Wind and terrain

The essentials in two minutes

The wind blowing on weather charts is the “free-air” wind, the wind aloft, far from the ground. The closer you get to the ground, the more it rubs against it and slows down: this is the wind gradient. Right at ground level, the wind is often only half the free-air wind; a few hundred metres up, it has almost recovered. In the afternoon, thermals mix the air and bring this strong wind down to the ground, in gusts.

When the wind meets a mountain, it behaves like water over a rock. On the windward side, the air is forced to rise: this is ridge lift, the lift used for ridge soaring. Above the crest, the streams of air squeeze together and the wind speeds up, to one and a half to two times its strength over the plain. In a pass, the air is constricted and speeds up too: this is the Venturi effect, the one that whips your cap off as you cross the pass.

Behind the ridge, in the lee, it’s the opposite: the air plunges, breaks away from the terrain and spins in rotors, like the eddies behind a rock in a river. This is the area to avoid, in the air as on the ground: sink, turbulence, and a ground wind sometimes blowing the wrong way. If the air is stable, it can even rush down the lee slope in violent gusts before bouncing back up in a hydraulic jump.

Two large-scale phenomena complete the picture. The foehn: moist air crossing a mountain range leaves its rain on the windward side, then comes down the other side dry, warm and gusty. Waves: behind a range, stable air starts to oscillate like the surface of a torrent behind a stone; lens-shaped clouds (lenticulars) are their signature, and the rotors under these waves are among the most dangerous kinds of turbulence.

For the pilot: you launch into wind, never in the lee, and you’re wary of a launch that “works” when the general wind is coming from behind. For the hiker: the ridge and the pass are the windiest places on the outing, and the wind multiplies the cold. At 0 °C, a 50 km/h wind feels like −8 °C.