The essentials in two minutes
It all starts from a simple idea: warm air rises, cold air flows downhill. By day, the sun heats the slopes, which heat the air in contact with them; this lighter air flows up the slope: this is the upslope breeze. At night, the slopes cool down, the air in contact with them becomes heavy and flows down to the valley floor like syrup: this is the downslope breeze. It fills the valley floors with a cold-air pool.
On a larger scale, the whole valley breathes. In the afternoon, the air in the valley is warmer than the air over the neighbouring plain: it “draws in” the air from the plain, and a wind blows up the valley towards its head. This is the valley breeze, commonly 10 to 20 km/h, sometimes more than 30 in large valleys. At night, the movement reverses: the mountain breeze blows down the valley towards the plain.
These two breezes run to a clock. In the morning, the slopes switch over first, as soon as the sun touches them; the valley follows two or three hours later. In the evening, the reversal starts on the slopes in shadow, then spreads to the whole valley. So each slope has its hour: the east-facing ones in the morning, the south-facing ones at midday, the west-facing ones in the afternoon.
Where two breezes meet — on a ridge between two valleys, where two valleys join, on the shore of a lake — the air has only one way out: up. These convergences produce the best lift and the first cumulus. Finally, the general wind is superimposed on the breezes: when it is light, it lets them be; when it is strong, it wipes them out; when it blows against them, it creates wind shear.
For the pilot: launch when the slope breeze is established on your face, and plan your landing in the afternoon valley breeze. For the hiker: the wind that picks up around midday and the clouds that cling to the ridges in the early afternoon are the normal breathing of the mountain — not necessarily a change in the weather.
Read on
The whole chapter is open to subscribers, with its animated figures: €3 a month or €25 a year. The book as an ePub, €6.99, comes with a month’s subscription. Contents:
- 7.1A breath that lasts twenty-four hours
- 7.2Slope breezes
- 7.3The valley breeze and the mountain breeze
- 7.4Each slope has its hour
- 7.5Confluences and convergences
- 7.6Lake and sea breezes
- 7.7Landing in the valley breeze
- 7.8When the general wind joins in
- 7.9Evening: reversal and katabatic winds
- 7.10A breeze day, from launch to trail
- Exercises
- Key figures to remember
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Chapter glossary#
- Upslope breeze (anabatic)
- Light wind that blows up a slope heated by the sun. It feeds the thermals and the cumulus above the ridges.
- Downslope breeze (katabatic)
- Downward flow of air cooled against a slope. In the evening and at night, or permanently over a glacier.
- Cold-air pool
- Accumulation of cold air on the floor of a valley or in a basin, capped by an inversion. Fog and frost form in it.
- Valley breeze
- Wind that blows up the valley by day, because the air in the valley warms more than the air over the plain.
- Mountain breeze (down-valley breeze)
- Wind that blows down the valley at night.
- Reversal
- Change in the direction of a breeze, in the morning or the evening, often preceded by a calm and accompanied by shifting winds.
- Convergence
- Meeting of two airflows which, unable to accumulate, rise. A line of lift and of cumulus.
- Lake breeze (sea breeze)
- Wind that blows from the cooler water towards the shore in the afternoon. Stable air over the water.
- Breeze front
- Boundary between the cool air of a lake or sea breeze and the warmer air inland; a convergence line.
- Glacier wind
- Katabatic wind that blows down a glacier all the time, even in sunshine, and emerges cold at the snout of the glacier tongue.
- Solar time
- Time set by the position of the sun (solar noon = sun at its highest). In summer in the southern French Alps, clock time ≈ solar time + 1 h 30 min.