APAD: Mackerel Sky

Meaning:

   A Mackerel sky is a sky that is streaked with rows of small white clouds

   which resemble the pattern of scales on a mackerel's back.

 

Background:

   There was no idiomatic dexterity involved in the derivation of this phrase;

   mackerel skies do look like the markings on a mackerel's back.

 

   The term has been in use since the 17th century and was first out into print

   by the appropriately named Thomas Sprat, in The History of the Royal-Society

   of London, for the improving of natural knowledge, 1667:

 

     Let Water'd signifie a Sky that has many high thin and small Clouds,

     looking almost like water'd Tabby, called in some places a Mackeril Sky.

 

   Meteorologically speaking, a mackerel sky is created when mid-level moisture

   is trapped between dry air below and cold dry above.

 

   The compressing of the cloud between the two air pockets forms the

   characteristic rippled formation.

 

- www.phrases.org.uk [edited]

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When it comes to clouds, I knew nimbus and cirrus and had always wondered what

to call those arrayed like fish scales. Also learned the word 'sprat' above.

Wonderful!