October 2023

Ripples on the Beach of the Sky

Of all the ten main cloud types, Cirrocumulus is probably the least common. This high cloud is composed of tiny clumps, or ‘cloudlets’, and it is a formation in transition, which means it doesn’t hang around for long. No sooner has Cirrocumulus formed then it starts to change into one of the other high clouds: the streaks of ice crystals called Cirrus or the delicate ice-crystal layers known as Cirrostratus. This cloud is also typically limited just to isolated patches of the sky. So a Cirrocumulus display like this one spotted over Alamos, Sonora, Mexico by Suzanne Winckler (Member 41,844) extending across the whole sky, the species called stratiformis, is an even rarer form of this already-uncommon cloud.

You’ll know you’re spotting Cirrocumulus when a cloud looks really high and has a granular appearance, resembling grains of white sand on a beach – a sky-blue coloured beach, of course. Its cloudlets look tiny because they are a long way away, forming in the high part of the troposphere (the region of our atmosphere where weather happens) at altitudes around 4.5 miles (7 km). Just like beach sand at the water’s edge, Cirrocumulus is often arranged into parallel ripples, which are known as undulatus. These are caused by shearing winds, when the wind speed or direction varies markedly with altitude. This causes the air in between to rise and dip in wavelike ridges that shape the arrangement of the cloudlets.

In addition to ice crystals, Cirrocumulus contains water droplets that are extremely cold. These droplets are described as ‘supercooled’ because they stay in liquid form despite the frigid conditions in the high troposphere, where the air temperatures are around -30 to -40 degrees Celsius and Fahrenheit. While water can remain in a supercooled liquid state at these temperatures, it can’t for long, and so the droplets soon freeze. Once the Cirrocumulus freezes entirely, it will be no longer – having made the transformation into Cirrus or Cirrostratus clouds. ‘I was impressed by the extent of the Cirrocumulus undulatus,’ Suzanne remembers. ‘It rippled across the entire sky. This was a very soothing end-of-day kind of cloud.’

Cirrocumulus stratiformis undulatus spotted over Alamos, Sonora, Mexico by Suzanne Winckler (Member 41,844). View it in the Photo Gallery.

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