The World From Your Airplane Window

A science writer’s guide for the inquisitive air traveler

(Pierre Selim)

Cloud 9

(Muhammad Mahdi Karim)

So far the view has been superb. But at some point on the journey you're likely to pass into cloud. Clouds are divided by type [here stratocumulus mix with cumulus in the foreground, with cumulus beyond]. These correspond both to the height at which the cloud is located and the shape and density of the cloud. There are technically a great number of cloud types—around 52.

The original classification identified three families of clouds. These were cirrus (from the Latin of 'hair'—hence wispy, thin clouds), cumulus (meaning a 'heap' or 'pile' for obvious reasons), and stratus (meaning a 'layer' or 'sheet').

This early structuring was done in 1802 by a pharmacist and amateur meteorologist from London, Luke Howard, and picked up by the likes of landscape painter John Constable, who produced reams of cloud studies. Later, in 1896, the clouds were grouped into nine basic forms, each given a number from 1 to 9. This was later revised to include ten cloud forms—1 to 10. But the World Meteorological Organization (WMO), the body responsible for the numbering, later changed the range again to be 0 to 9.

This final change of numbering was for a surprisingly romantic reason. The cloud type with the number 9 (which later briefly became 10) was the cumulonimbus. Although this is classified as a low cloud because its base starts well down, the peaks of a giant cumulonimbus climb higher than any other cloud. If you were perched on top of a cumulonimbus you could consider yourself on top of the world—and this is where the expression 'on cloud 9' comes from. The WMO realized they were being spoilsports turning a cloud 9 into a cloud 10, so they reversed their decision.


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