Crests of standing mountain waves may be marked by stationary, lens-shaped clouds known as what?

• A. Standing lenticular clouds
• B. Cirrus uncinus
• C. Cumulus congestus
• D. Altostratus

Answer: (A)

Crests of standing mountain waves are marked by stationary, lens-shaped clouds known as lenticular clouds. These form when stable air flows over a mountain and is lifted into the cooler, saturated layer, condensing into smooth, almond-shaped lenses that sit in roughly the same position as the wave pattern moves beneath the air. The standing nature comes from the wave crests remaining fixed in the airflow, so the lenticular caps appear stationary relative to the ground. This cloud form is a telltale sign of mountain-wave activity and potential lift in the area, though turbulence can accompany the wave and rotor regions may occur below the cloud. Cirrus uncinus are wispy cirrus with hooked ends, cumulus congestus are tall, vertically developed clouds, and altostratus is a mid-level gray sheet—none of these indicate standing mountain-wave crests in the same way.