an extensive group of cirrocumulus or altocumulus clouds, especially when well-marked in their arrangement: so called because of a resemblance to the scales on a mackerel.
Origin of mackerel sky
First recorded in 1660–70
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2019
a sky patterned with cirrocumulus or small altocumulus clouds
Word Origin for mackerel sky
from the similarity to the pattern on a mackerel's back
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012