Lenticular cloud

Lenticular clouds, also called standing lenticular clouds, are smooth, often lens- or saucer-shaped formations. They commonly develop downwind of a mountain range when relatively stable air is forced over a topographic barrier. The air then oscillates in gravity waves. If moisture is sufficient, cooling as air rises near a wave crest can produce cloud droplets or ice crystals; as air descends and warms, the cloud evaporates or sublimates. The cloud's shape therefore marks a region of wave activity rather than a fixed parcel of air.

In a persistent mountain wave, air flows continuously through the cloud. Cloud particles form on the rising side and disappear on the descending side, so the cloud can remain in roughly the same place despite fast winds. A National Weather Service account of a Hawaiian event describes lenticular clouds forming near atmospheric wave crests over Mauna Loa and Mauna Kea as strong winds passed over the mountains; the clouds appeared nearly motionless while continually reforming and dissipating.

Mountain waves can extend downwind of terrain, and aircraft encountering them may experience strong turbulence. The U.S. National Weather Service notes that lenticular clouds can be a visible clue to mountain waves, although waves and turbulence may still occur when the air is too dry for clouds to form. The clouds themselves do not cause turbulence; they indicate conditions associated with the wave. Accordingly, their presence is relevant to pilots, but a cloud-free sky does not rule out mountain-wave hazards.

Lenticular describes a cloud's form, not a single independent cloud genus. NOAA's satellite and education service describes lenticular clouds as special cloud forms and notes that they may occur with flow over hilly terrain or rising air over flatter terrain. Their appearance alone is not a reliable forecast of a specific weather event. Observation of the surrounding wind, moisture, terrain and atmospheric stability is needed to interpret them.