Thermals

The essentials in two minutes

The sun does not heat the air: it heats the ground, and the ground heats the thin layer of air resting on it. Some surfaces heat better than others: dark scree, a harvested field or a village heat up quickly; a lake, a damp forest or a snowfield stay cold. Above the good surfaces, a layer of warm air forms, swells, slides up the slopes, then suddenly breaks away at a terrain feature — a break of slope, a ridge, a summit — like a drop that grows at the end of a tap before it falls, but upside down.

Once it has left, the bubble rises as long as it stays warmer than the air around it. It has a strong core and soft edges, and around it the air sinks. The wind leans it over: the thermal is never directly above its source, but downwind of it, and the cloud it builds is even further away. When the bubble reaches the condensation level, it draws a cumulus — the pilot’s signpost, which lives for a quarter of an hour to half an hour.

During the day, everything follows the sun: the first thermals come off the east faces in mid-morning, convection is strongest and highest in the early afternoon on the south and south-west faces, then it calms down; in the evening, forests and rocks slowly give back their heat. To make use of all this, three steps: centre (shift your circle towards the side that climbs best), glide (leave high, speed up in what sinks, slow down in what rises), and always keep a landing field within reach.

For the hiker, thermals mean the heat and the afternoon breeze, then the growing cumulus: on humid, unstable days, the same engine builds the 15:00 thunderstorm (chapter 9).