It seems I've known what this is all my life. I just used it in conversation and Wally had never heard it. We looked up pictures so I could show him, and he immediately said "That's a buttermilk sky!"
Ok, I buy that, it looks like curdles. In my mind though, a buttermilk sky had been huge fluffy clouds, like biscuits.
I'm wondering if it's a region thing, as I grew up near the ocean.
So what do you call this image below?