What is the term for a cloud that forms like a smooth lens, often over mountains?

A cloud that forms like a smooth lens, often over mountains, is called a lenticular cloud. Its name comes from the Latin word lenticula, meaning “little lentil,” referring to its rounded, lens-like shape.

Lenticular clouds develop when moist, stable air flows over a mountain or ridge. The terrain produces standing atmospheric waves on the downwind side. Where air rises within one of these waves and cools to its dew point, water vapor condenses into a cloud. Although the cloud appears stationary, air is continuously moving through it.

These clouds can occur at low, middle, or high levels, including as stratocumulus, altocumulus, or cirrocumulus lenticularis. They are often mistaken for flying saucers because their smooth, flattened shapes can look remarkably artificial. Their presence can also signal strong turbulence and sinking air near mountains, so powered-aircraft pilots generally avoid them.

Glider pilots, however, seek the rising air associated with the wave system. The same atmospheric lift can allow gliders to climb high and travel long distances without an engine.