photo #5672554

The same wind shear that caused the curled fluctus features of the Cirrus clouds in yesterday’s Cloud-a-Day can also produce a pattern of parallel ridges known as undulatus. Anthony Skellern (Cloud Appreciation Society Member 19,011) spotted this ribbed sky carpet over Macclesfield, Cheshire, England. Not as high as yesterday’s Cirrus fluctus, the undulatus pattern appeared in the mid-level cloud Altocumulus. The same shearing effect of air flowing significantly faster above the cloud than below it (or in different directions) has caused the air between to rise and dip as it flowed along.Like a rug ruffling up as it is slid across the floor, the cloud developed a pattern of parallel ridges aligned perpendicular to the wind. This is because rising air encourages cloud to form, and dipping air suppresses it. We could, while we’re at it, add another term to this Altocumulus undulatus formation, radiatus, because the wind-shear-induced ridges appear to fan out. They are in fact parallel, but they cover enough of the sky to seem to radiate thanks to the effect of perspective.