Mountain waves

Can you see the air bouncing?

Waves in the sky

Five real photographs. Swipe or tap Next.

1. Ripples downwind of the peaks When an airstream crosses high ground, the ridge disturbs the flow. In the right conditions a train of waves forms on the lee side: a mountain wave or lee wave. From the space station these Aleutian peaks leave rows of wave clouds stretching downwind.
2. Cap, lenticular and rotor A cap cloud sits on the summit; strong wind can pour it down the lee slope like a waterfall, the Föhn wall. Smooth, lens-shaped lenticular clouds sit at the wave crests downwind. Under the first crest, a rotor (roll) cloud turns on a horizontal axis. The clouds keep forming on their upwind edge and dissolving on their downwind edge, so they seem to stand still.
3. The lens that warns you Lenticular clouds form a few thousand feet above the crests on the leeward side. Smooth edges mean smooth wave lift; ragged edges warn of turbulence.
4. Waves behind any obstacle Even a chain of islands can set up waves. Here, the Kuril Islands leave V-shaped ship-wave patterns in the cloud deck downwind.
5. Mother-of-pearl clouds Waves over big ranges can reach well into the stratosphere, carrying water vapour up to form nacreous (mother-of-pearl) clouds. These polar stratospheric clouds were photographed from NASA's DC-8 at about 39 000 ft over Norway.

Takeaway: Stable air pushed over a ridge springs back down and overshoots, like water over a rock in a stream. The wave train stands still while the wind blows through it, and clouds mark its crests.

When does a mountain range make waves?

What makes a wave

The wind and the air a mountain wave needs. Tap Next.

1. Across the ridge The wind must blow across the ridge, not along it. It can be up to 30° off square (perpendicular) to the ridge and still make waves. Wind blowing along the ridge just flows past it and makes no waves.
2. Strong enough The wind must also be strong: at least 7 m/s (about 14 kt) over small hills, and at least 15 m/s (about 30 kt) over large mountain ranges. A light wind does not push the air over the high ground hard enough to set it swinging up and down. The bigger the mountains, the stronger the wind they need.
3. Deep and steady The flow should be deep: the wind blows from much the same direction through a thick layer of air. It should also get stronger with height. Then the waves can build and stretch far downwind.
4. Stable below, less stable above Stable air resists being moved up or down, so when it is pushed over a ridge it springs back and swings like a spring. A stable layer at and near the crest gives these waves. Above it the air should be less stable.

Memory hook: Across (≤ 30°), strong (7 / 15 m/s), stronger with height; stable below, less stable above.

Takeaway: A mountain wave needs the right wind and the right air. The wind must blow across the ridge (within 30° of square to it), at least 7 m/s over small hills and 15 m/s over large ranges, from the same direction through a deep layer and getting stronger with height. The air must be stable near the crest, with less stable air above.