Altocumulus lenticularis: the lens cloud that holds still

A smooth outline is only the first clue. Check what the cloud is attached to: Altocumulus lenticularis is a detached lens or stack at middle level; pileus sits directly on a growing cumulus tower; a cap cloud clings to the terrain.

Then watch the lens for a few minutes. It can appear fixed even in strong wind because new droplets form on the upwind edge while others evaporate downwind. The air moves through the cloud while the wave crest holds its position. 1

Read it mainly as a sign of airflow aloft. Lenticular clouds reveal mountain-wave activity, which may extend well beyond the visible cloud and can bring strong gusts or aviation turbulence. Dry air can hide the cloud without removing the wave. 23

The formal identification follows the WMO definition: a lens- or almond-shaped patch, often elongated, with a well-defined outline. 4