A lenticular cloud (altocumulus lenticularis, or ACSL) is a distinctive lens- or saucer-shaped cloud that forms in standing waves on the lee side of mountains. When stable air flows over a mountain range, it oscillates vertically in a wave pattern downwind. At each wave crest, the air cools to its dew point and a cloud forms; at each trough, it warms and the cloud evaporates — creating stationary cloud lenses that hover in place despite strong winds.
Lenticular clouds can be stunningly beautiful, with smooth, polished edges and sometimes iridescent colours caused by diffraction of sunlight through uniform water droplets. They have been famously mistaken for UFOs due to their otherworldly appearance. In Spain, notable examples form over the Sierra Nevada, the Pyrenees, and Teide (Tenerife), where the so-called "Teide hat" (sombrero de Teide) is a well-known lenticular formation.
For aviation, lenticular clouds are a serious warning of mountain wave turbulence, which can be severe or even extreme. The rotor zones beneath lenticular clouds generate violent, turbulent eddies dangerous to all aircraft. Conversely, glider pilots actively seek mountain waves, using the smooth uplift to reach extraordinary altitudes — the glider altitude record exceeds 23,000 m, achieved in Patagonian mountain waves. Lenticular clouds are most common in winter and spring when strong upper-level winds combine with stable atmospheric layers.