How to Read a Thermal Forecast: Working Height, Climb Rate and Cloud Base
What thermal height, cloud base, usable working height and climb rate (w*) mean — and how to use them to judge whether a day is good for paragliding or hang gliding.
A thermal forecast answers three questions: how high can I get, how strong is the lift and when does it start? Four numbers cover this, and the thermal map has each of them as its own layer under “Show weather → Thermals”.
Thermal height: how far the thermals reach
The sun heats the ground, and the ground heats the air above it. This warmed layer — the convective boundary layer — grows through the morning until it hits a stable layer (an inversion). Its top is the thermal height. Above it, nothing lifts you any more; thermals weaken and break up near the top.
Cloud base: where the cumulus starts
Rising moist air cools until its water vapour condenses. That height is the cloud base. If it lies below the thermal height, cumulus clouds form — the best signposts to the next thermal. If it lies above, it’s a blue day: the thermals are there, but invisible.
Usable working height: the number to plan with
For pilots, whichever is lower counts: thermal height or cloud base — you don’t climb into the cloud. The map shows this as usable working height above ground. As a rough guide:
- below 800 m — short thermal flights at your local hill, little margin for valley crossings
- 800–1500 m — solid thermals, local cross-country possible
- above 1500 m — a good cross-country day, if wind and thunderstorm risk allow
Mind the reference: “above ground” means above the model terrain. Over a summit there’s less, over a deep valley more. That’s why a launch’s meteogram also shows working height above sea level.
Climb rate (w*): how strong the thermals are
w* (the “Lift (w*)” layer) is the average updraft speed in the boundary layer. Your vario shows more in the core and less at the edge — and your wing sinks by about 1 m/s on its own. Rule of thumb for what’s left:
- below 1 m/s — weak, takes patience to stay up
- 1–2 m/s — very usable, pleasant
- 2–3 m/s — strong, can be rough
- above 3 m/s — powerful; for newer pilots more of a watching day
When does it start?
Thermals need time. They’re usually good enough for first launches two to three hours after sunrise and peak in the early afternoon. Drag the time slider through the day or press ▶: you’ll see the working height grow — and whether thunderstorm risk or shading from cloud cover comes in later.
Model limits
ICON-D2 works on a 2.2 km grid. A ridge, a sunny south face or a shady side valley gets smoothed out. Treat the numbers as the picture for the region, not a promise for your slope — and compare them with what you see in the sky: where there’s cumulus, there’s the base.