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The Collaborative International Dictionary
mackerel sky

mackerel sky \mackerel sky\ n. A sky filled with rows of cirrocumulus or small altocumulus clouds.

Wiktionary
mackerel sky

n. a sky filled with a regular pattern of altocumulus clouds somewhat resembling the skin of a mackerel.

WordNet
mackerel sky

n. a sky filled with rows of cirrocumulus or small altocumulus clouds

Wikipedia
Mackerel sky

A mackerel sky is a common term for a sky with rows of cirrocumulus or altocumulus clouds displaying an undulating, rippling pattern similar in appearance to fish scales; this is caused by high altitude atmospheric waves.

Cirrocumulus appears almost exclusively with cirrus some way ahead of a warm front and is a reliable forecaster that the weather is about to change. When these high clouds progressively invade the sky and the barometric pressure begins to fall, precipitation associated with the disturbance is likely about 6 to 12 hours away. A thickening and lowering of cirrocumulus into middle-étage altostratus or altocumulus is a good sign the warm front or low has moved closer and it may start raining within less than six hours. The old rhymes "Mackerel sky, not twenty-four hours dry" and "Mares' tails and mackerel scales make lofty ships to carry low sails" both refer to this long-recognized phenomenon.

Other phrases in weather lore take mackerel skies as a sign of changeable weather. Examples include "Mackerel sky, mackerel sky. Never long wet and never long dry", and "A dappled sky, like a painted woman, soon changes its face".

It is sometimes known as a buttermilk sky, particularly when in the early cirrocumulus stage, in reference to the clouds' "curdled" appearance. In France it is sometimes called a ciel moutonné (fleecy sky); and in Spain a cielo empedrado ( cobbled sky); in Germany it is known as Schäfchenwolken (sheep clouds).

Usage examples of "mackerel sky".

The wind was no more than Force three, the sea calm enough to keep the San Andreas on an all but steady keel, and the cloud cover consisted of no more than a wide band of white, fleecy, mackerel sky against the pale blue beyond.

It was cloudNbreeches he I but with a thin-layered mackerel sky, through which the full moon shed plenty of light.

The sun was bright in the mackerel sky and it was warm as I drove through the dale between the black peat hills of Mid and East Kome.

The whole world was suddenly ablaze, the mackerel sky overhead flaming to a surrealist pattern.

The mare's tails thickened into a mackerel sky, ridge after ridge of gray-white against pale blue.

The mares tails thickened into a mackerel sky, ridge after ridge of gray-white against pale blue.

A mackerel sky still tinged with the reds and oranges of a fiery dawn glowed above the horizon.

Three or four Windships rode at anchor against a mackerel sky, above the far meadow.

No one will ever record my deeds, but I have been strength and determination and lie gray silent against the mackerel sky where the bulk of me reassures.

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

But now they were higher and smaller, settling at last into a mackerel sky like a beach at low tide.