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

Sayings related to this long-recognized phenomenon include:
"Mackerel sky, mackerel sky. Never long wet and never long dry"
"Mackerel sky, not twenty-four hours dry"
"Mares' tails and mackerel scales make lofty ships to carry low sails"
"A dappled sky, like a painted woman, soon changes its face"

In France it is sometimes called a ciel moutonné (fleecy sky);
in Spain a cielo empedrado (cobbled sky);
in Germany it is known as Schäfchenwolken (sheep clouds), and
in Italy the clouds are described as a pecorelle (like little sheep).
Reference: Wikipedia