radar.

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

1. Morning: check the CAPE forecast for the afternoon — is fuel expected?
2. Around midday: check simulated radar for the expected firing time and cell distribution.
3. 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).
4. 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.