A mackerel sky we used to call it - and I have only recently discovered that we didn't invent the term.
The "we" in the above is my family in Ireland.
From time to time an Irish sky, seldom without some cloudiness, would have a mottled cloud covering - reminiscent of the scales on a mackerel's body. I understood what it looked like from the time I was quite young - because mackerel was a favorite fish of my older sister and I. We used to get it fresh from the sea - Ireland being an island after all.
So, with our fondness for the fish, of course our description of the sky would be "ours".
Not so much, it would seem.
Mackerel sky is a real term, used by real people, far away from where I was reared.
Ah well, sometimes that is the way of things.
The sky this evening as we drove home from the train station put me in mind of Summer days in Ireland. 'Twas a mackerel sky.