Lenticular clouds (Altocumulus lenticularis) are stationary lens-shaped clouds that form at high altitudes, normally aligned perpendicular to the wind direction. Lenticular clouds can be separated into altocumulus standing lenticularis (ACSL), stratocumulus standing lenticular (SCSL), and cirrocumulus standing lenticular (CCSL). Due to their shape, they have been offered as an explanation for some Unidentified Flying Object (UFO) sightings.

Where stable moist air flows over a mountain or a range of mountains, a series of large-scale standing waves may form on the downwind side. If the temperature at the crest of the wave drops to the dew point, moisture in the air may condense to form lenticular clouds. As the moist air moves back down into the trough of the wave, the cloud may evaporate back into vapor.