Undulatus – A Sky for the Seashore

We use the undulatus cloud classification when one of the main cloud types, such as the this mid-level cloud Altocumulus, is arranged into a pattern of parallel ridges or undulations. This is a common, often stunning, cloud variety, which is a gentle reminder that our atmosphere is an ocean of air.  

The rows of undulatus clouds are always aligned perpendicular to the main flow of the wind up at the cloud level. Sometimes, they resemble a broad ocean swell. At other times, they look more like those little ridges of beach sand caused by the wind or the ebb and flow of the waves at the water’s edge. When the undulatus pattern appears in a cloud layer that is made up of lots of little cloudlets, they can even resemble the markings of scales on the sides of fish. This is why the more colloquial name for certain formations of Altocumulus undulatus and its higher cousin Cirrocumulus undulatus is a ‘mackerel sky’.

Just as ocean waves are generated by wind, so too are waves in the atmosphere. The distinctive patterns of undulatus are generally the result of vertical wind shear, when winds above and below the cloud flow at markedly different speeds and/or directions. The shearing effect in between where the cloud is causes the flowing air to rise and dip. Cloud formation is encouraged where the air rises and cools and discouraged where it dips and warms.

If you ever walk barefoot along the ocean shore beneath an Altocumulus undulatus sky, just remember what’s special about that moment and that place. You’re right where sea, land, and sky meet – where the ocean waves tumbling beside you, the ripples of sand pressing into your soles, and the row upon row of undulatus clouds drifting overhead have all, for one moment, come together.

Altocumulus perlucidus undulatus spotted over Gold Coast, Queensland, Australia by Matt Bennett, friend of Lucien Simon (Member 26,367). View this in the photo gallery.