Some people say they look like fingerprints, others like ripples on water or the patterns left on sandflats when the water has retreated.
Either way, they are one of Mother Nature’s more picturesque cloud formations. Altocumulus undulatus or Altocumulus stratiformis undulatus with the ‘unda’ referring to the Latin term for wave.
These clouds are created when two layers in the atmosphere, one on top of the other, have different speeds.
This is called wind shear and it can cause the cloud to bunch up or roll into ridges perpendicular to the flow of the wind.
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The lines you see in the cloud are perpendicular to the direction of the wind at that height in the atmosphere. This is a very similar process to wind over water making ripples.
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