Breezes: how the valleys breathe

The essentials in two minutes

1. Air heated by the slope: upslope breeze (1 to 3 m/s, a thin layer); cooled air: downslope breeze and a cold-air pool on the valley floor. Breezes rule when the general wind is light (under 10 km/h at the summits).
2. The whole valley breathes: an up-valley breeze in the afternoon (10 to 20 km/h, 25 to 35 in the large valleys), a down-valley mountain breeze at night (5 to 15 km/h).
3. The slopes switch over quickly, the valley follows two to three hours later; think in solar time (summer: clock time − 1 h 30 min).
4. Each side has its hour: east in the morning, south at midday, west in the afternoon.
5. Convergences (ridges, confluences, passes, breeze fronts) give the best lift and the first cumulus — and the first showers on unstable days.
6. Over a lake in the afternoon: stable air, no thermals; a breeze from the water towards the shore.
7. On landing: final approach into the valley breeze, away from the wake of obstacles, and watch out for the evening reversal.
8. General wind over the back at launch: the breeze can make it look like a good launch; it is the general wind that decides. For the hiker, the midday wind and the afternoon cumulus are normal; it is how fast they grow that is the warning.