Mackerel sky

A mackerel sky is a term for clouds made up of rows of cirrocumulus or altocumulus clouds displaying an undulating, rippling pattern similar in appearance to fish scales;[1][2] this is caused by high altitude atmospheric waves.[3]

Mackerel sky
Altocumulus mackerel sky
Abbreviation	Ac
Symbol
Genus	Alto- (mediumhigh) -cumulus (heaped)
Appearance	Clumps and rolls of clouds that resemble mackerel scales
Precipitation	No, but may signify approaching precipitation.

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.[4] 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 that the warm front or low front has moved closer and it may start raining within less than six hours.[5] The old rhymes "Mackerel sky, not twenty-four hours dry"[3] and "Mares' tails and mackerel scales make lofty ships to carry low sails"[6] 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".[4]

It is sometimes known as a buttermilk sky, particularly when in the early cirrocumulus stage, in reference to the clouds' "curdled" appearance.[7]