Spotting Thunderstorms and Overdevelopment: CAPE, Clouds and Warning Signs
Thunderstorms are the biggest weather hazard for paraglider and hang glider pilots. Learn to read CAPE in the forecast and spot overdevelopment in the sky in time.
A thunderstorm can suck a paraglider into the cloud at more than 10 m/s, and its gust front reaches landing fields still in sunshine. The good news: thunderstorms announce themselves — in the forecast and in the sky.
CAPE: how much energy is in the air
CAPE (Convective Available Potential Energy, in J/kg) measures how strongly a rising air parcel can accelerate once it gets going. On the map, CAPE is under “Show weather → Air mass”. Rough guide:
- below 300 J/kg — little thunderstorm potential
- 300–800 J/kg — showers and isolated storms possible, especially over mountains
- 800–1500 J/kg — storms likely if there’s a trigger; the meteogram shows a lightning bolt here
- above 1500 J/kg — strong storms possible, including hail and severe gusts
CAPE is a possibility, not a promise. It needs a trigger — sun on mountain flanks, a front, converging valley winds. And a stable layer (on the map as CIN, the “lid”) can hold development back for a long time and then suddenly release it.
The daily cycle
Heat thunderstorms follow the sun: harmless cumulus in the morning, growing upwards from midday, discharging in the afternoon. Drag through the day in the meteogram: if CAPE and cloud cover rise in the afternoon, plan to land well before. The earlier in the day the first towers grow, the more critical the afternoon.
Warning signs in the sky
- Cumulus growing taller rather than wider — towers with sharp, cauliflower-like edges
- Icing at the top — edges turn fibrous, an anvil spreads out
- The base darkens and lowers, with precipitation falling beneath it
- Widespread lift under the cloud that carries you everywhere — cloud suck
- A sudden wind shift or freshening cool wind — the gust front is coming
As soon as any of these appears: head out of the storm’s reach, land promptly and pack up. Gust fronts can arrive many kilometres ahead of the storm.
Freezing level and humidity
A high freezing level, muggy air at the ground and cool air aloft are the classic recipe for heat thunderstorms. On the map, the freezing level under “Air mass” shows how high the frost line is.
Before and during the flight
- Check CAPE and precipitation for the afternoon in the evening and again in the morning.
- Set a landing time — and stick to it, even when it’s working well.
- Keep an eye on a precipitation radar while flying (e.g. GeoSphere Austria or DWD).
- Better an hour too early on the ground than five minutes too late in the air.