Mackerel sky

1717, Dictionarium Rusticum, Urbanicum & Botanicum, London: J. Nicholson et al., 2nd edition, “CLOUDS,” […] in a fair day, if the Sky seem to be dappled with white Clouds, which is usually termed, A Mackerel-Sky, it commonly predicts rain:

The clouds were of the fleecy kind, which sailors denominate a mackerel sky.

The sky was what is called a mackerel sky, rows and rows of faint down-plumes of cloud, just tinted with the midsummer sunset.

The clouds came racing over the ridge from the south as they had on the May evening when Hazel first climbed the down. But now they were higher and smaller, settling at last into a mackerel sky like a beach at low tide.