The view from my bedroom window last week was spectacular. I found out later this cloud formation is known as a “Mackerel Sky”. I had to know more so I asked Wiki:
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.
But a little more reading revealed further gifts:
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), and in Italy the clouds are known as pecorelli (little sheep).