Mountain waves occur when stable, strong wind crosses an orographic barrier. Air oscillates vertically on the lee side, forming lenticular clouds at wave crests. Glider pilots exploit them to reach extreme altitudes (record: >23,000 m).
They generate clear-air turbulence dangerous for aviation, with downdrafts exceeding 10 m/s. In Spain, the Pyrenees, Sierra Nevada, and Cantabrian Range produce frequent mountain waves with northerly or north-westerly wind.