Thermals forecast — how to read it

The thermals map is a soaring-specific forecast built from ICON-D2's boundary-layer parameters. Glider pilots (Segelflug, vol à voile) use it for task planning; powered GA pilots can read the bumpiness index to anticipate low-level turbulence on a hot day.

What the map shows

• Thermal strength — average climb rate in m/s that a competent glider pilot could sustain.
• Usable top of climb — the altitude above which thermals no longer sustain climb (the top of the boundary layer, roughly).
• Cumulus base — where Cu clouds are expected to form, if they form at all. Blue-thermal days ("blaue Thermik") have no Cu at all.
• Bumpiness index — a scalar for how turbulent the boundary layer will feel to a fixed-wing aircraft.

How to read thermal strength (glider pilots)

• Below 1 m/s: weak thermals. Only worth flying if you're a glider club member with time to burn.
• 1 to 2 m/s: usable. Local flying, short tasks.
• 2 to 3 m/s: good day. Cross-country distances of 200–400 km are on the table.
• Above 3 m/s: strong day. Long distance possible, but also expect strong sink and possibly convective overdevelopment by afternoon.

How to read bumpiness (GA pilots)

The relationship to convection

Strong thermals + humid airmass = convective overdevelopment. On days where the thermals map is bright red across a wide region, the CAPE map is usually also elevated, and the day that started as classic soaring can turn into a thunderstorm day by 16:00 local. Watch both together.

Model caveats

Thermal strength depends strongly on surface heating, which the model forecasts well over open terrain but less well over cities, water and forest patches. Trust the regional pattern, but verify against real conditions once airborne. Glider club forecasts (e.g. TopMeteo, XCSkies) offer additional detail if you want to compare.