Thermals

The essentials in two minutes

1. A thermal is born from a source warmer than its surroundings (dry, dark, well-exposed ground: scree, a village, a harvested field) and from a trigger that releases the warm layer (a break of slope, a ridge, the edge of a forest).
2. It rises in bubbles or as a column; it has a core (3 to 4 m/s on a good day), edges and a ring of sink (−1 to −2 m/s); it is strongest in the middle of the convective layer.
3. The wind tilts it: look for it downwind of its source and upwind of its cloud (climbing at 2 m/s in an 18 km/h wind: 2.5 km of drift per 1,000 m).
4. A cumulus lives 15 to 30 min: head for the clouds that are being born (sharp outlines, flat base), avoid those that are fraying.
5. Cloud streets (spaced 2 to 3 times the depth of the layer) and convergences organise the lift over kilometres; convergences are also the cradle of the first thunderstorms.
6. The right face follows the sun one to two hours behind: east in the morning, south in the early afternoon, west in the evening; in the evening, forests and west faces give back their heat (0.5 to 1 m/s, smooth air).
7. Centring: shift your circle towards the strong side. Gliding: leave high, speed up in what sinks, slow down in what rises, keep a landing field within reach (a glide ratio of 8 to 9 in calm air, much less into wind).
8. For the hiker: set off early, check how the cumulus are growing every half hour, and bring the turnaround forward when they are building before 11:00.