Weather & Flight
Thermal & Updraft Risk Calculator
CAPE-based thermal turbulence risk for fixed-wing and glider pilots.
Planning aid only — not certified flight-planning software or airspace clearance. Always verify results against your own aircraft's manual and check current regulations before flying. Full disclaimer.
Atmospheric instability
Thermal / updraft risk
WeakLow instability — thermal lift and sink should be minor and easy to correct for.
What to do
- Normal flight operations — thermals are unlikely to be a factor today.
Who this is for
Thermals, the rising columns of warm air that form over sun-heated ground, matter most to fixed-wing drones, gliders and slope or thermal soaring models, whose flight path is pushed around by lift and sink. Multirotors correct small updrafts and downdrafts automatically, so for them thermals usually show up only as a bumpier flight on a hot afternoon. That is why the calculator is separate from UAV Meteo's main flight rating.
How the rating works
The calculator starts from CAPE (convective available potential energy) for your location, loaded from the forecast or entered by hand, in J/kg:
| CAPE (J/kg) | Rating | What it suggests |
|---|---|---|
| Under 300 | Weak | Little instability |
| 300–1,000 | Moderate | Noticeable convection possible |
| 1,000–2,500 | Strong | Vigorous convection; turbulent updrafts and downdrafts |
| 2,500 and above | Severe | Instability of the kind that feeds thunderstorms |
Two adjustments follow:
- Heavy cloud (over 85%) blocks the sun from heating the ground, which is what starts thermals. The calculator then halves the CAPE before rating it, and notes that conditions can change quickly if the sky clears.
- Surface wind over 8 m/s tends to break thermals up. The calculator adds a warning that the turbulence may be choppier and less predictable.
What CAPE does and doesn't tell you
CAPE measures how much energy a rising parcel of air could gain once it has formed cloud. That makes it a good guide to how vigorous showers and storms could become, and to the severe turbulence around them. It is not a direct forecast of thermal strength:
- Dry thermals form on sunny days with little or no cumulus cloud, and can be strong enough for soaring while CAPE is close to zero. A "Weak" rating doesn't mean the air is smooth on a hot, sunny afternoon.
- High CAPE doesn't guarantee storms. A warm layer higher up can stop air rising until the afternoon, or all day.
- CAPE is for a grid square several kilometres wide. Local thermals depend on the ground beneath you. Dark fields, car parks and sun-facing slopes heat up fastest.
For soaring flights, soaring forecasts built for glider pilots, which predict thermal height and strength directly, are the better tool. Use this calculator for the question it answers well: how unstable is the air, and is there a risk of storms?
Planning around convection
- Fly early or late. Thermals and afternoon showers are usually strongest from late morning to mid-afternoon, when the ground is hottest.
- Watch the sky. Cumulus clouds that grow taller through the day, with darkening bases, show instability turning into showers. Land before they arrive: storm outflow can bring sudden strong gusts kilometres ahead of the rain.
- Give light airframes more margin. Foam and small fixed-wing models are thrown around more by the same thermal than heavier aircraft.
- Check the whole day. Use Flight Window for wind and precipitation, and the Weather diagrams to see how conditions develop through the afternoon.