concept

Mountain wave

Standing gravity waves that form downwind of a ridge: when stable air is heaved up by the mountain, it overshoots its equilibrium, gets pushed back down, and goes on oscillating downstream as a train of waves that barely move relative to the ground. Lenticular clouds mark the wave crests, and beneath them churn dangerous rotors — reversed, tumbling eddies that have destroyed aircraft. The rising branch is the strongest and tallest updraft the atmosphere offers: on 2 September 2018 the Airbus Perlan Mission II sailplane rode mountain-wave lift over the Andes to 23,203 m, or 76,124 ft, entirely engineless.