A mackerel sky.
mackerel sky definition by Webster's New World
Webster's New World College Dictionary Copyright © 2010 by Wiley Publishing, Inc., Cleveland, Ohio. Used by arrangement with John Wiley & Sons, Inc.
a sky covered with rows of small, fleecy, cirrocumulus or altocumulus clouds, suggesting the streaks on a mackerel's back
mackerel sky definition by American Heritage Dictionary
noun Chiefly Northeastern U.S.
A sky covered with many small cirrocumulus or altocumulus clouds, resembling the markings found on a mackerel. Also called regionally buttermilk sky.