Mackerel sky

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.