Altocumulus lenticularis clouds are known for their smooth, lens-like appearance. They form at the crests of invisible airflows that rise and dip to form standing waves downwind of raised terrain. Lenticularis clouds owe their smoothness to the way the air’s moisture is consistently condensing and evaporating at the same points as the air flows through the peak of the wave. Sometimes, however, they can also display some very pointy features.

Tom Bean (Member 41,135) noticed one like this over Coconino County, Arizona, US. It seemed to have a serrated edge. This uncharacteristic jaggedness resulted from a strong shearing effect of wind flow at the top of the cloud. The air must have been flowing distinctly faster just above the lenticularis cloud than within it. Such an abrupt difference in windspeed with elevation can ruffle any cloud edge in between. In this way, Tom’s smooth cloud was teased out into parallel ridges, known as undulatus, just visible on its far side and licked up into sharp, curled spines, known as fluctus, along its top.