Altocumulus clouds are mid-level clouds found anywhere from two to seven km above the ground.  They often display visible rows or ripples of white that alternate with blue sky.  Of all the 10 different cloud types, altocumulus clouds are the most diverse in terms of appearance.  These clouds are commonly found between weather systems and are a sign of approaching instability in the atmosphere.  
Less formally, that layer of cloud is known as a “washboard sky,” and what’s a special sky without a rhyme?  
Grandma Says: “Washboard sky – not three days dry.” 
Reference: https://www.saltwire.com/weather/grandma-says/grandma-says-washboard-sky-be-quick-to-hang-your-wash-to-dry-314080/?location=newfoundland-labrador